Theoretical limits on polarization differential imaging for the GSMTs imposed by polarization aberrations

Abstract

Polarization differential imaging (PDI) is a key point-spread function subtraction technique that efficiently processes out starlight and reveals faint polarized structures, such as circumstellar disks and exoplanets. This technique operates by assuming that the signal measured from a star is unpolarized, and subtracts out of the measurement. However, in the presence of polarization aberrations the starlight will be slightly polarized by the telescope that observes it. This results in a spatially-varying polarized speckle field on the focal plane of high-contrast imaging polarimeters. Current high-contrast polarimeters are roughly an order of magnitude from achieving the photon noise limit, and polarization aberrations are a contributing factor. This effect will be stronger in the next-generation 30m Giant Segmented Mirror Telescopes (GSMTs) where polarization aberrations are stronger. In this work, we present a numerical model of a high-contrast imaging polarimeter behind an adaptive optics system subject to the polarization aberrations of an GSMT-class telescope. We use this model to understand the coupling of polarization aberrations into the adaptive optics residuals that leak through to the focal plane, and compare them to what has been observed on previous polarimetric instruments. We report on the fundamental limits imposed by polarization aberrations on PDI and discuss mitigation strategies to compensate for this effect.