Design of a telescope-occulter system for THEIA

TL;DR

This paper presents the design and optimization of occulters and coronagraphs for the THEIA space telescope mission, aiming to enhance direct imaging of terrestrial exoplanets with high starlight suppression.

Contribution

It introduces two optimized occulter designs and evaluates hybrid occulter/coronagraph systems, advancing the technical feasibility for exoplanet imaging missions.

Findings

Two optimized occulters achieve 10^-12 suppression over specified wavelength ranges.

An AIC coronagraph can improve performance but requires precise manufacturing tolerances.

Most other coronagraphs are infeasible due to non-plane wave residual starlight.

Abstract

The Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy (THEIA) is a mission concept study for a flagship-class telescope-occulter system to search for terrestrial planets and perform general astrophysics with a space-based 4m telescope. A number of design options were considered for the occulter and telescope optical systems; in this paper we discuss the design of occulters and coronagraphs for THEIA and examine their merits. We present two optimized occulters: a 25.6m-radius occulter with 19m petals that achieves 10^-12 suppression from 250-1000nm with a 75mas inner working angle, and a 20.0m-radius occulter with 10m petals that achieves 10^-12 suppression from 250-700nm with a 75mas inner working angle. For more widely separated planets (IWA > 108mas), this second occulter is designed to operate at a second closer distance where it provides 10^-12 suppression from 700-1000nm. We have also explored occulter/coronagraph hybrid systems, and found that an AIC coronagraph that exploits the symmetry of the PSF at the occulter can improve performance; however, it requires very accurate tolerances on the occulter manufacturing of the telescope/occulter system as the AIC does not cancel asymmetric terms. Other coronagraphs proved infeasible, primarily due to the fact that the residual starlight from the occulter is not a plane wave, and so is poorly suppressed by the coronagraph.