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 is dedicated to detect the very faint reflected and hence polarized visible light from extrasolar planets. The search for reflected light from extra-solar planets is very demanding because the signal decreases rapidly with the orbital separation. For a Jupiter-sized object and a separation of 1 AU the planet/star contrast to be achieved is on the order of 10-8 for a successful detection. This is much more demanding than the direct imaging of young self-luminous 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 2012. ZIMPOL is currently in the subsystem testing phase. We describe the results of verification and performance testing done at the NOVA-ASTRON lab. We will give an overview of the system noise performance, the polarimetric accuracy and the high contrast testing. For the high contrast testing we will describe the impact of crucial system parameters on the contrast performance. SPHERE is an instrument designed and built by a consortium consisting of IPAG, MPIA, LAM, LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA, ONERA and ASTRON in collaboration with ESO.