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 consistent basis and over extended periods of time using ground- based techniques. Phase diversity is a post-collection technique for restoring fine-resolution detail when imaging in the presence of phase aberrations such as atmospheric turbulence. Incident energy is split into two channels: one is collected at the conventional focal plane, the other is intentionally defocussed a known amount and collected by a second detector array. Phase-diverse speckle is an extension of phase diversity whereby a time sequence of short-exposure image pairs is collected. The maximum-likelihood estimate of a common object and a set of phase aberrations is performed jointly using all images. A phase-diverse speckle set of images of a plage region was collected over a span of 13.5 minutes using the 76-cm Vacuum Tower Telescope at the National Solar Observatory on Sacramento Peak. A phase- diverse pair of broad-band images at 6563 angstroms was collected along a third, narrow-band image in the wing of H- (alpha) . A set of restorations was made into a movie depicting the highly dynamic photosphere at scales below 0.3 arcsec. We conclude that the combination of fine spatial and temporal resolution achieved with phase-diverse speckle opens a new window to the study of the dynamics of the solar atmosphere from ground-based observatories.