Comparison of polarization aberrations from existing mirror coatings for coronagraphic imaging of habitable worlds

The Habitable Worlds Observatory (HWO) aims to directly image and spectroscopically characterize Earth-like exoplanets. This may be done with a coronagraph instrument, which can suppress the host star's light by a factor of ∼1010. One of the key factors limiting the performance of these instruments at that level is the aberration of the wavefront due to polarization. Changes in the angle of incidence across a beam result in spatially varying polarization state changes called polarization aberrations. Polarization aberrations present a unique problem in high-contrast imaging because the orthogonally polarized components of the wavefront experience different aberrations. This means that standard wavefront control techniques will be incapable of removing the aberration from all polarizations simultaneously. We report on the influence of polarization aberrations for an early-concept study in support of the HWO called the Six Meter Space Telescope operating with two different Apodized Pupil Lyot Coronagraph designs, which we developed for this study. Polarization aberrations from three different coatings studied set a mean uncompensated normalized intensity between 1 and 10×10-10 at the inner working angle. To minimize the influence of polarization aberrations, we split the coronagraph into orthogonal polarization channels and control the mean wavefront incident on each channel separately. This reduces the intensity at the inner working angle by an order of magnitude, restoring 10-10 contrast. We then outline strategies for further compensation of polarization aberrations that can be considered in future work.