Unleashing the power of physics-informed neural networks: time-dependent, 3-D radiative hydrodynamics models that reproduce movies of the solar photosphere
Current, realistic numerical simulations of the solar atmosphere reproduce observations in a statistical sense; they do not exactly reproduce observations such as a movie of solar granulation. Physics-informed neural networks (PINNs) offer a new approach to solving the time-dependent radiative hydrodynamics equations that easily includes observations as boundary conditions. PINNs approximate the solution of the integro-differential equations with a deep neural network. The parameters of this …
Polarization-dependent beam shifts upon metallic reflection in high-contrast imagers and telescopes
Context. To directly image rocky exoplanets in reflected (polarized) light, future space- and ground-based high-contrast imagers and telescopes aim to reach extreme contrasts at close separations from the star. However, the achievable contrast will be limited by reflection-induced polarization aberrations. While polarization aberrations can be modeled with numerical codes, these computations provide little insight into the full range of effects, their origin and characteristics, and possible …
InSPECtor: an end-to-end design framework for compressive pixelated hyperspectral instruments
Classic designs of hyperspectral instrumentation densely sample the spatial and spectral information of the scene of interest. Data may be compressed after the acquisition. In this paper, we introduce a framework for the design of an optimized, micropatterned snapshot hyperspectral imager that acquires an optimized subset of the spatial and spectral information in the scene. The data is thereby already compressed at the sensor level but can be restored to the full hyperspectral data cube by the …
Trade-offs in high-contrast integral field spectroscopy for exoplanet detection and characterisation. Young gas giants in emission
Context. Combining high-contrast imaging with medium- or high-resolution integral field spectroscopy has the potential to boost the detection rate of exoplanets, especially at small angular separations. Furthermore, it immediately provides a spectrum of the planet that can be used to characterise its atmosphere. The achievable spectral resolution, wavelength coverage, and FOV of such an instrument are limited by the number of available detector pixels. Aims: We aim to study the effect of the …
The Enhanced Resolution Imager and Spectrograph for the VLT
The Enhanced Resolution Imager and Spectrograph (ERIS) is an instrument that both extends and enhances the fundamental diffraction limited imaging and spectroscopy capability for the VLT. It replaces two instruments that were being maintained beyond their operational lifetimes, combines their functionality on a single focus, provides a new wavefront sensing module for natural and laser guide stars that makes use of the Adaptive Optics Facility, and considerably improves on their performance. The …
Improved Companion Mass Limits for Sirius A with Thermal Infrared Coronagraphy Using a Vector-apodizing Phase Plate and Time-domain Starlight-subtraction Techniques
We use observations with the infrared-optimized Magellan Adaptive Optics (MagAO) system and Clio camera in 3.9 ensuremathμm light to place stringent mass constraints on possible undetected companions to Sirius A. We suppress the light from Sirius A by imaging it through a grating vector-apodizing phase plate coronagraph with a 180° dark region (gvAPP-180). To remove residual starlight in postprocessing, we apply a time- domain principal-components-analysis-based algorithm we call PCA-Temporal, …
Estimating non-common path aberrations with an adaptive coronagraph
Context. The focal-plane contrast of exoplanet imagers is affected by non-common path aberrations (NCPAs) that the adaptive optics system cannot correct for because they occur after the wavefront has been measured. NCPA estimation is commonly based on the long-exposure science image. Phase retrieval algorithms are often used, and they mostly assume that the residual phase error right after the adaptive optics system and averaged over the integration time is zero. This assumption is not always …
Detecting life outside our solar system with a large high-contrast-imaging mission
In this White Paper, which was submitted in response to the European Space Agency (ESA) Voyage 2050 Call, we recommend the ESA plays a proactive role in developing a global collaborative effort to construct a large high-contrast imaging space telescope, e.g. as currently under study by NASA. Such a mission will be needed to characterize a sizable sample of temperate Earth-like planets in the habitable zones of nearby Sun-like stars and to search for extraterrestrial biological activity. We …
The European Solar Telescope
The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the- art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope …
Spectropolarimetry of life: airborne measurements from a hot air balloon
Does life exist outside our Solar System? A first step towards searching for life outside our Solar System is detecting life on Earth by using remote sensing applications. One powerful and unambiguous biosignature is the circular polarization resulting from the homochirality of biotic molecules and systems. We aim to investigate the possibility of identifying and characterizing life on Earth by using airborne spectropolarimetric observations from a hot air balloon during our field campaign in …
Imaging trace gas concentrations with a compact snapshot device that converts their spectral features into a polarization signal
While the concentrations of trace gases in Earth’s atmosphere were roughly constant for thousands of years, human activities such as the burning of fossil fuels have caused significant changes ever since the industrial revolution. The remote sensing of trace gases has proved to be a useful technique for mapping and monitoring on the global scale. We introduce a novel snapshot imaging instrument concept for measuring trace gases that is compact and lightweight compared to current …
End-to-end design framework for compressed on-chip pixel-wise spectro-polarimeters
Modern detector manufacturing allows spectral and polarimetric filters to be directly integrated on top of separate detector pixels. This enables the creation of CubeSat-sized spectro-polarimetric instruments that are not much larger than the detector and a lens. Redundancy inherent to the observed scene, offers the opportunity for sparse sampling in the form of not scanning all filters at every location. However, when there are fewer pushbroom steps than filters, data are missing in the …
Upgrading the high contrast imaging facility SPHERE: science drivers and instrument choices
SPHERE+ is a proposed upgrade of the SPHERE instrument at the VLT, which is intended to boost the current performances of detection and characterization for exoplanets and disks. SPHERE+ will also serve as a demonstrator for the future planet finder (PCS) of the European ELT. The main science drivers for SPHERE+ are 1/ to access the bulk of the young giant planet population down to the snow line (3 ensuremath- 10 au), to bridge the gap with complementary techniques (radial velocity, astrometry); …
MedRes: a new MEDium RESolution integral field spectrograph for SPHERE
MedRes is a proposed MEDium RESolution integral field spectrograph for upgrading SPHERE, the high contrast instrument for the ESO VLT telescope. MedRes is actually thought of as a potential Visitor Instrument with the scope to provide high contrast diffraction limited medium-high resolution spectra (R ensuremath≥ 1000) over a reasonably large field of view (a square with a side of at least 0.4) and across the spectral region 1.2-1.65 microns. Two main science objectives are driving the …
Joint optimization of wavefront sensing and reconstruction with automatic differentiation
High-contrast imaging instruments need extreme wavefront control to directly image exoplanets. This requires highly sensitive wavefront sensors which optimally make use of the available photons to sense the wavefront. Here, we propose to numerically optimize Fourier-filtering wavefront sensors using automatic differentiation. First, we optimize the sensitivity of the wavefront sensor for different apertures and wavefront distributions. We find sensors that are more sensitive than currently used …
ELVIS: the exoplanets at LBT with a visible IFS for SHARK-VIS
The Exoplanets at LBT with a Visible IFS for SHARK-VIS (ELVIS) is an add-on imaging spectrograph to be integrated in the new LBT high-contrast high-resolution AO-assisted imager SHARK-VIS. ELVIS is optimized for a medium/high spectral resolution of 10-20k with a limited bandwidth around the Hensuremathα, and it is planned fed by a small core (10-20 ensuremathøslash textmum) multi mode fiber bundle providing about 140 spaxels on a field of view around 300×300 sqmas. This instrument has a very …
Spatial polarization modulators: distinguishing diffraction effects from spatial polarization modulation
Are we alone? In our quest to find life beyond Earth, we use our own planet to develop and verify new methods and techniques to remotely detect life. Our Life Signature Detection polarimeter (LSDpol), a snapshot full-Stokes spectropolarimeter to be deployed in the field and in space, looks for signals of life on Earth by sensing the linear and circular polarization states of reflected light. Examples of these biosignatures are linear polarization resulting from O2-A band and vegetation, e.g. the …
Self-optimizing adaptive optics control with reinforcement learning for high-contrast imaging
Current and future high-contrast imaging instruments require extreme adaptive optics systems to reach contrasts necessary to directly imaged exoplanets. Telescope vibrations and the temporal error induced by the latency of the control loop limit the performance of these systems. One way to reduce these effects is to use predictive control. We describe how model-free reinforcement learning can be used to optimize a recurrent neural network controller for closed-loop predictive control. First, we …
Biosignatures of the Earth. I. Airborne spectropolarimetric detection of photosynthetic life
Context. Homochirality is a generic and unique property of life on Earth and is considered a universal and agnostic biosignature. Homochirality induces fractional circular polarization in the incident light that it reflects. Because this circularly polarized light can be sensed remotely, it can be one of the most compelling candidate biosignatures in life detection missions. While there are also other sources of circular polarization, these result in spectrally flat signals with lower magnitude. …
A MUSE view of the asymmetric jet from HD 163296
Context. Jets and outflows are thought to play important roles in regulating star formation and disk evolution. An important question is how the jets are launched. HD 163296 is a well- studied Herbig Ae star that hosts proto-planet candidates, a protoplanetary disk, a protostellar jet, and a molecular outflow, which makes it an excellent laboratory for studying jets. Aims: We aim to characterize the jet at the inner regions and check if there are large differences with the features at large …
The vector-apodizing phase plate coronagraph: design, current performance, and future development
Over the last decade, the vector-apodizing phase plate (vAPP) coronagraph has been developed from concept to on-sky application in many high-contrast imaging systems on 8-m class telescopes. The vAPP is an geometric-phase patterned coronagraph that is inherently broadband, and its manufacturing is enabled only by direct-write technology for liquid-crystal patterns. The vAPP generates two coronagraphic PSFs that cancel starlight on opposite sides of the point spread function (PSF) and have …
First light of a holographic aperture mask: Observation at the Keck OSIRIS Imager
We report on the design, construction, and commissioning of a prototype aperture masking technology implemented at the Keck OSIRIS Imager: the holographic aperture mask. Holographic aperture masking (HAM) aims at (i) increasing the throughput of sparse aperture masking (SAM) by selectively combining all subapertures across a telescope pupil in multiple interferograms using a phase mask, and (ii) adding low-resolution spectroscopic capabilities. Using liquid-crystal geometric phase patterns, we …
PCS — A Roadmap for Exoearth Imaging with the ELT
The Planetary Camera and Spectrograph (PCS) for the Extremely Large Telescope (ELT) will be dedicated to detecting and characterising nearby exoplanets with sizes from sub-Neptune to Earth-size in the neighbourhood of the Sun. This goal is achieved by a combination of eXtreme Adaptive Optics (XAO), coronagraphy and spectroscopy. PCS will allow us not only to take images, but also to look for biosignatures such as molecular oxygen in the exoplanets’ atmospheres. This article describes the …
A survey of the linear polarization of directly imaged exoplanets and brown dwarf companions with SPHERE-IRDIS. First polarimetric detections revealing disks around DH Tau B and GSC 6214-210 B
Context. Young giant planets and brown dwarf companions emit near- infrared radiation that can be linearly polarized up to several percent. This polarization can reveal the presence of an (unresolved) circumsubstellar accretion disk, rotation-induced oblateness of the atmosphere, or an inhomogeneous distribution of atmospheric dust clouds. Aims: We aim to measure the near-infrared linear polarization of 20 known directly imaged exoplanets and brown dwarf companions. Methods: We observed the …
LOUPE: observing Earth from the Moon to prepare for detecting life on Earth-like exoplanets
LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a small, robust spectro-polarimeter for observing the Earth as an exoplanet. Detecting Earth-like planets in stellar habitable zones is one of the key challenges of modern exoplanetary science. Characterizing such planets and searching for traces of life requires the direct detection of their signals. LOUPE provides unique spectral flux and polarization data of sunlight reflected by Earth, the only planet known to harbour …
Using the generalised-optical differentiation wavefront sensor for laser guide star wavefront sensing
Laser guide stars (LGS) are used in many adaptive optics systems to extend sky coverage. The most common wavefront sensor used in combination with a LGS is a Shack-Hartmann wavefront sensor (SHWFS). The ShackHartmann has a major disadvantage for extended source wavefront sensing because it directly samples the image. In this proceeding we propose to use the generalized-Optical Differentation Wavefront Sensor (g-ODWFS) a wavefront sensor for wavefront sensing of LGS. The g-ODWFS uses only 4 …
Self-optimizing adaptive optics control with reinforcement learning
Current and future high-contrast imaging instruments require extreme Adaptive Optics (XAO) systems to reach contrasts necessary to directly image exoplanets. Telescope vibrations and the temporal error induced by the latency of the control loop limit the performance of these systems. Optimization of the (predictive) control algorithm is crucial in reducing these effects. We describe how model-free Reinforcement Learning can be used to optimize a Recurrent Neural Network controller for …
Searching for proto-planets with MUSE
Context. Protoplanetary disks contain structures such as gaps, rings, and spirals, which are thought to be produced by the interaction between the disk and embedded protoplanets. However, only a few planet candidates are found orbiting within protoplanetary disks, and most of them are being challenged as having been confused with disk features. Aims: The VLT/MUSE discovery of PDS 70 c demonstrated a powerful way of searching for still-forming protoplanets by targeting accretion signatures with …
Prediction of the planet yield of the MaxProtoPlanetS high-contrast survey for H-alpha protoplanets with MagAO-X based on first light contrasts
Our past GAPplanetS survey over the last 5 years with the MagAO visible AO system discovered the first examples of accreting protoplanets (by direct observation of H-alpha emission). Examples include LkCa15 b (Sallum et al. 2015) and PDS70 b (Wagner et al. 2018). In this paper we review the science performance of the newly (Dec. 2019) commissioned MagAO-X extreme AO system. In particular, we use the vAPP coronagraphic contrasts measured during MagAO-X first light. We use the Massive Accreting …
Planet formation with all flavors of adaptive optics: VLT/MUSE's laser tomography adaptive optics to directly image young accreting exoplanets
We present recent results obtained with the VLT/MUSE Integral Field Spectrograph fed by the 4LGSF and its laser tomography adaptive optics module GALACSI. While this so-called narrow-field mode of MUSE was not designed to perform directly imaging of exoplanets and outflows, we show that it can be a game changer to detect and characterize young exoplanets with a prominent emission lines (i.e Hα, tracer of accretion), at moderate contrasts. These performances are achieved thanks to the combo of a …
Multi-core fibre-fed integral-field unit (MCIFU): overview and first-light
The Multi-Core Integral-Field Unit (MCIFU) is a new diffraction-limited near-infrared integral-field unit for exoplanet atmosphere characterization with extreme adaptive optics (xAO) instruments. It has been developed as an experimental pathfinder for spectroscopic upgrades for SPHERE+/VLT and other xAO systems. The wavelength range covers 1.0 um to 1.6um at a resolving power around 5000 for 73 points on-sky. The MCIFU uses novel astrophotonic components to make this very compact and robust …
MagAO-X first light
MagAO-X is a new ``extreme’’ adaptive optics system for the Magellan Clay 6.5 m telescope which began commissioning in December, 2019. MagAO-X is based around a 2040 actuator deformable mirror, controlled by a pyramid wavefront sensor operating at up to 3.6 kHz. When fully optimized, MagAO-X will deliver high Strehls ($łt$ 70%), high resolution (19 mas), and high contrast ($łt$ 1 × 10$^-4$) at Hensuremathα (656 nm). We present a brief review of the instrument design and operations, …
High contrast imaging for the enhanced resolution imager and spectrometer (ERIS)
ERIS is a diffraction limited thermal infrared imager and spectrograph for the Very Large Telescope UT4. One of the science cases for ERIS is the detection and characterization of circumstellar structures and exoplanets around bright stars that are typically much fainter than the stellar diffraction halo. Enhanced sensitivity is provided through the combination of (i) suppression of the diffraction halo of the target star using coronagraphs, and (ii) removal of any residual diffraction structure …
Focal plane wavefront sensing on SUBARU/SCExAO
Focal plane wavefront sensing is an elegant solution for wavefront sensing since near-focal images of any source taken by a detector show distortions in the presence of aberrations. Non- Common Path Aberrations and the Low Wind Effect both have the ability to limit the achievable contrast of the finest coronagraphs coupled with the best extreme adaptive optics systems. To correct for these aberrations, the Subaru Coronagraphic Extreme Adaptive Optics instrument hosts many focal plane wavefront …
Design of the life signature detection polarimeter LSDpol
Many biologically produced chiral molecules such as amino acids and sugars show a preference for left or right handedness (homochirality). Light reflected by biological materials such as algae and leaves therefore exhibits a small amount of circular polarization that strongly depends on wavelength. Our Life Signature Detection polarimeter (LSDpol) is optimized to measure these signatures of life. LSDpol is a compact spectropolarimeter concept with no moving parts that instantaneously measures …
Diffraction-limited integral-field spectroscopy for extreme adaptive optics systems with the multicore fiber-fed integral-field unit
Direct imaging instruments have the spatial resolution to resolve exoplanets from their host star. This enables direct characterization of the exoplanets atmosphere, but most direct imaging instruments do not have spectrographs with high enough resolving power for detailed atmospheric characterization. We investigate the use of a single-mode diffraction-limited integral-field unit that is compact and easy to integrate into current and future direct imaging instruments for exoplanet …
CS Cha B: A disc-obscured M-type star mimicking a polarised planetary companion
Context. Direct imaging provides a steady flow of newly discovered giant planets and brown dwarf companions. These multi-object systems can provide information about the formation of low-mass companions in wide orbits and/or help us to speculate about possible migration scenarios. Accurate classification of companions is crucial for testing formation pathways. Aims: In this work we further characterise the recently discovered candidate for a planetary-mass companion CS Cha b and determine if it …
Two Directly Imaged, Wide-orbit Giant Planets around the Young, Solar Analog TYC 8998-760-1
Even though tens of directly imaged companions have been discovered in the past decades, the number of directly confirmed multiplanet systems is still small. Dynamical analysis of these systems imposes important constraints on formation mechanisms of these wide-orbit companions. As part of the Young Suns Exoplanet Survey we report the detection of a second planetary-mass companion around the 17 Myr-old, solar-type star TYC 8998-760-1 that is located in the Lower Centaurus Crux subgroup of the …
On-sky verification of Fast and Furious focal-plane wavefront sensing: Moving forward toward controlling the island effect at Subaru/SCExAO
Context. High-contrast imaging (HCI) observations of exoplanets can be limited by the island effect (IE). The IE occurs when the main wavefront sensor (WFS) cannot measure sharp phase discontinuities across the telescope’s secondary mirror support structures (also known as spiders). On the current generation of telescopes, the IE becomes a severe problem when the ground wind speed is below a few meters per second. During these conditions, the air that is in close contact with the spiders …
A universal smartphone add-on for portable spectroscopy and polarimetry: iSPEX 2
Spectropolarimetry is a powerful technique for remote sensing of the environment. It enables the retrieval of particle shape and size distributions in air and water to an extent that traditional spectroscopy cannot. SPEX is an instrument concept for spectropolarimetry through spectral modulation, providing snapshot, and hence accurate, hyperspectral intensity and degree and angle of linear polarization. Successful SPEX instruments have included groundSPEX and SPEX airborne, which both measure …
On-sky verification of Fast and Furious focal-plane wavefront sensing: Moving forward toward controlling the island effect at Subaru/SCExAO
High-contrast imaging (HCI) observations of exoplanets can be limited by the island effect (IE). On the current generation of telescopes, the IE becomes a severe problem when the ground wind speed is below a few meters per second. This is referred to as the low- wind effect (LWE). The LWE severely distorts the point spread function (PSF), significantly lowering the Strehl ratio and degrading the contrast. In this article, we aim to show that the focal-plane wavefront sensing (FPWFS) algorithm, …
A universal smartphone add-on for portable spectroscopy and polarimetry: iSPEX 2
Spectropolarimetry is a powerful technique for remote sensing of the environment. It enables the retrieval of particle shape and size distributions in air and water to an extent that traditional spectroscopy cannot. SPEX is an instrument concept for spectropolarimetry through spectral modulation, providing snapshot, and hence accurate, hyperspectral intensity and degree and angle of linear polarization. Successful SPEX instruments have included groundSPEX and SPEX airborne, which both measure …
The Single-mode Complex Amplitude Refinement (SCAR) coronagraph. II. Lab verification, and toward the characterization of Proxima b
We present the monochromatic lab verification of the newly developed SCAR coronagraph that combines a phase plate (PP) in the pupil with a microlens-fed single-mode fiber array in the focal plane. The two SCAR designs that have been measured, create respectively a 360 degree and 180 degree dark region from 0.8-2.4lambda/D around the star. The 360 SCAR has been designed for a clear aperture and the 180 SCAR has been designed for a realistic aperture with central obscuration and spiders. The 360 …
SPHERE+: Imaging young Jupiters down to the snowline
SPHERE (Beuzit et al,. 2019) has now been in operation at the VLT for more than 5 years, demonstrating a high level of performance. SPHERE has produced outstanding results using a variety of operating modes, primarily in the field of direct imaging of exoplanetary systems, focusing on exoplanets as point sources and circumstellar disks as extended objects. The achievements obtained thus far with SPHERE (~200 refereed publications) in different areas (exoplanets, disks, solar system, stellar …
Original use of MUSE's laser tomography adaptive optics to directly image young accreting exoplanets
We present recent results obtained with the VLT/MUSE Integral Field Spectrograph fed by the 4LGSF and its laser tomography adaptive optics module GALACSI. While this so-called narrow-field mode of MUSE was not designed to perform directly imaging of exoplanets and outflows, we show that it can be a game changer to detect and characterize young exoplanets with a prominent emission lines (i.e Hα, tracer of accretion), at moderate contrasts. These performances are achieved thanks to the combo of a …
VizieR Online Data Catalog: A planetary-mass companion to a solar-type star (Bohn+, 2020)
Fits images corresponding to the imagery presented in the paper. The data are obtained with VLT/SPHERE/IRDIS. The data reduction is performed with version 0.8.1 of PynPoint. We detect a co-moving companion around The solar-type Sco-Cen member TYC 8998-760-1 with a mass of 14+/-3 Jupiter masses. The extracted astrometry and photometry is presented in this catalogue. (4 data files).
The Young Suns Exoplanet Survey: Detection of a wide-orbit planetary-mass companion to a solar-type Sco-Cen member
The Young Suns Exoplanet Survey consists of a homogeneous sample of 70 young, solar-mass stars located in the Lower Centaurus-Crux subgroup of the Scorpius-Centaurus association with an average age of 15 ensuremath± 3 Myr. We report the detection of a co-moving companion around the K3IV star TYC 8998-760-1 (2MASSJ13251211-6456207) that is located at a distance of 94.6 ensuremath± 0.3 pc using SPHERE/IRDIS on the VLT. Spectroscopic observations with VLT/X-SHOOTER constrain the mass of the star to …
RefPlanets: Search for reflected light from extrasolar planets with SPHERE/ZIMPOL
Aims: RefPlanets is a guaranteed time observation programme that uses the Zurich IMaging POLarimeter (ZIMPOL) of Spectro-Polarimetric High-contrast Exoplanet REsearch instrument at the Very Large Telescope to perform a blind search for exoplanets in wavelengths from 600 to 900 nm. The goals of this study are the characterisation of the unprecedented high polarimetic contrast and polarimetric precision capabilities of ZIMPOL for bright targets, the search for polarised reflected light around …
Polarimetric imaging mode of VLT/SPHERE/IRDIS. II. Characterization and correction of instrumental polarization effects
Context. Circumstellar disks and self-luminous giant exoplanets or companion brown dwarfs can be characterized through direct- imaging polarimetry at near-infrared wavelengths. SPHERE/IRDIS at the Very Large Telescope has the capabilities to perform such measurements, but uncalibrated instrumental polarization effects limit the attainable polarimetric accuracy.
Aims: We aim to characterize and correct the instrumental polarization effects of the complete optical system, that is, the telescope …
Aims: We aim to characterize and correct the instrumental polarization effects of the complete optical system, that is, the telescope …
Polarimetric imaging mode of VLT/SPHERE/IRDIS. I. Description, data reduction, and observing strategy
Context. Polarimetric imaging is one of the most effective techniques for high-contrast imaging and for the characterization of protoplanetary disks, and it has the potential of becoming instrumental in the characterization of exoplanets. The Spectro- Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument installed on the Very Large Telescope (VLT) contains the InfraRed Dual-band Imager and Spectrograph (IRDIS) with a dual-beam polarimetric imaging (DPI) mode, which offers the …
Overview of focal plane wavefront sensors to correct for the Low Wind Effect on SUBARU/SCExAO
The Low Wind Effect (LWE) refers to a phenomenon that occurs when the wind speed inside a telescope dome drops below $3$m/s creating a temperature gradient near the telescope spider. This produces phase discontinuities in the pupil plane that are not detected by traditional Adaptive Optics (AO) systems such as the pyramid wavefront sensor or the Shack-Hartmann. Considering the pupil as divided in 4 quadrants by regular spiders, the phase discontinuities correspond to piston, tip and tilt …
Focal-plane wavefront sensing with the vector-Apodizing Phase Plate
Context. One of the key limitations of the direct imaging of exoplanets at small angular separations are quasi-static speckles that originate from evolving non-common path aberrations (NCPA) in the optical train downstream of the instrument’s main wavefront sensor split-off.
Aims: In this article we show that the vector-Apodizing Phase Plate (vAPP) coronagraph can be designed such that the coronagraphic point spread functions (PSFs) can act as wavefront sensors to measure and correct the …
Aims: In this article we show that the vector-Apodizing Phase Plate (vAPP) coronagraph can be designed such that the coronagraphic point spread functions (PSFs) can act as wavefront sensors to measure and correct the …
Highly multiplexed Bragg gratings for large field of view gas sensing in planetary atmospheres
Detecting and monitoring gas species is an important part of remote sensing because the state of the environment can be retrieved from the state of the gas species. This can be used to track temperature and pressure structures in the atmosphere for weather predictions, or monitor the air quality. Discriminating different species is easier at higher spectral resolution when the spectral lines are clearly resolved. The need to do this at high spatial resolution and over large fields of view leads …
Spatial linear dark field control and holographic modal wavefront sensing with a vAPP coronagraph on MagAO-X
The Magellan Extreme Adaptive Optics (MagAO-X) Instrument is an extreme AO system coming online at the end of 2019 that will be operating within the visible and near-IR. With state-of-the-art wavefront sensing and coronagraphy, MagAO-X will be optimized for high-contrast direct exoplanet imaging at challenging visible wavelengths, particularly Hα. To enable high-contrast imaging, the instrument hosts a vector apodizing phase plate (vAPP) coronagraph. The vAPP creates a static region of high …
A snapshot full-Stokes spectropolarimeter for detecting life on Earth
We present the design of a point-and-shoot non-imaging full-Stokes spectropolarimeter dedicated to detecting life on Earth from an orbiting platform like the ISS. We specifically aim to map circular polarization in the spectral features of chorophyll and other biopigments for our planet as a whole. These non-zero circular polarization signatures are caused by homochirality of the molecular and supramolecular configurations of organic matter, and are considered the most unambiguous biomarker. To …
Search for gas from the disintegrating rocky exoplanet K2-22b
Context. The red dwarf star K2-22 is transited every 9.14 h by an object which is best explained by being a disintegrating rocky exoplanet featuring a variable comet-like dust tail. While the dust is thought to dominate the transit light curve, gas is also expected to be present, either from being directly evaporated off the planet or by being produced by the sublimation of dust particles in the tail.
Aims: Both ionized calcium and sodium have large cross-sections, and although present at low …
Aims: Both ionized calcium and sodium have large cross-sections, and although present at low …
Insights into Terrestrial Planet Compositions and Geophysics from Observations of Magma Worlds
There exists a remarkable population of short period transiting rocky exoplanets with temperatures >2,000 K, and masses ranging from about 8 Earth masses, such as the hot super-Earth 55 Cancri e, to that of Mercury or smaller, such as K2-22b. These planets are thought to have mineral atmospheres that are produced by the vaporisation of their magma surfaces, or large exospheres that are produced by sputtering of their atmospheres or exposed surfaces by intense stellar winds. Additionally, the …
ESA Voyage 2050 White Paper: Detecting life outside our solar system with a large high-contrast-imaging mission
In this white paper, we recommend the European Space Agency plays a proactive role in developing a global collaborative effort to construct a large high-contrast imaging space telescope, e.g. as currently under study by NASA. Such a mission will be needed to characterize a sizable sample of temperate Earth-like planets in the habitable zones of nearby Sun-like stars and to search for extraterrestrial biological activity. We provide an overview of relevant European expertise, and advocate ESA to …
Polarimetric imaging of circumstellar disks. I. Artifacts due to limited angular resolution
Context. Polarimetric images of circumstellar environments, even when corrected with adaptive optics, have a limited angular resolution. Finite resolution greatly affects polarimetric images because of the canceling of adjacent polarization signals with opposite signs. In radio astronomy this effect is called beam depolarization and is well known. However, radio techniques to mitigate beam depolarization are not directly applicable to optical images as a consequence of the inherent lack of phase …
Two accreting protoplanets around the young star PDS 70
Newly forming protoplanets are expected to create cavities and substructures in young, gas-rich protoplanetary disks$^1-3$, but they are difficult to detect as they could be confused with disk features affected by advanced image analysis techniques$^4,5$. Recently, a planet was discovered inside the gap of the transitional disk of the T Tauri star PDS 70$^6,7$. Here, we report on the detection of strong Hα emission from two distinct locations in the PDS 70 system, one corresponding to the …
Discovery of a directly imaged disk in scattered light around the Sco-Cen member Wray 15-788
Context. Protoplanetary disks are the birth environments of planetary systems. Therefore, the study of young, circumstellar environments is essential in understanding the processes taking place in planet formation and the evolution of planetary systems.
Aims: We detect and characterize circumstellar disks and potential companions around solar-type, pre-main sequence stars in the Scorpius-Centaurus association (Sco-Cen).
Methods: As part of our ongoing survey we carried out high-contrast …
Aims: We detect and characterize circumstellar disks and potential companions around solar-type, pre-main sequence stars in the Scorpius-Centaurus association (Sco-Cen).
Methods: As part of our ongoing survey we carried out high-contrast …
VizieR Online Data Catalog: Discovery of a resolved disk around Wray 15-788 (Bohn+, 2019)
Fits images corresponding to the imagery presented in the paper. The data are obtained with VLT/SPHERE/IRDIS. The data reduction is performed with the latest version of PynPoint. In both total and polarized intensity we resolve a disk around the K-type pre-main sequence star Wray 15-788. SED analysis suggests that this is a protoplanetary disk at a transition stage. (4 data files).
SPHERE/ZIMPOL high resolution polarimetric imager. I. System overview, PSF parameters, coronagraphy, and polarimetry
Context. The SPHERE planet finder is an extreme adaptive optics (AO) instrument for high resolution and high contrast observations at the Very Large Telescope (VLT). We describe the Zurich Imaging Polarimeter (ZIMPOL), the visual focal plane subsystem of SPHERE, which pushes the limits of current AO systems to shorter wavelengths, higher spatial resolution, and much improved polarimetric performance.
Aims: We present a detailed characterization of SPHERE/ZIMPOL which should be useful for an …
Aims: We present a detailed characterization of SPHERE/ZIMPOL which should be useful for an …
Chromatic transit light curves of disintegrating rocky planets
Context. Kepler observations have revealed a class of short-period exoplanets, of which Kepler-1520 b is the prototype, which have comet-like dust tails thought to be the result of small, rocky planets losing mass. The shape and chromaticity of the transits constrain the properties of the dust particles originating from the planet’s surface, offering a unique opportunity to probe the composition and geophysics of rocky exoplanets.
Aims: We aim to approximate the average Kepler …
Aims: We aim to approximate the average Kepler …
Review of high-contrast imaging systems for current and future ground- and space-based telescopes I: coronagraph design methods and optical performance metrics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. In this first installment of a series of three papers summarizing the outcomes of the OOC workshop, we present an overview of design methods and optical performance metrics developed for coronagraph instruments. The design and optimization of …
First direct detection of a polarized companion outside a resolved circumbinary disk around CS Chamaeleonis
Aims: To understand planet formation it is necessary to study the birth environment of planetary systems. Resolved imaging of young planet forming disks allows us to study this environment in great detail and find signs of planet-disk interaction and disk evolution. In the present study we aim to investigate the circumstellar environment of the spectroscopic binary T Tauri star CS Cha. From unresolved mid-to far-infrared photometry it is predicted that CS Cha hosts a disk with a large cavity. …
Review of high-contrast imaging systems for current and future ground-based and space-based telescopes: Part II. Common path wavefront sensing/control and coherent differential imaging
The Optimal Optical Coronagraph (OOC) Workshop held at the Lorentz Center in September 2017 in Leiden, the Netherlands, gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. In this second installment of a series of three papers summarizing the outcomes of the OOC workshop, we present an overview of common path wavefront sensing/control and Coherent Differential Imaging techniques, highlight the latest results, and …
Review of high-contrast imaging systems for current and future ground-based and space-based telescopes III: technology opportunities and pathways
The Optimal Optical CoronagraphWorkshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 30 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. This contribution is the final part of a series of three papers summarizing the outcomes of the workshop, and presents an overview of novel optical technologies and systems that are implemented or considered for high-contrast imaging instruments on both …
Optimization of contrast in adaptive optics for exoplanet imaging
Direct imaging of exoplanets requires overcoming the enormous contrast between the exoplanet and its host star, to distinguish the reflected light from the exoplanet, from the diffracted light of the star. Direct optimization of contrast, using nonlinear optimization techniques of the Adaptive Optics (AO) system coupled with coronagraphy, shows significant promise in achieving high contrast, beyond the limits of what can be achieved with traditional AO systems. Using a coronagraph optic as a …
On-sky results of the Leiden EXoplanet Instrument (LEXI)
The Leiden EXoplanet Instrument (LEXI) is a bench-mounted high-contrast spectrograph (HCS) and high- contrast imager(HCI). Both science instruments are mounted behind a common adaptive optics (AO) system. The AO can be controlled by several new wavefront sensors for which we will show the first on-sky results. There is a new pupil-plane wavefront sensors; the Generalized Optical Differentiation Wavefront sensor(g-ODWFS). LEXI can switch between two observing modes, the HCI mode or the HD-IFS …
Multiplexed holographic aperture masking with liquid-crystal geometric phase masks
Sparse Aperture Masking (SAM) allows for high-contrast imaging at small inner working angles, however the performance is limited by the small throughput and the number of baselines. We present the concept and first lab results of Holographic Aperture Masking (HAM) with extreme liquid-crystal geometric phase patterns. We multiplex subapertures using holographic techniques to combine the same subaperture in multiple non-redundant PSFs in combination with a non-interferometric reference spot. This …
MagAO-X: project status and first laboratory results
MagAO-X is an entirely new extreme adaptive optics system for the Magellan Clay 6.5 m telescope, funded by the NSF MRI program starting in Sep 2016. The key science goal of MagAO-X is high- contrast imaging of accreting protoplanets at Hensuremathα. With 2040 actuators operating at up to 3630 Hz, MagAO-X will deliver high Strehls (> 70%), high resolution (19 mas), and high contrast (< 1 × 10$^-4 $) at Hensuremathα (656 nm). We present an overview of the MagAO-X system, review the system …
Laboratory verification of Fast & Furious phase diversity: Towards controlling the low wind effect in the SPHERE instrument
Context. The low wind effect (LWE) refers to a characteristic set of quasi-static wavefront aberrations seen consistently by the SPHERE instrument when dome-level wind speeds drop below 3 ms$^-1$. The LWE produces bright low-order speckles in the stellar point-spread function (PSF), which severely limit the contrast performance of SPHERE under otherwise optimal observing conditions.
Aims: In this paper we propose the Fast & Furious (F&F) phase diversity algorithm as a viable …
Aims: In this paper we propose the Fast & Furious (F&F) phase diversity algorithm as a viable …
High contrast imaging for the enhanced resolution imager and spectrometer (ERIS)
ERIS is a diffraction limited thermal infrared imager and spectrograph for the Very Large Telescope UT4. One of the science cases for ERIS is the detection and characterization of circumstellar structures and exoplanets around bright stars that are typically much fainter than the stellar diffraction halo. Enhanced sensitivity is provided through the combination of (i) suppression of the diffraction halo of the target star using coronagraphs, and (ii) removal of any residual diffraction structure …
ERIS: revitalising an adaptive optics instrument for the VLT
ERIS is an instrument that will both extend and enhance the fundamental diffraction limited imaging and spectroscopy capability for the VLT. It will replace two instruments that are now being maintained beyond their operational lifetimes, combine their functionality on a single focus, provide a new wavefront sensing module that makes use of the facility Adaptive Optics System, and considerably improve their performance. The instrument will be competitive with respect to JWST in several regimes, …
A Planet with a Disc? A Surprising Detection in Polarised Light with VLT/SPHERE
With the Spectro-Polarimetric High- contrast Exoplanet REsearch (SPHERE) instrument at ESO’s Very Large Telescope (VLT) we can study the linear polarisation of directly detected planets and brown dwarfs, to learn about their atmospheres and immediate environments. We summarise here the recent discovery of a low- mass companion in polarised light by Ginski et al. (2018). The object shows an extreme degree of polarisation, indicating the presence of a circumplanetary disc.
The Single-mode Complex Amplitude Refinement (SCAR) coronagraph: II. Lab verification, and toward the characterization of Proxima b
We present the monochromatic lab verification of the newly developed SCAR coronagraph that combines a phase plate (PP) in the pupil with a microlens-fed single-mode fiber array in the focal plane. The two SCAR designs that have been measured, create respectively a 360 degree and 180 degree dark region from 0.8-2.4 λ/D around the star. The 360 SCAR has been designed for a clear aperture and the 180 SCAR has been designed for a realistic aperture with central obscuration and spiders. The 360 SCAR …
The Spectropolarimeter for Planetary Exploration: SPEX
SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact remotesensing instrument for measuring and characterizing aerosols in the atmosphere. The shoebox size instrument is capable of accurate full linear spectropolarimetry without moving parts or liquid crystals. High precision polarimetry is performed through encoding the degree and angle of linear polarization of the incoming light in a sinusoidal modulation of the spectrum. Measuring this intensity spectrum thus …
SPEX: the Spectropolarimeter for Planetary Exploration
We present SPEX, the Spectropolarimeter for Planetary Exploration, which is a compact, robust and low-mass spectropolarimeter designed to operate from an orbiting or in situ platform. Its purpose is to simultaneously measure the radiance and the state (degree and angle) of linear polarization of sunlight that has been scattered in a planetary atmosphere and/or reflected by a planetary surface with high accuracy. The degree of linear polarization is extremely sensitive to the microphysical …
SPEX: a highly accurate spectropolarimeter for atmospheric aerosol characterization
Global characterization of atmospheric aerosol in terms of the microphysical properties of the particles is essential for understanding the role aerosols in Earth climate [1]. For more accurate predictions of future climate the uncertainties of the net radiative forcing of aerosols in the Earth’s atmosphere must be reduced [2]. Essential parameters that are needed as input in climate models are not only the aerosol optical thickness (AOT), but also particle specific properties such as the …
Polarization modeling and predictions for DKIST part 2: application of the Berreman calculus to spectral polarization fringes of beamsplitters and crystal retarders
We outline polarization fringe predictions derived from an application of the Berreman calculus for the Daniel K. Inouye Solar Telescope (DKIST) retarder optics. The DKIST retarder baseline design used six crystals, single-layer antireflection coatings, thick cover windows, and oil between all optical interfaces. This tool estimates polarization fringes and optic Mueller matrices as functions of all optical design choices. The amplitude and period of polarized fringes under design changes, …
Three Years of SPHERE: The Latest View of the Morphology and Evolution of Protoplanetary Discs
Spatially resolving the immediate surroundings of young stars is a key challenge for the planet formation community. SPHERE on the VLT represents an important step forward by increasing the opportunities offered by optical or near-infrared imaging instruments to image protoplanetary discs. The Guaranteed Time Observation Disc team has concentrated much of its efforts on polarimetric differential imaging, a technique that enables the efficient removal of stellar light and thus facilitates the …
Investigating circular patterns in linear polarization observations of Venus
In this work, we report our observations of Venus using the polarized flux. Our observations show certain curious looking concentric rings around the sub-solar point of Venus. We use our radiative transfer model to explain these fluctuations and discuss what the possible explanations might be.
Combining angular differential imaging and accurate polarimetry with SPHERE/IRDIS to characterize young giant exoplanets
Young giant exoplanets emit infrared radiation that can be linearly polarized up to several percent. This linear polarization can trace: 1) the presence of atmospheric cloud and haze layers, 2) spatial structure, e.g. cloud bands and rotational flattening, 3) the spin axis orientation and 4) particle sizes and cloud top pressure. We introduce a novel high-contrast imaging scheme that combines angular differential imaging (ADI) and accurate near- infrared polarimetry to characterize self-luminous …
Rigorous vector wave propagation for arbitrary flat media
Precise modelling of the (off-axis) point spread function (PSF) to identify geometrical and polarization aberrations is important for many optical systems. In order to characterise the PSF of the system in all Stokes parameters, an end-to-end simulation of the system has to be performed in which Maxwell’s equations are rigorously solved. We present the first results of a python code that we are developing to perform multiscale end-to-end wave propagation simulations that include all …
Polarization modeling and predictions for DKIST part 2: application of the Berreman calculus to spectral polarization fringes of beamsplitters and crystal retarders
We outline polarization fringe predictions derived from a new application of the Berreman calculus for the Daniel K. Inouye Solar Telescope (DKIST) retarder optics. The DKIST retarder baseline design used 6 crystals, single layer anti-reflection coatings, thick cover windows and oil between all optical interfaces. This new tool estimates polarization fringes and optic Mueller matrices as functions of all optical design choices. The amplitude and period of polarized fringes under design changes, …
BP Piscium: its flaring disc imaged with SPHERE/ZIMPOL$^★$
Whether BP Piscium (BP Psc) is either a pre-main sequence T Tauri star at d ensuremath≈ 80 pc, or a post-main sequence G giant at d ensuremath≈ 300 pc is still not clear. As a first-ascent giant, it is the first to be observed with a molecular and dust disc. Alternatively, BP Psc would be among the nearest T Tauri stars with a protoplanetary disc (PPD). We investigate whether the disc geometry resembles typical PPDs, by comparing polarimetric images with radiative transfer models. Our Very Large …
The coronagraphic Modal Wavefront Sensor: a hybrid focal-plane sensor for the high-contrast imaging of circumstellar environments
The raw coronagraphic performance of current high-contrast imaging instruments is limited by the presence of a quasi-static speckle (QSS) background, resulting from instrumental Non-Common Path Errors (NCPEs). Rapid development of efficient speckle subtraction techniques in data reduction has enabled final contrasts of up to 10$^-6$ to be obtained, however it remains preferable to eliminate the underlying NCPEs at the source. In this work we introduce the coronagraphic Modal Wavefront Sensor …
On-sky Performance Analysis of the Vector Apodizing Phase Plate Coronagraph on MagAO/Clio2
We report on the performance of a vector apodizing phase plate coronagraph that operates over a wavelength range of 2-5 μm and is installed in MagAO/Clio2 at the 6.5 m Magellan Clay telescope at Las Campanas Observatory, Chile. The coronagraph manipulates the phase in the pupil to produce three beams yielding two coronagraphic point-spread functions (PSFs) and one faint leakage PSF. The phase pattern is imposed through the inherently achromatic geometric phase, enabled by liquid crystal …
The SPEX-airborne multi-angle spectropolarimeter on NASA's ER-2 research aircraft: capabilities, data processing and data products
A multi-angle spectropolarimeter payload, ``SPEX-airborne’’ has been developed for observing and characterizing aerosols from NASA’s high-altitude research aircraft ER-2. SPEX-airborne provides autonomously multi-angle snapshot measurements of spectral radiance and degree of linear polarization over a 7 degree swath in the visible part of the optical spectrum. The instrument is unique in the sense that it combines 30 highly accurate polarimetric measurements with hyperspectral …
Multiple rings in the transition disk and companion candidates around RX J1615.3-3255. High contrast imaging with VLT/SPHERE
Context. The effects of a planet sculpting the disk from which it formed are most likely to be found in disks that are in transition between being classical protoplanetary and debris disks. Recent direct imaging of transition disks has revealed structures such as dust rings, gaps, and spiral arms, but an unambiguous link between these structures and sculpting planets is yet to be found.
Aims: We aim to find signs of ongoing planet-disk interaction and study the distribution of small grains at …
Aims: We aim to find signs of ongoing planet-disk interaction and study the distribution of small grains at …
Dusty tails of evaporating exoplanets. II. Physical modelling of the KIC 12557548b light curve
Context. Evaporating rocky exoplanets, such as KIC 12557548b, eject large amounts of dust, which can trail the planet in a comet- like tail. When such objects occult their host star, the resulting transit signal contains information about the dust in the tail.
Aims: We aim to use the detailed shape of the Kepler light curve of KIC 12557548b to constrain the size and composition of the dust grains that make up the tail, as well as the mass loss rate of the planet.
Methods: Using a self- …
Aims: We aim to use the detailed shape of the Kepler light curve of KIC 12557548b to constrain the size and composition of the dust grains that make up the tail, as well as the mass loss rate of the planet.
Methods: Using a self- …
Direct detection of scattered light gaps in the transitional disk around HD 97048 with VLT/SPHERE
Aims: We studied the well-known circumstellar disk around the Herbig Ae/Be star HD 97048 with high angular resolution to reveal undetected structures in the disk which may be indicative of disk evolutionary processes such as planet formation.
Methods: We used the IRDIS near-IR subsystem of the extreme adaptive optics imager SPHERE at the ESO/VLT to study the scattered light from the circumstellar disk via high resolution polarimetry and angular differential imaging.
Results: We imaged the …
The Hera Saturn entry probe mission
The Hera Saturn entry probe mission is proposed as an M-class mission led by ESA with a contribution from NASA. It consists of one atmospheric probe to be sent into the atmosphere of Saturn, and a Carrier-Relay spacecraft. In this concept, the Hera probe is composed of ESA and NASA elements, and the Carrier-Relay Spacecraft is delivered by ESA. The probe is powered by batteries, and the Carrier-Relay Spacecraft is powered by solar panels and batteries. We anticipate two major subsystems to be …
Search for an exosphere in sodium and calcium in the transmission spectrum of exoplanet 55 Cancri e
Context. The atmospheric and surface characterization of rocky planets is a key goal of exoplanet science. Unfortunately, the measurements required for this are generally out of reach of present-day instrumentation. However, the planet Mercury in our own solar system exhibits a large exosphere composed of atomic species that have been ejected from the planetary surface by the process of sputtering. Since the hottest rocky exoplanets known so far are more than an order of magnitude closer to …
HERA: an atmospheric probe to unveil the depths of Saturn
The Hera Saturn entry probe mission is proposed as an M-class mission led by ESA with a significant collaboration with NASA. It consists of a Saturn atmospheric probe and a Carrier-Relay spacecraft. Hera will perform in situ measurements of the chemical and isotopic compositions as well as the dynamics of Saturn’s atmosphere, with the goal of improving our understanding of the origin, formation, and evolution of Saturn, the giant planets and their satellite systems, with extrapolation to …
Design trade-off and proof of concept for LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth
We provide a proof of the technical feasibility of LOUPE, the first integral-field snapshot spectropolarimeter, designed to monitor the reflected flux and polarization spectrum of Earth. These are to be used as benchmark data for the retrieval of biomarkers and atmospheric and surface characteristics from future direct observations of exoplanets. We perform a design trade-off for an implementation in which LOUPE performs snapshot integral-field spectropolarimetry at visible wavelengths. We used …
The Leiden EXoplanet Instrument (LEXI): a high-contrast high-dispersion spectrograph
The Leiden EXoplanet Instrument (LEXI) will be the first instrument designed for high-contrast, high-dispersion integral field spectroscopy at optical wavelengths. High-contrast imaging (HCI) and high-dispersion spectroscopy (HDS) techniques are used to reach contrasts of 10$^-7$. LEXI will be a bench-mounted, high dispersion integral field spectrograph that will record spectra in a small area around the star with high spatial resolution and high dynamic range. A prototype is being setup to The …
Novel instrument concepts for characterizing directly imaged exoplanets
Current high-contrast exoplanet imagers are optimized to find new exoplanets; they minimize diffracted starlight in a large area around a star. I present four novel instrumental approaches that are optimized to characterize these discoveries by minimizing starlight in a small area around the known location of an exoplanet: 1) coronagraphs that remove virtually all starlight over an octave in wavelength while transmitting more than 90% of the exoplanet signal; 2) holographic wavefront sensors …
The ZIMPOL high contrast imaging polarimeter for SPHERE: polarimetric high contrast commissioning results
SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a second generation VLT instrument aimed at the direct detection of exo-planets. It has received its first light in May 2014. ZIMPOL (Zurich Imaging Polarimeter) is the imaging polarimeter subsystem of the SPHERE instrument. It’s capable of both high accuracy and high sensitivity polarimetry but can also be used as a classical imager. It is located behind an extreme AO system and a stellar coronagraph. ZIMPOL operates at …
Focal-plane electric field sensing with pupil-plane holograms
The direct detection and spectral characterization of exoplanets requires a coronagraph to suppress the diffracted star light. Amplitude and phase aberrations in the optical train fill the dark zone of the coronagraph with quasi-static speckles that limit the achievable contrast. Focal-plane electric field sensing, such as phase diversity introduced by a deformable mirror (DM), is a powerful tool to minimize this residual star light. The residual electric field can be estimated by sequentially …
End-to-end simulations of the E-ELT/METIS coronagraphs
The direct detection of low-mass planets in the habitable zone of nearby stars is an important science case for future E-ELT instruments such as the mid-infrared imager and spectrograph METIS, which features vortex phase masks and apodizing phase plates (APP) in its baseline design. In this work, we present end-to-end performance simulations, using Fourier propagation, of several METIS coronagraphic modes, including focal-plane vortex phase masks and pupil-plane apodizing phase plates, for the …
Designing and testing the coronagraphic Modal Wavefront Sensor: a fast non-common path error sensor for high-contrast imaging
Non-Common Path Errors (NCPEs) are the dominant factor limiting the performance of current astronomical high-contrast imaging instruments. If uncorrected, the resulting quasi-static speckle noise floor limits coronagraph performance to a raw contrast of typically 10$^-4$, a value which does not improve with increasing integration time. The coronagraphic Modal Wavefront Sensor (cMWS) is a hybrid phase optic which uses holographic PSF copies to supply focal-plane wavefront sensing information …
A ``Fast and Furious''' solution to the low-wind effect for SPHERE at the VLT
We present a potential non-invasive solution to sensing the so-called low-wind effect (LWE) seen in the SPHERE instrument at the VLT, based on the ‘‘Fast and Furious (F&F) sequential phase diversity wavefront reconstruction algorithm. This uses non- coronagraphic focal-plane images available from the near-infra- red Differential Tip-Tilt Sensor (DTTS), with the closed-loop correction cycle itself providing the necessary phase diversity between frames required to reconstruct the …
FlySPEX: a flexible multi-angle spectropolarimetric sensing system
Accurate multi-angle spectropolarimetry permits the detailed and unambiguous characterization of a wide range of objects. Science cases and commercial applications include atmospheric aerosol studies, biomedical sensing, and food quality control. We introduce the FlySPEX spectropolarimetric fiber-head that constitutes the essential building block of a novel multi-angle sensing system. A combination of miniaturized standard polarization optics inside every fiber-head encodes the full linear …
Accurate spectrally modulating polarimeters for atmospheric aerosol characterization
Highly accurate multi-angle polarimeters are essential for taking the next step in global characterization of atmospheric aerosol. Spectral polarization modulation enables highly accurate snapshot polarimetry and is very suitable for ground-, air- and space-based instrumentation. In this paper we present two instruments that employ this technology, the SPEX prototype and groundSPEX. We have performed ground-based measurements at the CESAR Observatory in the Netherlands with these two …
Field Guide to Astronomical Instrumentation
This Field Guide is a concise, organized reference that explains the functions and configurations of astronomical instrumentation. It provides an overview of aspects of astronomical instrumentation from principles of general optics and optical design to optical manufacturing and systems engineering. Practitioners will also gain valuable information on polarimeters, interferometers, coronagraphs, cryogenics, software, and more.
Combining high-dispersion spectroscopy with high contrast imaging: Probing rocky planets around our nearest neighbors
Context. Ground-based high-dispersion (R ~ 100 000) spectroscopy (HDS) is proving to be a powerful technique with which to characterize extrasolar planets. The planet signal is distilled from the bright starlight, combining ral and time-differential filtering techniques. In parallel, high-contrast imaging (HCI) is developing rapidly, aimed at spatially separating the planet from the star. While HDS is limited by the overwhelming noise from the host star, HCI is limited by residual quasi-static …
Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049
Context. Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique opportunity to study the topology of stellar magnetic fields in detail and to investigate magnetically driven processes of spot formation.
Aims: Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049. This object hosts a surface field of åisebox-0.5ex~30 kG, one of the strongest known for any non-degenerate star.
Methods: We used …
Aims: Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049. This object hosts a surface field of åisebox-0.5ex~30 kG, one of the strongest known for any non-degenerate star.
Methods: We used …
Use of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observations
In this paper, the use of a neural network algorithm for the retrieval of the aerosol properties from ground-based spectropolarimetric measurements is discussed. The neural network is able to retrieve the aerosol properties with an accuracy that is almost comparable to that of an iterative retrieval. By using the outcome of the neural network as first guess in the iterative retrieval scheme, the accuracy of the retrieved fine- and coarse-mode optical thickness is further improved, while for the …
Surprising detection of an equatorial dust lane on the AGB star IRC+10216
Aims: Understanding the formation of planetary nebulae remains elusive because in the preceding asymptotic giant branch (AGB) phase these stars are heavily enshrouded in an optically thick dusty envelope.
Methods: To further understand the morphology of the circumstellar environments of AGB stars we observe the closest carbon-rich AGB star IRC+10216 in scattered light.
Results: When imaged in scattered light at optical wavelengths, IRC+10216 surprisingly shows a narrow equatorial density …
Atmospheric aerosol characterization with a ground-based SPEX spectropolarimetric instrument
Characterization of atmospheric aerosols is important for understanding their impact on health and climate. A wealth of aerosol parameters can be retrieved from multi-angle, multi-wavelength radiance and polarization measurements of the clear sky. We developed a ground-based SPEX instrument (groundSPEX) for accurate spectropolarimetry, based on the passive, robust, athermal, and snapshot spectral polarization modulation technique, and is hence ideal for field deployment. It samples the …
Mapping atmospheric aerosols with a citizen science network of smartphone spectropolarimeters
To assess the impact of atmospheric aerosols on health, climate, and air traffic, aerosol properties must be measured with fine spatial and temporal sampling. This can be achieved by actively involving citizens and the technology they own to form an atmospheric measurement network. We establish this new measurement strategy by developing and deploying iSPEX, a low- cost, mass-producible optical add-on for smartphones with a corresponding app. The aerosol optical thickness (AOT) maps derived from …
Use of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observations
In this paper, the use of a neural network algorithm for the retrieval of the aerosol properties from ground-based spectropolarimetric measurements is discussed. The neural network is able to retrieve the aerosol properties with an accuracy that is almost comparable to that of an iterative retrieval. By using the outcome of the neural network as a first guess of the iterative retrieval scheme, the accuracy of the fine and coarse mode optical thickness are further improved while for the other …
Focal-plane wavefront sensing with high-order adaptive optics systems
We investigate methods to calibrate the non-common path aberrations at an adaptive optics system having a wavefront-correcting device working with an extremely high resolution (larger than 150x150 correcting elements). We use focal-plane images collected successively, the corresponding phase-diversity information and numerically efficient algorithms to calculate the required wavefront updates. Different approaches are considered in numerical simulations, and laboratory experiments are shown to …
Drift scanning technique for mid-infrared background subtraction
Accurate calibration of ground-based, mid-infrared observations is challenging due to the strong and rapidly varying thermal background emission. The classical solution is the chopping/nodding technique where the secondary mirror and the telescope are being moved by several tens of arcseconds on the sky. However, chopping is generally inefficient and limited in accuracy and frequency by the mass and size of the secondary mirror. A more elegant solution is a drift scan where the telescope slowly …
The ZIMPOL high contrast imaging polarimeter for SPHERE: system test results
SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a new instrument for the VLT aimed at the direct detection of exo- planets. It has received its first light in May 2014. ZIMPOL (Zurich Imaging Polarimeter) is the imaging polarimeter subsystem of the SPHERE instrument. It’s capable of both high accuracy and high sensitivity polarimetry but can also be used as a classical imager. It is located behind an extreme AO system and a stellar coronagraph. ZIMPOL operates at visible …
Fast & Furious focal-plane wavefront sensing
We present two complementary algorithms suitable for using focal-plane measurements to control a wavefront corrector with an extremely high spatial resolution. The algorithms use linear approximations to iteratively minimize the aberrations seen by the focal-plane camera. The first algorithm, Fast & Furious (FF), uses a weak-aberration assumption and pupil symmetries to achieve fast wavefront reconstruction. The second algorithm, an extension to FF, can deal with an arbitrary pupil shape; it …
Characterizing instrumental effects on polarization at a Nasmyth focus using NaCo
We propose a new calibration scheme to determine the instrumental polarization (IP) and crosstalk induced by either the telescope or an instrument at Nasmyth focus. We measure the polarized blue sky at zenith with VLT/UT4/NaCo for different NaCo derotator and telescope azimuth angles. Taking multiple measurements after rotating both the instrument and the telescope with angles of 90° allows use to determine the IP and most crosstalk components separately for the telescope and the instrument. …
Atmospheric aerosol characterization with a ground-based SPEX spectropolarimetric instrument
Characterization of atmospheric aerosols is important for understanding their impact on health and climate. A wealth of aerosol parameters can be retrieved from multi-angle, multi-wavelength radiance and polarization measurements of the clear sky. We developed a ground-based SPEX instrument (groundSPEX) for accurate spectropolarimetry, based on the passive, robust, athermal and snapshot spectral polarization modulation technique, and hence ideal for field deployment. It samples the scattering …
Five-dimensional optical instrumentation: combining polarimetry with time-resolved integral-field spectroscopy
We present implementations of optical instrumentation that records five dimensions of light: polarization state as a function of wavelength, two spatial dimensions, and time. We focus on the optimal integration of polarimetry within microlens-based integral-field spectroscopy. The polarimetric analyzer (or beam- splitter) and dispersing element could be implemented separately, but also amalgamated in the form of a polarization grating. We present optimizations for stacking the polarization- …
LOUPE: Spectropolarimetry of the Earth from the surface of the Moon
We present our prototype for the LOUPE instrument: A small and robust imaging spectropolarimeter that can observe the Earth from the surface of the moon, with as primary objective to characterize the Earth’s linear polarization spectrum throughout the Earth’s daily rotation and monthly phase angle changes. The purpose of LOUPE is to provide benchmark data for future polarization observations of possibly habitable exoplanets. Our instrument concept has been proven to work in a …
Towards Polarimetric Exoplanet Imaging with ELTs
A prime science goal of Extremely Large Telescopes (ELTs) is the detection and characterization of exoplanets to answer the question: are we alone? ELTs will obtain the first direct images of rocky exoplanets in the habitable zone and search for atmospheric biomarkers. However, the required instrumental technologies are not yet at a level where an instrument could be built that would achieve this goal. Polarimetry will be an important ingredient in future high-contrast instruments as it will …
Instrumental polarisation at the Nasmyth focus of the E-ELT
The åisebox-0.5ex~39-m European Extremely Large Telescope (E-ELT) will be the largest telescope ever built. This makes it particularly suitable for sensitive polarimetric observations, as polarimetry is a photon-starved technique. However, the telescope mirrors may severely limit the polarimetric accuracy of instruments on the Nasmyth platforms by creating instrumental polarisation and/or modifying the polarisation signal of the object. In this paper we characterise the polarisation effects of …
Imaging the circumstellar environment of the young T Tauri star SU Aurigae
The circumstellar environments of classical T Tauri stars are challenging to directly image because of their high star-to-disk contrast ratio. One method to overcome this is by using imaging polarimetry where scattered and consequently polarised starlight from the star’s circumstellar disk can be separated from the unpolarised light of the central star. We present images of the circumstellar environment of SU Aur, a classical T Tauri star at the transition of T Tauri to Herbig stars. The …
Analysis and interpretation of 15 quarters of Kepler data of the disintegrating planet KIC 12557548 b
Context. The Kepler object KIC 12557548 shows irregular eclipsing behaviour with a constant 15.685 h period, but strongly varying transit depth. The object responsible for this is believed to be a disintegrating planet forming a trailing dust cloud transiting the star. A 1D model of an exponentially decaying dust tail was found to reproduce the average eclipse in intricate detail. Based on radiative hydrodynamic modelling, the upper limit for the planet mass was found to be twice the mass of the …
Remote sensing of atmospheric aerosols with the SPEX spectropolarimeter
Characterizing atmospheric aerosols is key to understanding their influence on climate through their direct and indirect radiative forcing. This requires long-term global coverage, at high spatial (åisebox-0.5ex~km) and temporal (i̊sebox-0.5ex~days) resolution, which can only be provided by satellite remote sensing. Aerosol load and properties such as particle size, shape and chemical composition can be derived from multi-wavelength radiance and polarization measurements of sunlight that is …
Crowdsourced aerosol measurements using smartphone spectropolarimeters
We present the development, organisation and results of a large citizen science project with the goal to measure and characterise atmospheric aerosols using a network of smartphone spectropolarimeters. The project, called ‘;iSPEX’, was conceived and carried out in the Netherlands, and organised the first National iSPEX measurement day on July 8th 2013. During this day, more than 3000 people performed over 6000 measurements with their own smartphones using a special add-on and a …
Calibrating a high-resolution wavefront corrector with a static focal-plane camera
We present a method to calibrate a high-resolution wavefront-correcting device with a single, static camera, located in the focal plane; no moving of any component is needed. The method is based on a localized diversity and differential optical transfer functions (dOTF) to compute both the phase and amplitude in the pupil plane located upstream of the last imaging optics. An experiment with a spatial light modulator shows that the calibration is sufficient to robustly operate a focal-plane …
Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 . I. Spectropolarimetric observations in all four Stokes parameters
Context. High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. High- quality spectra in the Stokes IQUV parameters are currently available for very few early-type magnetic chemically peculiar stars. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712 with a recently commissioned …
SPHERE-ZIMPOL system testing: status report on polarimetric high contrast results
SPHERE (Spectro-Polarimetric High Contrast Exoplanet Research) is one of the first instruments which aim for the direct detection from extra-solar planets. SPHERE commissioning is foreseen in 2013 on the VLT. ZIMPOL (Zurich Imaging Polarimeter) is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL is dedicated to detect the very faint reflected and hence polarized visible light (600-900 nm) from extrasolar planets. It is located behind an extreme AO system …
The color dependent morphology of the post-AGB star HD 161796
Context. Many protoplanetary nebulae show strong asymmetries in their surrounding shells, pointing to asymmetries during the mass loss phase. Questions concerning the origin and the onset of deviations from spherical symmetry are important for our understanding of the evolution of these objects. Here we focus on the circumstellar shell of the post-AGB star HD 161796.
Aims: We aim to detect signatures of an aspherical outflow, and to derive its properties.
Methods: We used the imaging …
Aims: We aim to detect signatures of an aspherical outflow, and to derive its properties.
Methods: We used the imaging …
Are there tangled magnetic fields on HgMn stars?
Context. Several recent spectrophotometric studies failed to detect significant global magnetic fields in late-B HgMn chemically peculiar stars, but some investigations have suggested the presence of strong unstructured or tangled fields in these objects.
Aims: We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-quality observed spectra of eight slowly rotating HgMn stars and one normal late-B star. We also evaluated recent sporadic detections …
Aims: We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-quality observed spectra of eight slowly rotating HgMn stars and one normal late-B star. We also evaluated recent sporadic detections …
Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae
We present high spectral resolution (R ensuremath≈ 108, 000) Stokes V polarimetry of the classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 ̊A and 6678 Å. The He I lines in these CTTSs contain both narrow emission cores, believed to come from near …
Sterrekundig Instituut Utrecht: The Last Years
I describe the last years of the 370-year long life of the Sterrekundig Instituut Utrecht, which was the second-oldest university observatory in the world and was closed in early 2012 after the Faculty of Science and the Board of Utrecht University decided, without providing qualitative or quantitative arguments, to remove astrophysics from its research and education portfolio.
Observing Circumstellar Neighbourhoods with the Extreme Polarimeter
The study of circumstellar environments at ever higher contrasts has generated considerable interest in recent years. One method to increase the contrast is to observe the linearly polarized light scattered by the circumstellar material while suppressing the unpolarized stellar flux. This paper presents some sample imaging polarimetry results obtained with the Extreme Polarimeter (ExPo). ExPo operates in the visible part of the spectrum, and currently achieves a polarimetric sensitivity of …
HARPS Spectropolarimetry of the Classical T Tauri Stars GQ Lup and TW Hya
We present high spectral resolution Stokes V polarimetery of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 A and 6678 A. The He I lines in both these CTTS contain both narrow emission cores, believed to come from near the accretion shock …
Astronomical Polarimetry: Polarized Views of Stars and Planets
Polarization is a fundamental property of light from astronomical objects, and measuring that polarization often yields crucial information, which is unobtainable otherwise.This chapter reviews the useful formalisms for describing polarization in the optical regime, the mechanisms for the creation of such polarization, and methods for measuring it. Particular emphasis is given on how to implement a polarimeter within an astronomical facility, and on how to deal with systematic effects that often …
Observing the Earth as an exoplanet with LOUPE, the lunar observatory for unresolved polarimetry of Earth
The detections of small, rocky exoplanets have surged in recent years and will likely continue to do so. To know whether a rocky exoplanet is habitable, we have to characterize its atmosphere and surface. A promising characterization method for rocky exoplanets is direct detection using spectropolarimetry. This method will be based on single pixel signals, because spatially resolving exoplanets is impossible with current and near-future instruments. Well-tested retrieval algorithms are essential …
Unusual Stokes V profiles during flaring activity of a delta sunspot
Aims: We analyze a set of full Stokes profile observations of the flaring active region NOAA 10808. The region was recorded with the Vector-Spectromagnetograph of the Synoptic Optical Long-term Investigations of the Sun facility. The active region produced several successive X-class flares between 19:00 UT and 24:00 UT on September 13, 2005 and we aim to quantify transient and permanent changes in the magnetic field and velocity field during one of the flares, which has been fully captured. …
SPICES: spectro-polarimetric imaging and characterization of exoplanetary systems. From planetary disks to nearby Super Earths
SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) is a five-year M-class mission proposed to ESA Cosmic Vision. Its purpose is to image and characterize long-period extrasolar planets and circumstellar disks in the visible (450-900 nm) at a spectral resolution of about 40 using both spectroscopy and polarimetry. By 2020/2022, present and near-term instruments will have found several tens of planets that SPICES will be able to observe and study in detail. Equipped …
The extreme polarimeter: design, performance, first results and upgrades
Well over 700 exoplanets have been detected to date. Only a handful of these have been observed directly. Direct observation is extremely challenging due to the small separation and very large contrast involved. Imaging polarimetry offers a way to decrease the contrast between the unpolarized starlight and the light that has become linearly polarized after scattering by circumstellar material. This material can be the dust and debris found in circumstellar disks, but also the atmosphere or …
Searching for signs of habitability with LOUPE, the Lunar Observatory of Unresolved Polarimetry of Earth
We present LOUPE, a novel type of spectropolarimeter to measure the flux and state of polarization of sunlight that is reflected by the Earth from 0.4 to 0.8 ensuremathμm. LOUPE has been designed as payload of a lunar lander. From the moon, the Earth can be observed as a whole, during its daily rotation and at all phase angles, just as if it were an exoplanet. LOUPE will provide benchmark data for the development of instruments for Earth-like exoplanet characterization, and for the testing of …
Potential of phase-diversity for metrology of active instruments
We investigate the potential of phase-diversity (PD) and Gerchberg- Saxton (GS) algorithms in the calibration of active instruments. A set of images is recorded with the focal-plane scientific camera, each image having a known and unique defocus. The phase- retrieval algorithms are used, with those images, to estimate the non-common path aberration that needs to be compensated by correct alignment of the instrument. We demonstrate by numerical simulations that such algorithms, in particular GS, …
Modeling the instrumental polarization of the VLT and E-ELT telescopes with the M&m's code
Polarimetry is a particularly powerful technique when imaging circumstellar environments. Currently most telescopes include more or less advanced polarimetric facilities and large telescopes count on it for their planet-finder instruments like SPHERE-ZIMPOL on the VLT or EPICS on the future E-ELT. One of the biggest limitations of this technique is the instrumental polarization (IP) generated in the telescope optical path, which can often be larger than the signal to be measured. In most cases …
Evidence for the disintegration of KIC 12557548 b
Context. The Kepler object KIC 12557548 b is peculiar. It exhibits transit-like features every 15.7 h that vary in depth between 0.2% and 1.2%. Rappaport et al. (2012, ApJ, 752, 1) explain the observations in terms of a disintegrating, rocky planet that has a trailing cloud of dust created and constantly replenished by thermal surface erosion. The variability of the transit depth is then a consequence of changes in the cloud optical depth.
Aims: We aim to validate the disintegrating-planet …
Aims: We aim to validate the disintegrating-planet …
A spectro-polarimetric integral field spectrograph for EPICS-EPOL
Imaging polarimetry offers a way to increase the contrast of light scattered from circumstellar material, enabling direct observation of exoplanets -possibly rocky- with the E-ELT. To actually characterize these planets, some spectral resolution is essential. With sufficient resolution -both spectral and spatial- the spectral differential imaging technique can be used in addition to the polarimetry to detect circumstellar point sources. We present the concept for a spectro-polarimetric integral …
Linear analytical solution to the phase diversity problem for extended objects based on the Born approximation
In this paper we give a new wavefront estimation technique that overcomes the main disadvantages of the phase diversity (PD) algorithms, namely the large computational complexity and the fact that the solutions can get stuck in a local minima. Our approach gives a good starting point for an iterative algorithm based on solving a linear system, but it can also be used as a new wavefront estimation method. The method is based on the Born approximation of the wavefront for small phase aberrations …
FOAM: the modular adaptive optics framework
Control software for adaptive optics systems is mostly custom built and very specific in nature. We have developed FOAM, a modular adaptive optics framework for controlling and simulating adaptive optics systems in various environments. Portability is provided both for different control hardware and adaptive optics setups. To achieve this, FOAM is written in C++ and runs on standard CPUs. Furthermore we use standard Unix libraries and compilation procedures and implemented a hardware abstraction …
Extremely fast focal-plane wavefront sensing for extreme adaptive optics
We present a promising approach to the extremely fast sensing and correction of small wavefront errors in adaptive optics systems. As our algorithm’s computational complexity is roughly proportional to the number of actuators, it is particularly suitable to systems with 10,000 to 100,000 actuators. Our approach is based on sequential phase diversity and simple relations between the point-spread function and the wavefront error in the case of small aberrations. The particular choice of …
Experimental validation of optimization concepts for focal-plane image processing with adaptive optics
We show experimental results demonstrating the feasibility of an extremely fast sequential phase-diversity (SPD) algorithm for point sources. The algorithm can be implemented on a typical adaptive optics (AO) system to improve the wavefront reconstruction beyond the capabilities of a wavefront sensor by using the information from the imaging camera. The algorithm is based on a small-phase approximation enabling fast numerical implementation, and it finds the optimal wavefront correction by …
Constraining the circumbinary envelope of Z Canis Majoris via imaging polarimetry
Context. Z CMa is a complex binary system composed of a Herbig Be and an FU Ori star. The Herbig star is surrounded by a dust cocoon of variable geometry, and the whole system is surrounded by an infalling envelope. Previous spectropolarimetric observations have reported a preferred orientation of the polarization angle, perpendicular to the direction of a very extended, parsec-sized jet associated with the Herbig star.
Aims: The variability in the amount of polarized light has been associated …
Aims: The variability in the amount of polarized light has been associated …
Bilinear solution to the phase diversity problem for extended objects based on the Born approximation
We propose a new approach for the joint estimation of aberration parameters and unknown object from diversity images with applications in imaging systems with extended objects as astronomical ground-based observations or solar telescopes. The motivation behind our idea is to decrease the computational complexity of the conventional phase diversity (PD) algorithm and avoid the convergence to local minima due to the use of nonlinear estimation algorithms. Our approach is able to give a good …
Data driven identification and aberration correction for model-based sensorless adaptive optics
Wavefront sensorless adaptive optics methodologies are considered in many applications where the deployment of a dedicated wavefront sensor is inconvenient, such as in fluorescence microscopy. In these methodologies, aberration correction is achieved by sequentially changing the settings of the adaptive optical element until a predetermined imaging quality metric is optimised. Reducing the time required for this optimisation is a challenge. In this paper, a two stage data driven optimisation …
HARPS spectropolarimetry of classical T Tauri stars
We present high spectral resolution Stokes V polarimetery of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 A and 6678 A. The He I lines in both these CTTS contain both narrow emission cores, believed to come from near the accretion shock …
SPEX2Earth, a novel spectropolarimeter for remote sensing of aerosols and clouds
Multi-angle spectro-polarimetry is the tool for the remote detection and characterization of aerosol and clouds in the Earth’s atmosphere. Using a novel technique to measure polarization, we have developed a 30 kg instrument design to simultaneously measure the intensity and state of linear polarization of scattered sunlight, from 400 to 800 nm and 1200 to 1600 nm, for 30 viewing directions, each with a 30° swath. Aerosols affect the climate directly by scattering and absorption of solar …
Observing the Earth as an exoplanet
Observations of Solar System planets, including the Earth, have shown the power of polarimetry for the characterization of planetary atmospheres and surfaces, and its ability to break degeneracies in retrievals from flux observations only and is thus essential for the full characterization of atmospheres and surfaces of (exo-) planets. With the discoveries of the first rocky exoplanets, the quest for Earth-like exoplanets and signs of their habitability has started. Since exoplanet observations …
Multiwavelength imaging polarimetry of Venus at various phase angles
Venus is the only planet with an atmosphere that we can observe from the ground at a large range of phase angles. Therefore it constitutes an important benchmark for direct observations of exoplanets, which will soon become available. Moreover, polarimetric observations at various phase angles and wavelengths provide a unique way to characterize any (exo-)planetary atmosphere. For instance, the famous study by Hansen & Hovenier (1974) which combines disk-integrated polarimetric observations …
iSPEX: the creation of an aerosol sensor network of smartphone spectropolarimeters
An increasing amount people carry a mobile phone with internet connection, camera and large computing power. iSPEX, a spectropolarimetric add-on with complementary app, instantly turns a smartphone into a scientific instrument to measure dust and other aerosols in our atmosphere. A measurement involves scanning the blue sky, which yields the angular behavior of the degree of linear polarization as a function of wavelength, which can unambiguously be interpreted in terms of size, shape and …
iSPEX: everybody can measure atmospheric aerosols with a smartphone spectropolarimeter
An increasing amount people carry a mobile phone with internet connection, camera and large computing power. iSPEX, a spectropolarimetric add-on with complementary app, instantly turns a smartphone into a scientific instrument to measure dust and other aerosols in our atmosphere. A measurement involves scanning the blue sky, which yields the angular behavior of the degree of linear polarization as a function of wavelength, which can unambiguously be interpreted in terms of size, shape and …
Planetary science: In search of biosignatures
An analysis of the intensity and polarization of sunlight reflected by Earth reveals signatures of life on our planet. What prospects are there for using similar measurements to find life on planets outside the Solar System? Planetary scientists offer some answers. See Letter p.64
Magnetism, chemical spots, and stratification in the HgMn star ensuremathϕ Phoenicis
Context. Mercury-manganese (HgMn) stars have been considered as non- magnetic and non-variable chemically peculiar (CP) stars for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key role in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded …
Direct imaging of a massive dust cloud around R Coronae Borealis
We present recent polarimetric images of the highly variable star R CrB using ExPo and archival WFPC2 images from the HST. We observed R CrB during its current dramatic minimum where it decreased more than 9 mag due to the formation of an obscuring dust cloud. Since the dust cloud is only in the line-of-sight, it mimics a coronograph allowing the imaging of the star’s circumstellar environment. Our polarimetric observations surprisingly show another scattering dust cloud at approximately …
Coherence-gated wavefront sensing for microscopy using fringe analysis
We have implemented a coherence-gated wavefront sensor on a two-photon excitation microscope. We used the backscattered near-infrared light from the sample to interfere with an optically flat reference beam. By applying a known waverfront tilt in the reference beam, a fringe pattern emerged on the camera. The deformmation of the wavefront due to the turbid media under study warps the fring pattern, similar to frequency modulation. Through Fourier transform analysis of the modulated fringe …
The effects of disk and dust structure on observed polarimetric images of protoplanetary disks
Context. Imaging polarimetry is a powerful tool for imaging faint circumstellar material. It is a rapidly developing field with great promise for diagnostics of both the large-scale structures and the small-scale details of the scattering particles.
Aims: For a correct analysis of observations we need to fully understand the effects of dust particle parameters, as well as the effects of the telescope, atmospheric seeing, and assumptions about the data reduction and processing of the observed …
Aims: For a correct analysis of observations we need to fully understand the effects of dust particle parameters, as well as the effects of the telescope, atmospheric seeing, and assumptions about the data reduction and processing of the observed …
Signatures of Water Clouds on Exoplanets: Numerical Simulations.
Clouds are of crucial importance for a planetary climate, because they store atmospheric volatiles, and because they scatter and absorb incident starlight and absorb, emit and scatter thermal radiation. Consequently, the detection and characterization of clouds on a planet can provide us with a wealth of information on the conditions on the surface. Here, we present numerically simulated flux and polarization spectra, from 0.3 ensuremathμm to 1.0 ensuremathμm, of starlight reflected by …
New Insights into Stellar Magnetism from the Spectropolarimetry in All Four Stokes Parameters
Development of high-resolution spectropolarimetry has stimulated a major progress in our understanding of the magnetism and activity of late-type stars. During the last decade magnetic fields were discovered and mapped for various types of active stars using spectropolarimetric methods. However, these observations and modeling attempts are inherently incomplete since they are based on the interpretation of the stellar circular polarization alone. Taking advantage of the recently commissioned …
Innovative Imaging of Young Stars: First Light ExPo Observations
We have developed an innovative imaging polariemter, ExPo, that excels in the imaging of the circumstellar environments of young stars. The basic physics that ExPo exploits is that starlight reflected from a star’s circumstellar environment becomes linearly polarised, making it easily separable from unpolarised starlight. Our preliminary results, from the William Herschel Telescope in La Palma, show that ExPo has successfully detected several known protoplanetary disks out to a much larger …
Spectropolarimeter for planetary exploration (SPEX): performance measurements with a prototype
SPEX (Spectropolarimeter for Planetary Exploration) was developed in close cooperation between scientific institutes and space technological industries in the Netherlands. It is used for measuring microphysical properties of aerosols and cloud particles in planetary atmospheres. SPEX utilizes a number of novel ideas. The key feature is that full linear spectropolarimetry can be performed without the use of moving parts, using an instrument of approximately 1 liter in volume. This is done by …
Simulating Polarized Light from Exoplanets
In Utrecht we are building an imaging polarimeter, ExPo (Extreme Polarimeter), to image circumstellar disk and characterize extra-solar planets. To test and calibrate ExPo, we have built a laboratory-based simulator that mimicks a star with a Jupiter- like exoplanet as seen by the 4.2 m William-Herschel Telescope. The star and planet are simulated using two single-mode fibres in close proximity that are fed with a broadband arc lamp. The unpolarized star has a flux of 10$^11$ photons s$^-1$, to …
Design and Prototype Results of the ExPo Imaging Polarimeter
We present the design and prototype laboratory results of ExPo, an imaging polarimeter for the study of circumstellar disks and possibly exoplanet detection currently under development at the University of Utrecht. The instrument is designed to achieve a contrast ratio of 10$^-9$ between the unpolarized starlight and the polarized source. First light is scheduled for the second half of 2008 at the 4.2 m William Herschel telescope at La Palma. The instrument is based on the dual beam-exchange …
Data Reduction Approach for the Extreme Polarimeter
ExPo (Extreme Polarimeter) is an imaging polarimeter that we are building at Utrecht University, The Netherlands. It will detect polarized light from circumstellar disks and extrasolar planets, initially at the 4.2 m WHT and later at other telescopes. We have developed a data reduction approach that minimizes the influence of instrumental and atmospherical effects by using a partially transmitting coronagraph focal-plane mask. The approach has been tested with a laboratory simulator and an ExPo …
The ZIMPOL high contrast imaging polarimeter for SPHERE: sub-system test results
SPHERE (Spectro-Polarimetric High Contrast Exoplanet Research) is one of the first instruments which aim for the direct detection from extra-solar planets. The instrument will search for direct light from old planets with orbital periods of several months to several years as we know them from our solar system. These are planets which are in or close to the habitable zone. ZIMPOL (Zurich Imaging Polarimeter) is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL …
Prototyping for the Spectropolarimeter for Planetary EXploration (SPEX): calibration and sky measurements
We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (åisebox-0.5ex~ 1 liter), robust and lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, optimized for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and a multiple-order retarder, followed by a polarizing …
No magnetic field in the spotted HgMn star ensuremathμ Leporis
Context. Chemically peculiar stars of the mercury-manganese (HgMn) type represent a new class of spotted late-B stars, in which evolving surface chemical inhomogeneities are apparently unrelated to the presence of strong magnetic fields but are produced by some hitherto unknown astrophysical mechanism.
Aims: The goal of this study is to perform a detailed line profile variability analysis and carry out a sensitive magnetic field search for one of the brightest HgMn stars - ensuremathμ Lep. …
Aims: The goal of this study is to perform a detailed line profile variability analysis and carry out a sensitive magnetic field search for one of the brightest HgMn stars - ensuremathμ Lep. …
M&m's: an error budget and performance simulator code for polarimetric systems
Although different approaches to model a polarimeter’s accuracy have been described before, a complete error budgeting tool for polarimetric systems has not been yet developed. Based on the framework introduced by Keller & Snik, in 2009, we have developed the M&m’s code as a first attempt to obtain a generic tool to model the performance and accuracy of a given polarimeter, including all the potential error contributions and their dependencies on physical parameters. The main …
Fast horizontal flows in a quiet sun MHD simulation and their spectroscopic signatures
Numerical simulations of solar surface convection have predicted the existence of supersonic horizontal flows in the photospheric granulation. Recently, the detection of such flows in data from the Hinode satellite was reported. We study supersonic granular flows in detail to understand their signatures in spectral lines and to test the observational detection method used to identify these flows in the Hinode observations. We perform time- dependent 3D radiative MHD numerical simulations and …
The search for magnetic fields in mercury-manganese stars
Mercury-manganese (HgMn) stars were considered to be non-magnetic, showing no evidence of surface spots. However, recent investigations revealed that some stars in this class possess an inhomogeneous distribution of chemical elements on their surfaces. According to our current understanding, the most probable mechanism of spot formation involves magnetic fields. Taking the advantage of a newly-built polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we performed a …
Spectral and polarimetric characterization of gazeous and telluric planets with SEE COAST
SEE COAST stands for Super Earth Explorer - Coronagraphic Off-Axis Space Telescope. The concept was initially proposed to ESA for Cosmic Vision. None of the direct detection exoplanet proposals were selected in 2007 and we are now pursuing our efforts to consolidate the astrophysical program and the technical developments for the next call for proposal. The prime objective of SEE COAST is to contribute to the understanding of the formation and evolution of planetary systems. Exploring the …
Data-reduction techniques for high-contrast imaging polarimetry. Applications to ExPo
Context. Imaging polarimetry is a powerful tool for detecting and characterizing exoplanets and circumstellar environments. Polarimetry allows a separation of the light coming from an unpolarized source such as a star and the polarized source such as a planet or a protoplanetary disk. Future facilities like SPHERE at the VLT or EPICS at the E-ELT will incorporate imaging polarimetry to detect exoplanets. The Extreme Polarimeter (ExPo) is a dual-beam imaging polarimeter that can currently reach …
First Detection of Linear Polarization in the Line Profiles of Active Cool Stars
The application of high-resolution spectropolarimetry has led to major progress in understanding the magnetism and activity of late- type stars. During the last decade, magnetic fields have been discovered and mapped for many types of active cool stars using spectropolarimetric data. However, these observations and modeling attempts are fundamentally incomplete since they are based on the interpretation of the circular polarization alone. Taking advantage of the newly built HARPS polarimeter, we …
Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani
Context. According to our current understanding, a subclass of the upper main-sequence chemically peculiar stars, called mercury- manganese (HgMn), is non-magnetic. Nevertheless, chemical inhomogeneities were recently discovered on their surfaces. At the same time, no global magnetic fields stronger than 1-100 G are detected by systematic studies.
Aims: The goals of our study are to search for a magnetic field in the HgMn binary system 66 Eri and to investigate chemical spots on the stellar …
Aims: The goals of our study are to search for a magnetic field in the HgMn binary system 66 Eri and to investigate chemical spots on the stellar …
The Polarization Optics for the European Solar Telescope
EST, the European Solar Telescope, is a 4-m class solar telescope, which will be located at the Canary Islands. It is currently in the conceptual design phase as a European funded project. In order to fulfill the stringent requirements for polarimetric sensitivity and accuracy, the polarimetry has been included in the design work from the very beginning. The overall philosophy has been to use a combination of techniques, which includes a telescope with low (and stable) instrumental polarization, …
The HARPS Polarimeter
We recently commissioned the polarimetric upgrade of the HARPS spectrograph at ESO’s 3.6-m telescope at La Silla, Chile. The HARPS polarimeter is capable of full Stokes spectropolarimetry with large sensitivity and accuracy, taking advantage of the large spectral resolution and stability of HARPS. In this paper we present the instrument design and its polarimetric performance. The first HARPSpol observations show that it can attain a polarimetric sensitivity of ensuremath∼10$^-5$ (after …
HARPSpol — The New Polarimetric Mode for HARPS
The HARPS spectrograph can now perform a full polarisation analysis of spectra. It has been equipped with a polarimetric unit, HARPSpol, which was jointly designed and produced by Uppsala, Utrecht and Rice Universities and by the STScI. Here we present the new instrument, demonstrate its polarisation capabilities and show the first scientific results.
The search for magnetic fields in mercury-manganese stars
Context. A subclass of the upper main-sequence chemically peculiar stars, mercury-manganese (HgMn) stars were traditionally considered to be non-magnetic, showing no evidence of variability in their spectral line profiles. However, discoveries of chemical inhomogeneities on their surfaces imply that this assumption should be investigated. In particular, spectroscopic time-series of AR Aur, ensuremathα And, and five other HgMn stars indicate the presence of chemical spots. At the same time, no …
Imaging polarimetry of protoplanetary disks: feasibility and usability
Imaging polarimetry is one of the most promising tools to map the structure of faint protoplanetary disks. In this contribution we discuss the feasibility of imaging polarimetry of protoplanetary disks and the usability to answer the scientific questions in the field. From the theoretical side we do this by simulations of disks of various geometries and dust properties. We model the expected signal and detailed predictions for current and upcoming imaging polarimeters. This way we can address …
Imaging polarimetry of circumstellar environments with the Extreme Polarimeter
Three successful observation campaigns have been conducted with the Extreme Polarimeter, an imaging polarimeter for the study of circumstellar environments in scattered light at visible wavelengths. A contrast ratio between the central star and the circumstellar source of 10-5 can be achieved with polarimetry, with a Lyot coronograph capable of increasing this contrast by several orders of magnitude. The instrument currently operates without an adaptive optics system. An Adaptive Optics system …
EPOL: the exoplanet polarimeter for EPICS at the E-ELT
EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging Camera and Spectrograph) for the 42-m E-ELT. It is based on sensitive imaging polarimetry to differentiate between linearly polarized light from exoplanets and unpolarized, scattered starlight and to characterize properties of exoplanet atmospheres and surfaces that cannot be determined from intensity observations alone. EPOL consists of a coronagraph and a dual-beam polarimeter with a liquid-crystal retarder to exchange the …
Observations of solar scattering polarization at high spatial resolution
Context. The weak, turbulent magnetic fields that supposedly permeate most of the solar photosphere are difficult to observe, because the Zeeman effect is virtually blind to them. The Hanle effect, acting on the scattering polarization in suitable lines, can in principle be used as a diagnostic for these fields. However, the prediction that the majority of the weak, turbulent field resides in intergranular lanes also poses significant challenges to scattering polarization observations because …
The ZIMPOL high-contrast imaging polarimeter for SPHERE: design, manufacturing, and testing
ZIMPOL is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL is dedicated to detect the very faint reflected and hence polarized visible light from extrasolar planets. ZIMPOL is located behind an extreme AO system (SAXO) and a stellar coronagraph. SPHERE is foreseen to have first light at the VLT at the end of 2011. ZIMPOL is currently in the manufacturing, integration and testing phase. We describe the optical, polarimetric, mechanical, thermal and electronic …
The polarization optics for the European Solar Telescope (EST)
EST (European Solar Telescope) is a 4-m class solar telescope, which is currently in the conceptual design phase. EST will be located at the Canary Islands and aims at observations with the best possible spectral, spatial and temporal resolution and best polarimetric performance, of the solar photosphere and chromosphere, using a suite of instruments that can efficiently produce two-dimensional spectropolarimetric information of the thermal, dynamic and magnetic properties of the plasma over …
SPEX: the spectropolarimeter for planetary exploration
SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact instrument for spectropolarimetry, and in particular for detecting and characterizing aerosols in planetary atmospheres. With its åisebox-0.5ex~1-liter volume it is capable of full linear spectropolarimetry, without moving parts. The degree and angle of linear polarization of the incoming light is encoded in a sinusoidal modulation of the intensity spectrum by an achromatic quarter-wave retarder, an athermal …
EPOL: the exoplanet polarimeter for EPICS at the E-ELT
EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging Camera and Spectrograph) for the 42-m E-ELT. It is based on sensitive imaging polarimetry to differentiate between linearly polarized light from exoplanets and unpolarized, scattered starlight and to characterize properties of exoplanet atmospheres and surfaces that cannot be determined from intensity observations alone. EPOL consists of a coronagraph and a dual-beam polarimeter with a liquid-crystal retarder to exchange the …
EPICS: direct imaging of exoplanets with the E-ELT
Presently, dedicated instruments at large telescopes (SPHERE for the VLT, GPI for Gemini) are about to discover and explore self- luminous giant planets by direct imaging and spectroscopy. The next generation of 30m-40m ground-based telescopes, the Extremely Large Telescopes (ELTs), have the potential to dramatically enlarge the discovery space towards older giant planets seen in reflected light and ultimately even a small number of rocky planets. EPICS is a proposed instrument for the European …
Inversions of High-Cadence SOLIS-VSM Stokes Observations
We have processed full-Stokes observations made with the SOLIS-VSM using Fe I 630.15 and Fe I 630.25 nm. The data have high spectral and temporal resolution, moderate spatial resolution, and large polarimetric sensitivity and accuracy. We use the code LILIA, an LTE inversion code written by Socas-Navarro (2001), in order to invert the data in vector magnetic fields. The 180-degree ambiguity in magnetic field orientation is solved by using the Non-Potential Field Calculation (NPFC) method of …
EPICS, the exoplanet imager for the E-ELT
Very soon, dedicated instruments developments at large telescopes (SPHERE for the VLT, GPI for Gemini) are about to discover and explore self-luminous giant planets by direct imaging and spectroscopy in significant numbers. The next generation of 30m- 40m ground-based telescopes, the Extremely Large Telescopes (ELTs), have the potential to dramatically enlarge the discovery space towards older giant planets seen in reflected light and ultimately even a small number of rocky planets. EPICS is a …
Two Ways of Improving Stokes Inversions
Performing Stokes inversion on single pixels can be difficult due to seeing and instrumental smearing. We present two different techniques to improve Stokes inversions. The first method, REGIONS, is a code that finds pixels that are likely to contain signal from the same magnetic element. The second method, mCLEAN, reconstructs a magnetogram if the point spread function (PSF) of the instrument and seeing is known.
Statistics of Convective Collapse Events in the Photosphere and Chromosphere Observed with the HINODE SOT
Convective collapse, a theoretically predicted process that intensifies existing weak magnetic fields in the solar atmosphere, was first directly observed in a single event by Nagata et al. (2008) using the high resolution Solar Optical Telescope (SOT) of the Hinode satellite. Using the same space telescope, we observed 49 such events and present a statistical analysis of convective collapse events. Our data sets consist of high resolution time series of polarimetric spectral scans of two iron …
Statistics of convective collapse events in the photosphere and chromosphere observed with the Hinode SOT
Convective collapse, a theoretically predicted process that intensifies existing weak magnetic fields in the solar atmosphere, was first directly observed in a single event by Nagata et al. (2008, ApJ, 677, L145) using the high resolution Solar Optical Telescope (SOT) of the Hinode satellite. Using the same space telescope, we observed 49 such events and present a statistical analysis of convective collapse events. Our data sets consist of high resolution time series of polarimetric spectral …
Polarimetric Measurements of Protoplanetary Disks with ExPo
Polarimetry is a powerful tool for detecting and characterizing exoplanets and protoplanetary disks as light scattered from circumstellar material is linearly polarized. We present the first light results of ExPo (Extreme Polarimeter) [1], a sensitive imaging polarimeter developed at Utrecht University that works in the visible part of the spectrum. Our first light observations at the 4.2-meter William Herschel Telescope (WHT) show that ExPo can reach the high-contrast ratios that are necessary …
An IFU for diffraction-limited 3D spectroscopic imaging: laboratory and on-site tests
We have developed a state-of-the-art image slicer Integral Field Unit (IFU) for the McMath-Pierce Solar Telescope (McMP) located at Kitt Peak National Solar Observatory. The IFU will be used for high-resolution 3-dimensional spectroscopy and polarimetry over a small field of view that is well corrected by adaptive optics. It consists of 19 effective slices that correspond to a field of view of 6.27``x 7’’. The IFU delivers a 152’’ long slit to an existing spectrograph …
Vector Magnetic Field Inversions of High Cadence SOLIS-VSM Data
We have processed full Stokes observations from the SOLIS VSM in the photospheric lines Fe I 630.15 nm and 630.25 nm. The data sets have high spectral and temporal resolution, moderate spatial resolution, and large polarimetric sensitivity and accuracy. We used the LILIA, an LTE code written by i̧tetfischer_Navarro2001 to invert the data. We also applied the non-potential magnetic field calculation method of ţetfischer_Manolis2005 in order to resolve the 180 degree ambiguity. The output are maps …
The Prototype of the Small Synoptic Second Solar Spectrum Telescope (S5̂T)
We present the design and the prototype of the Small Synoptic Second Solar Spectrum Telescope (S5̂T), which can autonomously measure scattering polarization signals on a daily basis with large sensitivity and accuracy. Its data will be used to investigate the nature of weak, turbulent magnetic fields through the Hanle effect in many lines. Also the relation between those fields and the global solar dynamo can be revealed by spanning the observations over a significant fraction of a solar cycle. …
SOLIS Vector Spectromagnetograph: Status and Science
The Vector Spectromagnetograph (VSM) instrument has recorded photospheric and chromospheric magnetograms daily since August 2003. Full-disk photospheric vector magnetograms are observed at least weekly and, since November 2006, area-scans of active regions daily. Quick-look vector magnetic images, plus X3D and FITS formated files, are now publicly available daily. In the near future, Milne-Eddington inversion parameter data will also be available and a typical observing day will include three …
Polarimetry from the Ground Up
Ground-based solar polarimetry has made great progress over the last decade. Nevertheless, polarimetry is still an afterthought in most telescope and instrument designs, and most polarimeters are designed based on experience and rules of thumb rather than using more formal systems engineering approaches as is common in standard optical design efforts. Here we present the first steps in creating a set of systems engineering approaches to the design of polarimeters that makes sure that the final …
A Brief History of the Second Solar Spectrum
The development of the ZIMPOL principle at ETH Zurich led to a dramatic increase in our observational knowledge of the solar scattering polarization spectrum, now known as the Second Solar Spectrum. This contribution summarizes the historical developments of this field with a special focus on the observational events between 1994 and 1996 when many interesting things happened that cannot be found in the literature.
Polarization Properties of Real Aluminum Mirrors, I. Influence of the Aluminum Oxide Layer
In polarimetry, it is important to characterize the polarization properties of the instrument itself to disentangle real astrophysical signals from instrumental effects. This article deals with the accurate measurement and modeling of the polarization properties of real aluminum mirrors, as used in astronomical telescopes. Main goals are the characterization of the aluminum oxide layer thickness at different times after evaporation, and its influence on the polarization properties of the mirror. …
Super earth explorer: a coronagraphic off-axis space telescope
The Super-Earth Explorer is an Off-Axis Space Telescope (SEE-COAST) designed for high contrast imaging. Its scientific objective is to make the physico-chemical characterization of exoplanets possibly down to 2 Earth radii. For that purpose it will analyze the spectral and polarimetric properties of the parent starlight reflected by the planets, in the wavelength range 400-1,250 nm.
Spectral modulation for full linear polarimetry
Linear (spectro) polarimetry is usually performed using separate photon flux measurements after spatial or temporal polarization modulation. Such classical polarimeters are limited in sensitivity and accuracy by systematic effects and noise. We describe a spectral modulation principle that is based on encoding the full linear polarization properties of light in its spectrum. Such spectral modulation is obtained with an optical train of an achromatic quarter-wave retarder, an athermal …
An analytical model to demonstrate the reliability of reconstructed `active longitudes'.
Photometric light curves show apparent spot concentrations separated by 180 degrees in longitude that are commonly referred to as
active longitudes'. These spot concentrations have been observed to change in strength resulting in the flip-flop’ effect. We use a simple analytical model to calculate the light curve of a star with an arbitrary spot pattern to show that `active longitudes’ are a likely consequence of the limited information content contained in a light curve. We also …
Polarimetry of Mars with SPEX, an Innovative Spectropolarimeter
We present SPEX, an innovative, compact, and robust spectropolarimeter that measures fluxes and polarization of sunlight reflected by Mars from 400 to 800 nm. With simulations we’ll show how with SPEX atmospheric dust and the surface can be studied.
Diversity among other worlds: characterization of exoplanets by direct detection
The physical characterization of exoplanets will require to take spectra at several orbital positions. For that purpose, a direct imaging capability is necessary. Direct imaging requires an efficient stellar suppression mechanism, associated with an ultrasmooth telescope. We show that before future large space missions (interferometer, 4-8 m class coronograph, external occulter or Fresnel imager), direct imaging of giant planets and close-by super-Earth are at the cross-road of a high scientific …
A New Era in Solar Thermal-IR Astronomy: the NSO Array Camera (NAC) on the McMath-Pierce Telescope
The U.S. National Solar Observatory Array Camera (NAC) is a cryogenically cooled 1Kx1K InSb
Aladdin array that recently became operational at the McMath-Pierce facility on Kitt Peak, a high dry site in the southwest U.S. (Arizona). The new camera is similar to those already incorporated into instruments on nighttime telescopes, and has unprecedented sensitivity, low noise, and excellent cosmetics compared with the Amber Engineering (AE) device it replaces. (The latter was scavenged from a …
The upgrade of HARPS to a full-Stokes high-resolution spectropolarimeter
We present the design of a compact module that converts the HARPS instrument at the 3.6-m telescope at La Silla to a full-Stokes high-resolution spectropolarimeter. The polarimeter will replace the obsolete Iodine cell inside the HARPS Cassegrain adapter. Utilizing the two fibers going into the spectrograph, two dual- beam systems can be positioned in the beam: one with a rotating superachromatic quarter-wave plate for circular polarimetry and one with a rotating superachromatic half-wave plate …
The Extreme Polarimeter (ExPo): design of a sensitive imaging polarimeter
The Extreme Polarimeter (ExPo) is approaching its first deployment at the 4.2 m William Herschel Telescope at La Palma. This imaging polarimeter, developed at the Astronomical Institute of Utrecht University, aims to study circumstellar material at a contrast ratio with the central star of 10$^-9$. Working at visible wavelengths, it will provide an inner working angle down to 0.5 arcsec and a field of view of 20 arcsec diameter. ExPo employs a dual beam-exchange technique based on polarimeter …
SPHERE ZIMPOL: overview and performance simulation
The ESO planet finder instrument SPHERE will search for the polarimetric signature of the reflected light from extrasolar planets, using a VLT telescope, an extreme AO system (SAXO), a stellar coronagraph, and an imaging polarimeter (ZIMPOL). We present the design concept of the ZIMPOL instrument, a single-beam polarimeter that achieves very high polarimetric accuracy using fast polarization modulation and demodulating CCD detectors. Furthermore, we describe comprehensive performance simulations …
SPEX: an in-orbit spectropolarimeter for planetary exploration
SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact remote-sensing instrument for detecting and characterizing aerosols. With its 1-liter volume it is capable of full linear spectropolarimetry, without moving parts. High precision polarimetry is performed through encoding the degree and angle of linear polarization of the incoming light in a sinusoidal modulation of the intensity spectrum. This is achieved by using an achromatic quarter-wave retarder, an athermal …
Design of a laboratory simulator to test exoplanet imaging polarimetry
Research on extrasolar planets is one of the most rapidly advancing fields of astrophysics. In just over a decade since the discovery of the first extra-solar planet orbiting around 51 Pegasi, 289 extrasolar planets have been discovered. This breakthrough is the result of the development of a wide range of new observational techniques and facilities for the detection and characterisation of extrasolar planets. In Utrecht we are building the Extreme Polarimeter (ExPo) to image extra-solar planets …
New Observations of the Magnetic Vector Field across the Solar Disk
Full disk solar magnetograms have been available for more than three decades. However, those maps only show the line-of-sight magnetic flux. The physical quantity we really want to know is the magnetic field vector along with the filling factor, i.e. the fractional area of the resolution element that is occupied by the magnetic field. Since August 2003, the SOLIS Vector- SpectroMagnetograph has been recording the photospheric magnetic field vector across the full solar disk with high sensitivity …
Science Goals of the Extreme Polarimeter (ExPo)
To advance our understanding of the formation, evolution and structure of extra-solar planetary systems we are building a high- precision imaging polarimeter (ExPo). ExPo will initially be located at the 4.2m William Herschel Telescope on La Palma. We will use polarimetric techniques similar to those developed for high-precision solar polarimetry to reach a sensitivity of 10-̂5, to polarimetrically image and characterize planets and protoplanetary debris discs. I will present a review of the …
Design Options for the Extreme Polarimeter (ExPo)
The Extreme Polarimeter, ExPo, is being developed for the detailed study of circumstellar disks and exoplanet characterization at the 4.2-m William Herschel Telescope at La Palma. This imaging polarimeter is designed to measure linear polarization at the 10-5 level around bright stars at distances outward of about 0.5 arcsec. We will discuss the design options for this instrument as well as the advantages and disadvantages of specific components such as the Atmospheric Dispersion Compensator, …
Seething Horizontal Magnetic Fields in the Quiet Solar Photosphere
The photospheric magnetic field outside of active regions and the network has a ubiquitous and dynamic line-of-sight component that strengthens away from disk center as expected for a nearly horizontal orientation. This component shows a striking time variation with an average temporal rms of 1.7 G at 3 arcsec resolution. In our moderate resolution observations the nearly horizontal component has a frequency variation power law exponent of -1.4 below 1.5 mHz and is spatially patchy on scales up …
Seething Horizontal Magnetic Fields in the Quiet Solar Photosphere
The photospheric magnetic field outside of active regions and the network has a ubiquitous and dynamic line-of-sight component that strengthens from disk center to limb as expected for a nearly horizontal orientation. This component shows a striking time variation with an average temporal rms near the limb of 1.7 G at ~3`` resolution. In our moderate-resolution observations the nearly horizontal component has a frequency variation power-law exponent of -1.4 below 1.5 mHz and is spatially patchy …
The Ba II 4554 / Hensuremathβ Imaging Polarimeter for the Dutch Open Telescope
In order to expand the high-resolution, multi-wavelength imaging capabilities of the Dutch Open Telescope (DOT), an additional polarimetric channel based on a 80 m̊A tunable Lyot filter for Ba II 4554 and Hensuremathβ has been designed and constructed. The large atomic mass and the resulting steep line wings, make Ba II 4554 particularly suitable for the creation of photospheric Dopplergrams and Stokes-V magnetograms. The line also yields a significant degree of linear (scattering) polarization …
SOLIS-VSM Solar Vector Magnetograms
The Vector SpectroMagnetograph (VSM) instrument has recorded full-disk photospheric vector magnetograms weekly since August 2003 as part of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project. After the full deployment of the VSM data processing system, a typical observing day will include three Fe I 630.2 nm full-disk photospheric vector magnetograms, one full- disk photospheric and three Ca II 854.2 nm chromospheric longitudinal magnetograms, along with three He I 1083 nm …
High-Resolution Surface Observations and What They Can Tell MHD Simulations
I review selected state-of-the art observations of the solar photosphere at the highest possible spatial resolution with a special emphasis on properties that can directly be compared with current MHD simulations. Observations of the solar surface have made big advances during the last decade thanks to new telescopes and new data processing approaches. I will provide an overview of the latest observations and discuss parameters such as the rms contrast and other statistical parameters that can …
Solar Carbon Monoxide, Thermal Profiling, and the Abundances of C, O, and Their Isotopes
A solar photospheric
thermal profiling analysis is presented, exploiting the infrared (2.3-4.6 ensuremathμm) rovibrational bands of carbon monoxide (CO) as observed with the McMath-Pierce Fourier transform spectrometer (FTS) at Kitt Peak, and from above the Earth’s atmosphere by the Shuttle-borne ATMOS experiment. Visible continuum intensities and center-limb behavior constrained the temperature profile of the deep photosphere, while CO center-limb behavior defined the thermal structure at …
Development of an IFU for diffraction-limited 3D spectropolarimetry
Ground-based telescopes can achieve diffraction-limited images when equipped with adaptive optics (AO). A major limitation of AO is the small field of view, which is due to the limited isoplanatic patch size. Nevertheless, conventional long-slit spectrographs cannot sample the entire AO-corrected field of view in a single exposure. However, equipped with a modern, large detector array, the Integral Field Unit (IFU) technique will allow a 3-dimensional (3-D) data cube to be recorded …
Design of a polarimeter for extrasolar planetary systems characterization
Unpolarized light from the central star that is reflected by exoplanets, protoplanetary disks, and debris disks becomes partially polarized by the reflection process. Imaging polarimetry is therefore the ideal way to discriminate between the polarized light from circumstellar environments and the unpolarized light from the nearby central star. A sensitivity of 10 $^-5$ (fraction of polarized intensity to the total intensity) must be achieved to detect exoplanets; 10 $^-4$ is sufficient for …
Mapping sodium distribution in the exosphere of Mercury with tip-tilt image stabilization
Mapping the sodium emission about Mercury is a difficult observational problem, since Mercury is seen either against the bright daytime sky, or against a dark sky at very high air masses. The distribution of sodium emission over the surface of Mercury is non-uniform, and changes over time. These effects give clues to the processes that produce the sodium and control its distribution, so that improved mapping of sodium emissions over the Mercury surface will help clarify their relative …
C3Po --- The Ultimate Focal Plane Array for Imaging Polarimetry from 200 nm to 20 ensuremathμm
C3̂Po is a novel array detector concept that is optimized for highly sensitive and precise differential imaging. Chopping between two or more independent image states (such as four linearly independent polarization states) can be performed at speeds in the kHz domain to provide virtually simultaneous images without the need to read out the array at kHz frame rates. This allows the technology to be applied to large arrays with precise, slow readouts. All independent image planes are observed with …
High precision polarimetry with the Advanced Technology Solar Telescope
This paper addresses the issue of calibrating the Advanced Technology Solar Telescope for high-precision polarimetry, in particular of the optical train above the Gregorian station (where suitable calibration optics will be placed). Conventional techniques would not be adequate for this telescope given its large aperture. Here we explore two different methods that are currently being considered by the design team. The first one is the ``sub-aperture’’ method, which uses small …
The Solar Oxygen Abundance, and the Rare Isotopes of C and O, Derived from Infrared Spectra of Carbon Monoxide
A detailed abundance analysis is presented for solar oxygen based on the ensuremathΔV=1 fundamental (4.6åisebox-0.5ex~ensuremathμm) and ensuremathΔV=2 (2.3i̊sebox-0.5ex~ensuremathμm) first- overtone rovibrational bands of carbon monoxide observed above the Earth’s atmosphere at very high spectral resolution and high signal-to-noise by the Shuttle-borne ATMOS Fourier transform spectrometer (FTS). Additional observations to define the reference photospheric thermal structure were taken of …
SOLIS-VSM Synoptic Products
Since August 2003, the Vector Spectromagnetograph (VSM) instrument has recorded full-disk photospheric and chromospheric magnetograms along with He I 1083 nm spectroheliograms daily as part of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project. In addition, the VSM measured full-disk photospheric vector magnetograms weekly. After the full deployment of the VSM data processing system, a typical observing day will include three full-disk photospheric vector magnetograms. …
Reduction of SOLIS/Vector Spectromagnetograph He I 1083 nm Observations
The SOLIS/Vector Spectromagnetograph (VSM) now continues the three- decade record of full-disk He I 1083 nm spectroheliograms from NSO/Kitt Peak (see poster by Henney et al.). Although the sensitivity of the VSM interim cameras at 1083 nm is far greater than that of previous NSO/KP instruments, the observations are compromised by strong interference fringes produced within the detectors. We discuss in some detail the extent to which this difficulty is overcome in the simple algorithm used to …
First-Light Instrumentation for the Advanced Technology Solar Telescope
The 4m Advanced Technology Solar Telescope (ATST) is the next generation ground based solar telescope. In this paper we provide an overview of the ATST post-focus instrumentation. The majority of ATST instrumentation is located in an instrument Coude lab facility, where a rotating platform provides image de-rotation. A high order adaptive optics system delivers a corrected beam to the Coude lab facility. Alternatively, instruments can be mounted at the Nasmyth focus. For example, instruments for …
Are You Being Served? - Solar Data Management at the National Solar Observatory
Solar physics is a data driven field, where no progress can be made without multi-wavelength and multi-cadence information about the sun’s behavior. The National Solar Observatory (NSO) has provided data to the research community for over 30 years and thus has considerable experience in the areas of data processing, archiving, and distribution. This presentation will describe the development of NSO’s data services, outline what lessons have been learned along the way, and attempt to …
Helicity and the SOLIS Vector-Spectromagnetograph
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments that greatly improve ground- based synoptic solar observations. The main instrument the Vector Spectro-Magnetograph (VSM) is a compact high-throughput vector-polarimeter that measures the magnetic field strength and direction over the full solar disk within 15 minutes. Helicity can be directly calculated from the full-disk vector field data provided by the Stokes inversion of the observed …
An integrated multiangle, multispectral, and polarimetric imaging concept for aerosol remote sensing from space
Techniques for passive remote sensing of aerosol optical and microphysical properties from space include visible, near- and shortwave-infrared imaging (e.g., from MODIS), multiangle intensity imaging (e.g., ATSR-2, AATSR, MISR), near-ultraviolet mapping (e.g., TOMS/OMI), and polarimetry (e.g., POLDER, APS). Each of these methods has unique strengths. In this paper, we present a concept for integrating these approaches into a unified sensor. Design goals include spectral coverage from the near-UV …
Adaptive Optics Observations of the Sun
Modern solar observations are performed close to the diffraction limit of the existing solar telescopes since important, fundamental processes on the Sun occur on scales of a tenth of an arc-second and smaller. Spatial resolution is therefore paramount in solar observations. Even though solar telescopes have relatively small apertures as compared to modern night-time telescopes, their spatial resolution is still limited by seeing, which is worse during the day as compared to at night. Adaptive …
Instrumentation for the Advanced Technology Solar Telescope
The 4-m aperture Advanced Technology Solar Telescope (ATST) is the next generation ground based solar telescope. In this paper we provide an overview of the ATST post-focus instrumentation. The majority of ATST instrumentation is located in an instrument Coude lab facility, where a rotating platform provides image de- rotation. A high order adaptive optics system delivers a corrected beam to the Coude lab facility. Alternatively, instruments can be mounted at Nasmyth or a small Gregorian area. …
Evaluation of Seeing-Induced Cross Talk in Tip-Tilt-Corrected Solar Polarimetry
We reanalyze the effects of atmosphere-induced image motions on the measurement of solar polarized light using a formalism developed by Lites. Our reanalysis is prompted by the advent of adaptive optics (AO) systems that reduce image motion and higher-order aberrations, by the availability of liquid crystals as modulation devices, and by the need to understand how best to design polarimeters for future telescopes such as the Advanced Technology Solar Telescope. In this first attempt to …
SOLIS-VSM Longitudinal Magnetogram Calibration
The VSM (Vector SpectroMagnetograph) instrument started recording daily full-disk magnetograms during August, 2003 at a temporary site in Tucson, Arizona. Along with the other instruments that constitute the SOLIS (Synoptic Optical Long-term Investigations of the Sun) project, the VSM will be relocated to Kitt Peak in April, 2004. The VSM instrument provides a unique record of solar full-disk vector magnetograms along with the high sensitivity photospheric and chromospheric longitudinal …
On the Origin of Solar Faculae
Solar faculae appear as bright small features close to the solar limb. Recent high-resolution images show these brightenings in unprecedented detail. Our analysis of numerical MHD simulations reproduces the observed small-scale features. The simulations reveal that faculae originate from a thin layer within granules just below largely transparent magnetic flux concentrations. This is basically the ``bright wall’’ model of Spruit. The dark, narrow lanes often associated with faculae …
Measurement Scale of the SOLIS Vector Spectromagnetograph
Longitudinal magnetograms obtained with the SOLIS Vector Spectromagnetograph (VSM) during a cross-calibration period are compared with similar data from the NASA/NSO Spectromagnetograph (SPM) at the NSO/Kitt Peak Vacuum Telescope as well as with SOHO/MDI and GONG magnetograms. The VSM began observations at the University of Arizona agricultural test site and collaborative observations were obtained with both the VSM and SPM from 2003 Aug 05 through 2003 Sep 21 when the SPM was officially …
Innovative Information Technology for Space Weather Research
Solar activity is closely related to the near earth environment – summarized descriptively as space weather. Changes in space weather have adverse effect on many aspects of life and systems on earth and in space. Real-time, high-quality data and data processing would be a key element to forecast space weather promptly and accurately. Recently, we obtained a funding from US National Science Foundation to apply innovative information technology for space weather prediction. (1) We use the …
Improved Spatial Resolution of the Sodium Distribution on Mercury
The distribution of sodium emission over the surface of Mercury is non- uniform, and changes over time. These non-uniformities and time- dependent changes give clues to the processes that produce the sodium. However, observations of the distribution of sodium on Mercury are hampered by the bad seeing that accompanies the need to observe either in daytime, or at extremely large air masses in twilight. In an effort to improve this situation, we have developed an image stabilizer utilizing a …
High-accuracy Multiangle Spectropolarimetric Imaging Concept for Aerosol Remote Sensing from Space
Satellite remote sensing has a key role in measuring the distribution, radiative impact, and regional and global spatial context of tropospheric aerosols. A synergistic combination of multispectral, multiangle, and polarimetric approaches would improve the accuracies of aerosol optical depth and particle property characterizations compared to what is achievable using each method by itself. In this paper we discuss the science benefits and technical feasibility of combining key attributes of …
Evershed Flow of CO at Different Depths in a Sunspot Penumbra.
CO lines of different strengths near 4.7 microns have been used to observe Evershed flow of molecular gas in the penumbra of a large sunspot near to the solar limb. An adaptive optics system and IR camera on the main spectrograph of the McMath-Pierce Solar Telescope produced a sequence of diffraction-limited spectral-spatial images to 0.8 arc-second resolution as the telescope was scanned across this sunspot. Dopplergrams constructed from this sequence for several of these CO lines have been …
Early Results from SOLIS
SOLIS (Synoptic Optical Long-Term Investigations of the Sun) is a project that is replacing antiquated synoptic observing equipment at the National Solar Observatory. SOLIS consists of a suite of three instruments on an equatorial mount that will be installed on Kitt Peak in April 2004. The major SOLIS instrument is a vector spectromagnetograph (VSM) that maps magnetic fields across the full solar disk using a slit spectrograph and one arc sec pixels. Limited daily observations started at a …
High-speed cameras and solar observing
The National Solar Observatory operates two facilities with demanding needs for rapid image collection (i.e. > television frame rates). The first is GONG, a global network of six identical small telescopes devoted to nearly continuous observations of the sun’s surface vibrations in order to study its internal properties by helioseismology. The second, SOLIS, is a suite of three instruments that collects images and spectra of the sun needed to study the behavior of solar activity on time …
Charge caching CMOS detector for polarimetry (C$^3$Po)
C$^3$Po is a concept for a novel array detector concept that is optimized for highly sensitive and precise differential imaging such as needed for astrophysical polarimetry. Chopping between two or more independent image states (such as four linearly independent polarization states) can be performed at speeds in the kHz domain to provide virtually simultaneous images without the need to read out the array at kHz frame rates. This allows the technology to be applied to large arrays with precise, …
Advanced Technology Solar Telescope: a progress report
The 4m ATST will be the most powerful solar telescope in the world, providing a unique scientific tool to study the Sun and other astronomical objects. The design and development phase for the Advance Technology Solar Telescope (ATST) is progressing. The conceptual design review (CoDR) for the ATST is scheduled for August 2003. We present a brief description of the science requirements of ATST, and remind the reader of some of the technical challenges of building a 4-m solar telescope. We will …
Radio Observations of the Quiet Sun
While radio observations of the Sun have mostly focused on active region phenomena, they also contribute unique data to our knowledge of the quiet Sun, in particular through accurate measurements of the temperature as a function of height in the atmosphere and through the measurement of nonthermal emissions from chromospheric and coronal heating events. Here we review observations of the quiet Sun using radio telescopes and discuss current science problems that will be addressed with future …
Mapping sodium distribution on the surface of Mercury
The distribution of sodium emission over the surface of Mercury is non- uniform, and changes over time. These non-uniformities and time- dependent changes give clues to the processes that produce sodium. Photon-stimulated desorption may be the major sodium source process on Mercury, so the sodium density might be expected to maximize at the subsolar point. Another source process may be sputtering by solar particles that impact the surface at high latitudes in the polar cusps, producing sodium …
First Results from SOLIS
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a project to replace antiquated synoptic observing equipment at the National Solar Observatory. SOLIS includes a suite of three instruments on an equatorial mount originally installed at a site in Tucson and will be moved to Kitt Peak before the end of 2003. The major SOLIS instrument is a vector spectromagnetograph that maps magnetic fields across the full solar disk using a slit spectrograph and one arc sec pixels. Daily …
Linear Polarization Measurements of Chromospheric Emission Lines
We have used the Zurich Imaging Stokes Polarimeter (ZIMPOL I) with the McMath-Pierce 1.5 m main telescope on Kitt Peak to obtain linear polarization measurements of the off-limb chromosphere with a sensitivity better than 1×10$^-5$. We found that the off-disk observations require a combination of good seeing (to show the emission lines) and a clean heliostat (to avoid contamination by scattered light from the Sun’s disk). When these conditions were met, we obtained the following principal …
The SOLIS Vector-Spectromagnetograph (VSM)
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments that will greatly improve ground-based synoptic solar observations. The main instrument, the Vector Spectro-Magnetograph (VSM), is a compact, high- throughput vector-polarimeter with an active secondary mirror, an actively controlled grating spectrograph, and two high-speed cameras based on silicon-on-CMOS-multiplexer hybrid focal plane arrays. It measures the photospheric magnetic field …
Calibration and Data Analysis for the SOLIS-VSM
The Vector Spectromagnetograph (VSM) instrument of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project will provide a unique 25-year record of synoptic solar observations with daily full-disk photospheric vector and high sensitivity longitudinal magnetograms. In addition, the VSM will produce daily full-disk chromospheric longitudinal magnetograms along with various He I 1083 nm parameter images. The VSM is being prepared for installation at a temporary site in Tucson for a …
Advanced Technology Solar Telescope - Approach to a Four-meter Diffraction Limited Solar Telescope
The Advanced Technology Solar Telescope (ATST) is intended to be the next major step in ground based solar observatories. The ATST will provide a laboratory for ultra high resolution, polarimetric measurements of all layers of the solar atmosphere. Currently the project is preparing a conceptual design to fulfill this mission, including plans for the design, development, construction and operation of this facility. Given the nearly three-fold increase in aperture size over the largest existing …
Technical challenges of the Advanced Technology Solar Telescope
The 4m Advance Technology Solar Telescope (ATST) will be the most powerful solar telescope in the world, providing a unique scientific tool to study the Sun and possibly other astronomical objects, such as solar system planets. We briefly summarize the science drivers and observational requirements of ATST. The main focus of this paper is on the many technical challenges involved in designing a large aperture solar telescope. The ATST project has entered the design and development phase. …
SOLIS: an innovative suite of synoptic instruments
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments under construction that will greatly improve ground-based synoptic solar observations. The Vector Spectromagnetograph (VSM) is a compact, high-throughput vector-polarimeter with an active secondary mirror, an actively controlled grating spectrograph, and two high-speed cameras with silicon-on-CMOS-multiplexer hybrid focal plane arrays. It will measure the magnetic field strength and direction …
Solar polarimetry close to the diffraction limit
Much progress has been made during the last years in obtaining polarimetric observations of the Sun close to the diffraction limit. Here I summarize the problems one encounters when observing close to the diffraction limit of a large solar telescope, review techniques, present examples of recent observations, and discuss the need for further developments of instruments and image reconstruction techniques.
Site testing issues for the frequency agile solar radiotelescope (FASR)
The Frequency Agile Solar Radiotelescope (FASR) will be a broadband synthesis-imaging array with 3-km or larger baselines, operating over a broad frequency range of 0.1-30 GHz. The instrument demands a site with low levels of Radio Frequency Interference (RFI) over this entire band. The site also must be large enough to accommodate the expected size of the array configuration and ideally would provide room to grow with future upgrades. The site must have a benign environment in which at least …
PEPSI spectro-polarimeter for the LBT
PEPSI (Postham Echelle Polarimetric and Spectroscopic Instrument) is to use the unique feature of the LBT and its powerful double mirror configuration to provide high and extremely high spectral resolution full-Stokes four-vector spectra in the wavelength range 450-1100nm. For the given aperture of 8.4m in single mirror mode and 11.8m in double mirror mode, and at a spectral resolution of 40,000-300,000 as designed for the fiber-fed Echelle spectrograph, a polarimetric accuracy between 10$^-4$ …
Low-cost solar adaptive optics in the infrared
We have developed a low-cost adaptive optics system for solar observations in the infrared between 1 and 28 ensuremathμm with the 1.5-m McMath-Pierce solar telescope. The 37-actuator membrane mirror and a fast tip-tilt mirror are controlled by a PC running Linux RedHat 7.1 that analyzes images from a 256 by 256 pixel, 1 kHz frame rate CCD camera. The total hardware cost is less than $25,000, and the system provides diffraction-limited performance under median seeing conditions above 2.3 …
Design and development of the Advanced Technology Solar Telescope (ATST)
High-resolution studies of the Sun’s magnetic fields are needed for a better understanding of solar magnetic fields and the fundamental processes responsible for solar variability. The generation of magnetic fields through dynamo processes, the amplification of fields through the interaction with plasma flows, and the destruction of fields are still poorly understood. There is still incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes …
Solar observing facilities
An overview is given of current and planned ground-based solar telescopes and instruments, balloon-borne and suborbital solar telescopes, and solar and heliospheric space missions. These observing facilities operate in all areas of solar physics, from the solar interior to interplanetary space and from regimes of high energy to observations requiring high resolution. The next generation of solar telescopes and instruments promise us the ability to investigate solar processes on their fundamental …
Helicity and the SOLIS Vector-Spectromagnetograph
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of three innovative instruments that greatly improve ground- based synoptic solar observations. The main instrument the Vector Spectro-Magnetograph (VSM) is a compact high-throughput vector-polarimeter that measures the magnetic field strength and direction over the full solar disk within 15 minutes. Helicity can be directly calculated from the full-disk vector field data provided by the Stokes inversion of the observed …
Design and development of the Advanced Technology Solar Telescope
Led by the National Solar Observatory, plans have been made to design and to develop the Advanced Technology Solar Telescope (ATST). The ATST will be a 4-m general-purpose solar telescope equipped with adaptive optics and versatile post-focus instrumentation. Its main aim will be to achieve an angular resolution of 0.03 arcsec (20 km on the solar surface). The project and the telescope design are briefly described.
Data analysis for the SOLIS Vector Spectromagnetograph
The National Solar Observatory’s SOLIS Vector Spectromagnetograph, which will produce three or more full-disk maps of the Sun’s photospheric vector magnetic field every day for at least one solar magnetic cycle, is in the final stages of assembly. Initial observations, including cross-calibration with the current NASA/NSO spectromagnetograph (SPM), will soon be carried out at a test site in Tucson. This paper discusses data analysis techniques for reducing the raw data, calculation …
Spectroscopic Detection of the 3.934 Micron Line of Si IX in the Solar Corona
We report the detection of the 2s$^2$2p$^2$$^3$P$_1–>0$ line of Si IX using the McMath-Pierce telescope on Kitt Peak. Observations were made of the solar disk and at various heights above the limb between 2002 May 13 and 17, under nonideal sky conditions, using the 13.5 m vertical spectrograph and an InSb single-diode detector. We report a new rest wavelength for the line and discuss its potential use as a diagnostic of coronal magnetic fields using ground-based telescopes. Our …
The Advanced Technology Solar Telescope
The Advanced Technology Solar Telescope is the largest solar optical facility currently under development. The National Solar Observatory and its partners have just started the design and development phase with first light being planned for late this decade. The 4-m telescope will provide an angular resolution down to 0.025 arcsec, a large photon flux for precise magnetic and velocity field measurements, and access to a broad set of diagnostics from 0.3 to 28 mu m. We summarize the currently …
Spatial mapping of the Hanle and Zeeman effects on the Sun
Spatial mapping of the Hanle and Zeeman effects on the Sun has been done for the first time, through Stokes vector imaging with a narrow- band (0.2 ̊A) universal filter. It is shown how the polarization signatures of the Hanle and Zeeman effects can be cleanly distinguished from each other by comparing the Stokes images recorded at different, specially selected wavelengths within the Na I D$_2$-D$_1$ line system. Examples of the polarization signatures of sunspots, faculae, the supergranulation …
The Advanced Technology Solar Telescope
The 4m aperture Advance Technology Solar Telescope (ATST) will be the most powerful solar telescope in the world and a unique scientific tool to study the Sun and other astronomical objects, such as planets. The ATST will replace major existing national solar facilities at the end of this decade. The ATST project has entered the design and development phase. We present an overview of the ATST science drivers and discuss preliminary design concepts and technical challenges. The ATST science goals …
Stokes Inversion Techniques for the SOLIS-VSM
The Vector Spectromagnetograph (VSM) instrument of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) project will begin operation in 2002 and provide a 25 year record of synoptic solar observations. The 50-cm aperture VSM will provide daily full- disk photospheric vector and high sensitivity longitudinal magnetograms. In addition, the VSM will produce daily full-disk chormospheric longitudinal magnetograms, along with 1083 nm equivalent width images. Current data transmission and …
Preliminary tests of a low-cost solar infrared adaptive optics system
Images produced by the National Solar Observatory’s McMath-Pierce telescope on Kitt Peak, the largest solar telescope in the world, have been at the mercy of atmospheric turbulence for decades. Work is currently underway to install a low-cost adaptive optics system with the goal of correction in the infrared for a total hardware cost of $25k. As a preliminary step, a slow AO system was constructed in the lab to demonstrate the feasibility of the low-cost approach. The design is a simple …
Observational Signatures of a Solar Small-Scale Global Dynamo
There is ample theoretical and observational evidence for the existence of a dynamo operating in the solar convection zone that produces small-scale, weak magnetic fields. The next generation of solar telescopes such as the 4-m Advanced Technology Solar Telescope and the 1.5-m GREGOR will be able to provide observational data on these magnetic fields. In order to guide the development of instruments and observational procedures to investigate these small-scale magnetic fields, we have calculated …
Instrumentation for astrophysical spectropolarimetry
Astronomical spectropolarimetry is performed from the X-ray to the radio regimes of the electromagnetic spectrum. The following chapter deals with instruments and their components that are used in the wavelength range from 300 nm to 20 μm. After introducing the terminology and formalisms that are used in the context of astronomical spectropolarimeters, I discuss the most widely used optical components. These include crystal and sheet polarizers, fixed monochromatic and achromatic retarders, and …
The Advanced Technology Solar Telescope -- Science Goals and Instrument Description.
High-resolution studies of the Sun’s magnetic fields are needed for a better understanding of solar magnetic fields and the fundamental processes responsible for solar variability. The generation of magnetic fields through dynamo processes, the amplification of fields through the interaction with plasma flows, and the destruction of fields are still poorly understood. There is still incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes …
Polarization Modulation for Solar Vector-Polarimeters using Ferroelectric Liquid Crystals
Polarization modulation for solar vector polarimetry can be performed in various ways. Ferroelectric liquid crystals (FLC) have the advantage of allowing modulation frequencies of up to several kHz, therefore avoiding spurious signals due to seeing. A new modulation scheme using two FLC modulators was designed for the SOLIS Vector-Spectromagnetograph (VSM). Here we present experimental confirmation of this modulation scheme from a laboratory setup using a fast CCD camera running at 300 frames …
Influence of magnetic fields on the coherence effects in the Na I D$_1$ and D$_2$ lines
To clarify the physical nature of the enigmatic scattering polarization in the Na i D$_1$ and D$_2$ line cores we have explored their behavior with full Stokes vector polarimetry in regions with varying degree of magnetic activity near the solar limb. These observations represent the first time that ZIMPOL II, the second generation of our CCD based imaging polarimeter systems, has been used for a scientific program. With ZIMPOL II the four Stokes images can be demodulated and recorded with a …
The Advanced Technology Solar Telescope: Science Goals and Instrument Description
High-resolution studies of the Sun’s magnetic fields are needed for a better understanding of solar magnetic fields and the fundamental processes responsible for solar variability. For example, the generation of magnetic fields through dynamo processes is still poorly understood. There is still incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes variations in the radiative output of the Sun. Progress in answering these critical questions …
The Advanced Solar Telescope
The planned Advanced Technology Solar Telescope (ATST) will be a 4-m aperture general-purpose solar telescope with integrated adaptive optics and versatile post focus instrumentation. The ATST will achieve an angular resolution of 0.03 arcsec (20 km on the solar surface) in the visible, which is almost an order of magnitude better than what is achieved with current solar telescopes. This will make it possible to resolve the fundamental astrophysical hydrodynamic and magnetohydrodynamic processes …
Results from a revisit to the K$_2V$ bright points
We have used pairs of temporally simultaneous CaII K-line spectroheliograms and magnetic area scans to search for spatial correlation between the CaII K$_2V$ bright points in the interior of the network and corresponding magnetic elements. We find that about 60% of the K$_2V$ bright points spatially coincide with magnetic elements of flux density > 4 Mx cm$^-2$. About 25% of the K$_2V$ bright points with equally enhanced emission lie over bipole elements where the fields are > 4 Mx cm$^-2$ …
New Results from the Flare Genesis Experiment
From January 10 to 27, 2000, the Flare Genesis solar telescope observed the Sun while suspended from a balloon in the stratosphere above Antarctica. The goal of the mission was to acquire long time series of high-resolution images and vector magnetograms of the solar photosphere and chromosphere. Images were obtained in the magnetically sensitive Ca I line at 6122 Angstroms and at H-alpha (6563 Angstroms). The FGE data were obtained in the context of Max Millennium Observing Campaign #004, the …
Evolution of small-scale magnetic fields from combined adaptive optics and phase-diverse speckle imaging
We have obtained movies of the photospheric magnetic field at a sustained resolution of 0.2 arcsec by combining the adaptive optics system at the Dunn Solar Telescope with the Zurich Imaging Polarimeter I (ZIMPOL) and processing the data with Phase-Diverse Speckle Imaging and speckle deconvolution. The adaptive optics was correcting the low-order aberrations with an update rate of about 1.5 kHz and fed a narrow-band channel through the Universal Birefringent Filter in the wing of the CaI 610.3 …
Center-to-limb variation of the enigmatic NaI D_1 and D_2 polarization profiles
The remarkable polarization structure of the Na I D_1 and D_2 lines that is due to coherent scattering has remained an enigma, since it has not yet been possible to find an explanation that is consistent with both current understanding of quantum mechanics and the astrophysical properties of the Sun’s atmosphere. To guide future theoretical efforts we have here explored the detailed center-to-limb variation of the linearly polarized profiles in non-magnetic regions. In particular we find …
Anomalous polarization effects due to coherent scattering on the Sun
The richly structured linearly polarized spectrum that is produced by coherent scattering in the Sun’s atmosphere contains a number of spectral features for which no explanation has been found within the standard scattering theory. According to this quantum- mechanical framework, the intrinsic polarizability of a given line should be determined by the total angular momentum quantum numbers of the atomic levels involved in the scattering transition (which may be resonant or fluorescent). …
Deconvolution of narrowband solar images using aberrations estimated from phase-diverse imagery
Phase-Diverse Speckle (PDS) is a short-exposure data- collection and processing technique that blends phase- diversity and speckle- imaging concepts. PDS has been successfully used for solar astronomy to achieve near diffraction-limited resolution in ground-based imaging of solar granulation. Variants of PDS that involve narrow-band, spectroscopic, and polarimetric data provide more information observations. We present results from processing data collected with the 76-cm Richard B. Dunn Solar …
Infrared lines as probes of solar magnetic features. XV. Evershed flow in cool, weak penumbral fields
Observations of Ti I lines at 2.2 micrometer show that the Evershed flow takes place in cool, almost horizontal channels with a low magnetic field strength (~500-900 G) that does not appear to change significantly across the penumbra. This property might allow an outward directed siphon flow to exist along such cool flux tubes.
Infrared lines as probes of solar magnetic features. XIV. TI i and the cool components of sunspots
The first systematic observations of sunspot umbrae using the lines of the Ti Ii multiplet at 2.2 mu m are presented. Their diagnostic capabilities are investigated, developed and used to investigate the magnetic and velocity structure of a sunspot. These lines are most sensitive to cool plasma. In addition, they are extremely Zeeman sensitive. We find that a sunspot is composed of two distinct cool magnetic components. One of them is fairly vertical, has a large magnetic field strength and is …
SOLIS: a modern facility for synoptic solar observations
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of instruments that will modernize and greatly improve synoptic solar observations carried out by the National Solar Observatory. It will provide fundamental data necessary to understand the solar activity cycle, sudden energy releases in the solar atmosphere, and solar spectral irradiance changes. State-of-the-art instrumentation and data collection techniques will be employed to enhance both the quality and quantity of …
The Procyon campaign: Observations from Kitt Peak
Time series spectra of the F5IV star Procyon (alpha CMi) were obtained at the Kitt Peak National Observatory during a 35-night observing run in January-February 1997. The observations were obtained as part of an international collaboration to detect and study acoustic oscillations in solar-type stars. Spectra covered the wavelength range from 4000 to 5300 AA , with a resolving power of approximately 3500 (1.3 AA resolution). The sampling rate was one observation per minute, and the typical S/N …
SOLIS - A Modern Facility for Synoptic Solar Observations
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a proposed suite of instruments that will modernize and greatly improve synoptic solar observations carried out by the National Solar Observatory. The primary scientific goal is to provide fundamental data necessary to understand the solar activity cycle, sudden energy releases in the solar atmosphere, and solar spectral irradiance changes. An operational goal is to produce real-time and near real-time data for forecasting space …
Differential Hanle effect and the spatial variation of turbulent magnetic fields on the Sun
While diagnostic techniques based on the ordinary Zeeman effect (e.g. magnetograms) are almost ``blind’’ to a turbulent magnetic field with mixed magnetic polarities within the spatial resolution element, the Hanle effect is sensitive to this domain of solar magnetism. We present observational evidence that the turbulent magnetic field that fills the 99ts% of the volume between the kG flux tubes in quiet solar regions does not have a unique field-strength distribution, but the rms …
Complex magnetic fields in an active region
High-resolution observations of the full Stokes vector in FeI spectral lines around 5250 Angstroms obtained at the Swedish Vacuum Solar Telescope on La Palma with the ZIMPOL I Stokes polarimeter in a complex active region reveal the presence of anomalously shaped Stokes profiles indicating the coexistence of at least two magnetic components within the same spatial resolution element. These Stokes profiles have been analyzed with an inversion code based on a 3-component atmospheric model with two …
Asteroseismology from equivalent widths: a test of the sun
Kjeldsen et al. (1995) reported a probable detection of solar-like, low- amplitude, p-mode oscillations of eta Bootes using equivalent width measurements from low-resolution spectra of the hydrogen Balmer lines. We tested the usefulness of this technique using observations of the Sun in integrated light. Despite the very high signal-to-noise ratio of the data stretching over six continuous days, no solar oscillation signal was found so far in the equivalent width of Hensuremathβ. Spatially …
Center-to-limb variation of the second solar spectrum.
The linear polarization that is caused by scattering processes in the solar atmosphere has been refered to as the ``second solar spectrum’’, since it is structurally as rich as the ordinary intensity spectrum but quite different in appearance and information contents. One of the most used and theoretically best understood lines in the second solar spectrum is the SrI 4607̊A line, which has served as a diagnostic tool for determinations of spatially unresolved, turbulent magnetic …
The second solar spectrum. A new window for diagnostics of the Sun.
The Sun’s radiation becomes linearly polarized by coherent scattering processes in the solar atmosphere. With a novel polarimetry system that achieves a precision of 1e-5 in the degree of polarization, the previously largely unexplored territory of scattering physics on the Sun is now fully accessible. The observations reveal a polarized spectrum that looks very different as compared with the ordinary, unpolarized solar spectrum but has an astounding wealth of spectral structures. It is …
ISOON: The Improved Solar Observing Optical Network
Efforts are under way to replace the existing SOON system, which was designed in the 1970s, with a new system (ISOON) based on a fully tunable narrow-band filter and CCD detector. ISOON would feature autonomous, rapid-cadence solar imaging and remote operation at four sites, and would transmit solar images in near real time to central facilities at Falcon AFB and Boulder CO, for use in space weather forecasting. The ISOON technical approach is to retain the front end of the existing SOON …
Chromospheric Events in the Quiet Network
Time sequences of a quiet network region close to disk center have been simultaneously recorded with the VLA, various instruments on SOHO, and the solar telescopes on Kitt Peak. The analysis of the Hensuremathα spectra obtained at the McMath-Pierce telescope revealed down-flows with apparent velocities of more than 2.5 km/s associated with magnetic field structures in the quiet network. During such events, the Hensuremathα spectra show a pronounced asymmetry. The photospheric magnetic field was …
Time series restoration from ground-based solar observations
Many processes of interest in the solar atmosphere have spatial scales of much less than one second of arc. If the processes are related to magnetic fields, the relevant scales are even smaller. Noticeable evolutions of solar features occur on time- scales of less than a minute if a spatial resolution of better than one second of arc is reached. It is, therefore, of great interest to recover time-series imagery with near diffraction- limited spatial resolution and good temporal resolution on a …
Observations of the quiet Sun's magnetic field.
The profiles of Stokes I and V of FeI 5247A, CrI 5247A, FeI 5250.2A and Fe I 5250.6A have been measured in the quiet photosphere with ZIMPOL, a new polarimeter, with very low noise level. The spatial resolution element of about 1Mm was essentially determined by seeing. In about 15% of the observed area, the signal-to-noise ratio was sufficiently large to determine the strength of the magnetic field and to derive various properties of the Stokes V profiles (asymmetries, zero-crossing shift and …
Flare Genesis Experiment
In January 1996, the Flare Genesis Experiment was carried for 19 days by a 29.4 M cu. ft helium-filled balloon in the stratosphere above Antarctica, during which over 14000 images of the Sun were recorded. Long-duration ballooning provides a relatively inexpensive means to observe the Sun under near-space conditions and to develop instrumentation and techniques that will be used on future solar space missions. The purpose of the flight was to improve understanding of the mechanisms involved in …
Concept for a miniature solar magnetograph
We present a novel concept for a solar magnetograph that uses a photo- refractive crystal to reflect and focus the light from the wings of many spectral lines onto a camera. The crystal acts simultaneously as multiple, narrow-band filters and as an off- axis telescope. Polarization measurements are performed close to the final focus. Since this approach uses the light from many spectral lines simultaneously, the required telescope aperture is substantially reduced and exposure times can be so …
Evaluation of Phase-Diversity Techniques for Solar-Image Restoration
Phase-diversity techniques provide a novel observational method for overcoming the effects of turbulence and instrument-induced aberrations in ground-based astronomy. Two implementations of phase-diversity techniques that differ with regard to noise model, estimator, optimization algorithm, method of regularization, and treatment of edge effects are described. Reconstructions of solar granulation derived by applying these two implementations to common data sets are shown to yield nearly …
Imaging Coronal Emission Lines under High Sky-Background Conditions
We have obtained images in solar coronal emission lines under high sky- background conditions by making precise differential measurements between the coronal emission line and the near-by continuum, which is primarily due to scattered light from the solar disk. Chopping between the two wavelengths was performed at 100 kHz to avoid artifacts from fast-flying dust particles and other aerosols, and also from seeing effects. The differential signal was detected with a novel CCD camera that …
The Second Solar Spectrum
The Sun’s radiation becomes linearly polarized by coherent scattering processes in the solar atmosphere. With a novel imaging polarimeter (Zurich Imaging Stokes Polarimeter I) that achieves a precision of 10(-5) in the degree of polarization, the previously largely unexplored territory of scattering physics on the Sun is now fully accessible. The observations reveal a polarized spectrum that looks very different from the ordinary, unpolarized solar spectrum but has an astounding wealth of …
The Flare Genesis Experiment
The goal of the Flare Genesis Experiment is to make solar observations at the highest practicable resolution in order to improve understanding of the mechanisms involved in many different types of solar activity, particularly flares and solar filament eruptions. Achieving this goal demanded the development of a balloon-borne platform for an 80-cm F/1.5 optical telescope that could maintain 10 arcsec pointing stability. The first flight of the Flare Genesis Experiment took place in January 1996. …
SOLIS - A Modern Facility for Synoptic Solar Observations
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a proposed suite of instruments that will modernize and greatly improve synoptic solar observations carried out by the National Solar Observatory on behalf the solar and solar-terrestrial physics communities. The primary scientific goal is to provide fundamental data necessary to understand the solar activity cycle, sudden energy releases in the solar atmosphere, and solar spectral irradiance changes. An operational goal is to …
NIM-2 -- A Near Infrared Imaging Vector Magnetograph
NIM-1 is a spectrograph-based Stokes polarimeter for measuring the strength and orientation of magnetic fields in the solar photosphere using two Zeeman-sensitive Fe I lines (g = 3 and g_geff = 1.53) near 1565 nm. NIM-2, now under construction, also uses these spectral lines but is based on a high-resolution Fabry-Perot etalon. NIM-2 will eliminate the image scanning and consequent spatial distortions of NIM-1 and will be compact and light enough to serve as a prototype for balloon or space …
Asteroseismology via Equivalent Widths -- Tests on Procyon, Eta Bootes, and Alpha Triangulum
Recently, Kjeldsen et al. reported a probable detection of solar-like low-amplitude p-mode oscillations of Eta Bootes using equivalent width measurements from low-resolution spectra of the H Balmer lines. This technique has the potential to provide stellar oscillation measurements good enough to allow the asteroseismic inference of stellar structure. Here we report on the preliminary analysis of data from three observing runs with the Kitt Peak Coude Feed and 2.1-m telescope in November 1995 …
Recent Progress in Imaging Polarimetry
Recent instrumental developments in imaging polarimetry allow array detectors to reach a polarimetric sensitivity of 1 × 10$^-4$ of the intensity. New instrumental effects appear at these levels of sensitivity and generate spurious polarization signals with amplitudes of up to 5 × 10$^-4$. Here I discuss these effects and present methods to avoid them. Polarized spectra with an rms noise of 6 × 10$^-6$ may then be obtained. Furthermore a method is brought to the reader’s attention that …
Measurement of the full Stokes vector of He I 10830 ̊A
First observations of the full Stokes vector in the upper chromosphere are presented. The He I 10830 ̊A line, which has been shown to give reliable measurements of the line-of-sight component of the magnetic field vector, has been used for this purpose. It is shown that the difference between the appearance of chromospheric and photospheric magnetic structures observed close to the solar limb is largely due to the difference in height to which they refer and projection effects. The observations …
Balloon-Borne Polarimetry
For about two weeks in 1995, the balloon-borne Flare Genesis Experiment will continuously observe the Sun well above the turbulent, image-blurring layers of the Earth’s atmosphere. The polarization-free 80 cm telescope will supply images to a liquid-crystal based vector magnetograph, which will measure magnetic features at a resolution of 0.2 arcsec. An electrically tunable lithium-niobate Fabry-Perot provides a spectral resolution of about 0.015 nm. In a follow-up series of Antarctic …
Direct measurements of flux tube inclinations in solar plages.
Observations of the full Stokes vector in three spectral lines indicate that flux tubes in solar plages have an average inclination in the photosphere of 14deg with respect to the local vertical. Most flux tubes are inclined in the eastwards direction, i.e., opposite to the solar rotation. We have recorded the Stokes vector of the FeI 5247.1A, FeI 5250.2A, and FeI 5250.7A lines in nine different plages with the polarization-free 20cm Zeiss coronagraph at the Arosa Astrophysical Observatory of …
Persistent spectral hole burning used for spectrally high-resolved imaging of the sun
Persistent spectral hole burning (PSHB) makes it possible to store images of the sun spectrally and spatially in a single exposure step at very high resolution. The current system consists of a chlorin-doped polymer film (polyvinylbutyral), cooled to 2 K. It has a spectral resolution of 300 MHz (0.0004 nm) and may be used in the range of about 628 to 638 nm. Theoretically the spatial resolution is confined to molecular dimensions. In solar observations, however, it is determined by the optical …
Handling of huge multispectral image data volumes from a spectral hole burning device (SHBD)
We use chlorin-doped polymer films at low temperatures as the primary imaging detector. Based on the principles of persistent spectral hole burning, this system is capable of storing spatial and spectral information simultaneously in one exposure with extremely high resolution. The sun as an extended light source has been imaged onto the film. The information recorded amounts to tens of GBytes. This data volume is read out by scanning the frequency of a tunable dye laser and reading the images …
Speckle spectrography of extended objects.
We present a method to obtain diffraction limited spectrograms of extended sources from a series of ground-based slit- spectrograms. The method is a combination of speckle deconvolution (Keller & von der Luehe 1992) and a rapid spectrograph scanning scheme (Johannesson et al. 1992). The slit of a spectrograph is scanned over the solar surface while simultaneous slit-jaw images and spectrograms are recorded. The exposure time is short with respect to seeing-induced variations. A Knox-Thompson …
The spectral hole-burning device: a 3-dimensional photon detector.
We present the principles of a new device that combines a high-resolution spectrometer (0.001nm spectral resolution, 10nm bandwidth) with maximum efficiency, a large detector (pixel size 1μm by 1μm, overall size 2cm by 2cm), and a very high density data storage device (up to 400TB) in a single optical device. This device is based on persistent spectral hole-burning in a dye-doped polymer at temperatures of a few K. It is called a Spectral Hole-Burning Device (SHBD) in analogy to Charge Coupled …
Visible and near-infrared polarimetry with LEST.
This document describes the LEST vector polarimeters for the visible and the near-infrared part of the solar spectrum. After some general remarks on precise polarimetry with large telescopes, the authors present the specifications based on scientific reasons and some general design considerations. The proposed instrument design for the visible is based on the ZIMPOL II concept. They present two different concepts for vector polarimetry in the near infrared. One is based on a beam-splitter system …
Zurich Imaging Stokes polarimeters I and II
We present the design of the Zurich Imaging Stokes Polarimeters (ZIMPOL) I and II along with the first measurements and scientific observations. ZIMPOL I which was developed for precision solar vector polarimetry, uses two piezoelastic modulators and CCD arrays that have every other row covered with an opaque mask. During exposure the charges are shifted back and forth between covered and light-sensitive rows in synchrony with the modulation. In this way Stokes I and one of Q, U, or V can be …
First light for an astronomical 3-D photon detector.
Spectroscopic studies of extended astronomical sources at optical wavelengths are hampered by the two-dimensional nature of current photon detectors. Most photons of interest are not utilized because a spectrometer must scan in the spatial or spectral domain. We present the first solar spectrum recorded with a three-dimensional, highly wavelength-sensitive photon detector based on a dye-doped polymer film at 1.6K. The achieved spectral resolution is comparable to the best solar spectra obtained …
Diffraction-limited slit spectrography
We present a method to obtain diffraction limited spectrograms of two- dimensional extended objects from a series of ground-based slit- spectrograms. The method is a combination of differential speckle interferometry and a rapid spectrograph scanning scheme. The slit of a spectrograph is scanned over the solar surface while simultaneous images of the reflective slit plate (slit-jaw images) and spectrograms are recorded with an exposure time that is short with respect to seeing-induced …
On the strength of solar intra-network fields.
The combination of the German Vacuum Tower Telescope and the prototype of ZIMPOL I (Zuerich Imaging Stokes Polarimeter I), a novel, very sensitive imaging polarimeter, has resulted in the first spectra of solar intra-network (IN) fields in circular polarization. The sensitivity in terms of flux density is 0.7 Mx/cm^2. While magnetic fields in solar spots, pores, plages, and in the network have predominantly kG field strengths, the magnetic field strength of the IN flux is a controversial subject …
Application of multiframe iterative blind deconvolution for diverse astronomical imaging
We present applications of a recently developed iterative blind deconvolution algorithm to both simulated and real data. The applications demonstrate the algorithm’s performance for a wide range of astronomical imaging. We demonstrate the effectiveness of using multiple observations of the same object convolved with different point spread functions. We also show the extension of the algorithm to phase retrieval when the object Fourier amplitude is available.
Resolution of magnetic flux tubes on the Sun
Magnetic flux at the surface of the Sun is predominantly concentrated in discrete areas with kilogauss field strengths. Except for sunspots, these areas are too small to have been resolved by conventional observations. These magnetic flux tubes are an essential part of the physics of the activity and heating of the outer atmosphere of the Sun and other late-type stars, but although their average properties have been studied in considerable detail, direct observations of them have been lacking …
Solar speckle polarimetry
The combination of a polarimeter with real-time frame selection and differential speckle imaging results in diffraction-limited magnetograms that provide new insight into the morphology of solar small-scale magnetic fields. The method to record diffraction limited narrow-band filtergrams of solar features is based on two cameras taking simultaneous short exposure images through a broad-band and a narrow-band filter, respectively. Speckle imaging reconstructs the image in the broad-band channel. …
High Spatial Resolution Magnetograms of Solar Active Regions
Using the Universal Birefringent Filter at the Sacramento Peak Vacuum Tower Telescope we have obtained simultaneous observations of left and right circular polarization in various solar magnetic features with a resulting spatial resolution of 0’'.7 in the magnetograms. We describe the data reduction in some detail and discuss the various instrumental effects. In particular we show that seeing can create features in magnetograms. A penumbra near disk center shows small-scale features in the …
Zürich Imaging Stokes Polarimeter - ZIMPOL I. Design review.
This document describes the design of ZIMPOL I, the first Zürich Imaging Stokes Polarimeter. This solar vector polarimeter will mainly be used for observations of the solar magnetic field at high spatial and/or spectral resolution. A brief overview of ZIMPOL I is given in the preface. The scientific requirements are then specified in detail. They lead to an instrument concept which consists of several parts: the optical system, the camera system, the real-time image processing system, and the …
Demodulation of all four Stokes parameters with a single CCD - ZIMPOL II. Conceptual design.
It is shown how it is possible to simultaneously record images of all four Stokes parameters with a single CCD detector chip when fast (50 kHz), piezoelastic modulation of the polarization state is used. As the four image planes use the identical pixels of the CCD, all gain-table or flat-field effects vanish when forming the fractional polarization images. For each group of four pixel rows, one row collects the photons, while the other three are used for fast buffer storage. There are no light …
Multicolor Continuum Analysis of the Solar Granulation in Quiet and Active Regions
A set of narrow-band filtergrams of the solar photosphere recorded in three widely separated true continuum windows and in Mg I b1 with a spatial resolution of about 0.5 arcsec is analyzed. The influence of small-scale magnetic fields on the granulation is studied by comparing various statistical parameters in selected active and quiet regions, and the temperature stratification of faculae at the level of continuum formation is investigated. In active regions there exist more points with …
Two-dimensional polarimeter with CCD image sensors and piezo-elastic modulators.
A new type of polarimeter for application in solar physics is described, which combines fast polarization modulation and high spatial resolution using piezoelastic modulators and charge coupled device (CCD) image sensors. The problem of incompatibility between the slow read-out of CCD sensors and fast modulation has been solved. First two-dimensional Stokes Q and V images, free from gain-table noise, and with an rms noise level of about 10$^-3$ have been obtained.
Inversion of Stokes V profiles: The structure of solar magnetic fluxtubes and its dependence on the filling factor.
The authors present results from an inversion procedure that derive the temperature stratification, the turbulent velocity, and the magnetic field strength of the photospheric layers of magnetic fluxtubes from observed Stokes V spectra near disk center. In a first step the inversion has been applied to 10 Fe I and Fe II Stokes V profiles of a plage and a network region to obtain reliable models of the fluxtubes. In a second step the dependence of the fluxtube structure on the filling factor has …
High spatial resolution polarimetry using filtergrams.
Using tunable filters at the Sacramento Peak Vacuum Tower Telescope Stokes V images of various solar magnetic features have been observed. Simultaneous recording of left and right circular polarization leads to high spatial resolution magnetograms. Various instrumental effects of these observations are discussed in detail.
Dependence of the properties of solar magnetic flux tubes on filling factor. II - Results of an inversion approach
The dependence of the properties of solar magnetic elements on the magnetic filling factor is studied using Stokes V spectra of three lines observed near the center of the solar disk. The inversion technique developed by Keller et al. (1990) is applied to three neighboring spectral lines, and the average temperature difference, the magnetic field strength, and the nonstationary velocity in the relevant line-forming layers are quantitatively determined. Quantitative evidence is provided for the …
Solar magnetic field strength determinations from high spatial resolution filtergrams
Circularly polarized images with high spatial resolution (better than 1 arcsec) of a solar active region, obtained with a tunable filter in the wings of Fe I 5247.1 A and Fe I 5250.2 A, have been analyzed in terms of the magnetic line ratio technique introduced by Stenflo (1973). Whenever a measurable amount of polarization is present, the distribution of the observed magnetic-line ratio is compatible with a unique value, which is randomly blurred by noise due to the photon statistics, the CCD …
Structure of solar magnetic fluxtubes from the inversion of Stokes spectra at disk center
The paper presents an inversion procedure that derives the temperature stratification, the turbulent velocity, and the magnetic field strength of the photospheric layers of small-scale magnetic fields from observed Stokes V spectra and the continuum intensity. The inversion is based on the determination of a small number of model flux parameters by a nonlinear least squares fitting algorithm. The minimization of the sum of the squared differences between observed and synthetic observables makes …