Polarization aberration modeling of internal occulters for coronagraphs

TL;DR

This paper models polarization aberrations caused by internal occulters in coronagraphs using FDTD simulations and assesses their impact on contrast performance for exoplanet imaging, informing the design of future space telescopes.

Contribution

It introduces a detailed polarization aberration modeling approach for internal occulters using FDTD and evaluates contrast degradation with wavefront control simulations.

Findings

Polarization aberrations depend on spot size, thickness, and incidence angle.

Aberrations can significantly degrade contrast performance.

Modeling informs design choices for high-contrast exoplanet imaging instruments.

Abstract

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.