Estimation of polarization aberrations and their effect on the coronagraphic performance for future space telescopes

TL;DR

This paper investigates how polarization aberrations from mirror reflections affect the performance of space telescope coronagraphs, analyzing different mirror coatings to optimize starlight suppression for exoplanet imaging.

Contribution

It provides a detailed simulation of polarization aberrations for a 6.5 m space telescope and evaluates the impact of various mirror coatings on coronagraphic contrast.

Findings

Retardance-defocus and retardance-tilt are the main polarization aberrations.

Coating choice significantly influences aberration strength.

Bare/oxi-aluminum and Al+18nm LiF coatings outperform others by an order of magnitude.

Abstract

A major goal of proposed future space observatories, such as the Habitable World Observatory, is to directly image and characterize Earth-like planets around Sun-like stars to search for habitability signatures requiring the starlight suppression (contrast) of 1e-10. One of the significant aspects affecting this contrast is the polarization aberrations generated from the reflection from mirror surfaces. The polarization aberrations are the phase-dependent amplitude and phase patterns originating from the Fresnel reflections of the mirror surfaces. These aberrations depend on the angle of incidence and coating parameters of the surface. This paper simulates the polarization aberrations for an on-axis and off-axis TMA telescope of a 6.5 m monolithic primary mirror. We analyze the polarization aberrations and their effect on the coronagraphic performance for eight different recipes of mirror coatings for Astronomical filter bands g-I: three single-layer metal coatings and five recipes of protective coatings. First, the Jones pupils are estimated for each coating and filter band using the polarization ray tracing in Zemax. Then, we propagate these Jones pupils through a Vector Vortex Coronagraph and Perfect Coronagraphs using hcipy, a physical optics-based simulation framework. The analysis shows that the two main polarization aberrations generated from the four mirrors are the retardance-defocus and retardance-tilt. The simulations also show that the coating plays a significant role in determining the strength of the aberrations. The bare/oxi-aluminum and Al+18nm LiF coating outperforms all the other coatings by one order of magnitude.