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 designs for solar studies. A partially transmitting coronagraph mask placed in the first focus reduces the light of the star. The beam is modulated using three ferro-electric liquid crystals in a Pancharatnam configuration, then split in a polarizing beamsplitter. Both beams are re-imaged onto the same Electron- Multiplying CCD camera. We present the design of the ExPo instrument, highlighting the elements that are critical to the polarimetric performance. Some prototype laboratory experiments demonstrating the instrument concept are discussed. These have been performed using our realistic exoplanet laboratory simulator.