Polarization aberration modeling of internal occulters for coronagraphs

The Habitable Worlds Observatory (HWO) is a flagship mission concept proposing to characterize earth-like exoplanets at high contrast with a coronagraph instrument. The most in-depth, validated contrast error budgets made to date have been at the $10^{-9}$ planet-to-star contrast levels for the Roman Space Telescope. To obtain the raw contrast levels of $\leq 10^{-10}$ contrast needed for HWO, more modeling is needed of the vector diffraction effects within a coronagraph instrument. Several coronagraph architectures are based on the Lyot coronagraph, which utilizes a small occulting spot to block most of the on-axis starlight at a focal plane. In this paper, we present a study of the polarization aberrations generated by internal occulting spots. First, we use finite-difference time-domain (FDTD) modeling to produce the complex transmission of the occulter in each polarization state, and then we separately run those through a closed-loop wavefront control model in FALCO\cite{rigss2018falco} to determine the degradation in achievable contrast. We perform sweeps of such parameters as spot diameter, spot thickness, and angle of incidence to inform the HWO instrument design.