Polarization Aberrations in Coronagraphs

Abstract

In the search for habitable extrasolar planets, the ability to separate and detect the dim planet's light from its much brighter host star is paramount. The inherent polarization properties of optical systems can lead to small but significant deviations from ideal imaging behavior, possibly hindering the ability of that system to detect exoplanets. In this work, the polarization aberrations in a telescope/coronagraph optical system are modeled with polarization ray tracing software. The effect of these polarization aberrations on the coronagraph's ability to suppress on-axis starlight is analyzed, and an algorithm for mitigation of some of the polarization aberrations is provided. This dissertation lays the foundation for modeling and analyzing polarization aberrations in telescope/coronagraph systems and shows that, in the absence of adaptive optics, both isotropic and anisotropic polarization aberrations degrade a coronagraph's ability to suppress starlight. The contrast is a measure of on-axis starlight suppression. With isotropic polarization aberrations, the contrast is degraded by two orders of magnitude relative to the ideal, polarization aberration-free case. Anisotropy modeled on the primary mirror further degraded the contrast by two orders of magnitude relative to the isotropic case. Modeling polarization aberrations shows how coatings negatively affect the possible contrast of a coronagraph.