Polarization compensating protective coatings for TPF-Coronagraph optics to control contrast degrading cross polarization leakage

摘要

The Terrestrial Planet Finder Coronagraph (TPF-C) for observing and characterizing exo-solar planets requiring star light suppression to 10~(-10) level demands optical aberrations and instrument stability to sub-nm levels. Additionally, wavefront polarization has to be tightly controlled over the 8m x 3.5m primary mirror aperture and 500nm — 500nm minimum bandwidth because the Deformable Mirror (DM) employed to control the wavefront can not correct simultaneously for the different wavefronts presented by two orthogonal uncorrected polarization fields. Further, leakage of cross polarization fields introduced by the various optical surfaces can degrade the image contrast. The study reported here shows mirror coating designs that reduce the phase difference between orthogonal polarizations reflected by a mirror surface to less than 0.6 deg over the bandwidth and aperture which may encounter a maximum angle of incidence of about 12 deg at a curved mirror. Such designs mitigate the contrast degradation due to cross polarization leakage. Simulations show that required contrast levels can be achieved with such coatings.