Ray-based Physical Optics for the Design of Astronomical Observatories

Abstract

For diffraction-limited optical systems an accurate physical optics model is necessary to properly evaluate instrument performance. Industry-standard commercial optical design packages as well as existing open-source physical optics tools achieve this by the propagation of a complex-valued scalar optical field through the system. This requires the evaluation of diffraction integrals via Fourier transforms of large arrays and can consequently be computationally intensive. The scalar assumption also obfuscates polarization-dependent behavior, which has been shown to be a considerable performance limiter for astronomical observatories that seek to image earth-like exoplanets. These observatories are outfitted with coronagraphs, which are designed to image faint stellar companions at high contrast using specialized pupil-plane and image-plane masks. When imaging Earth-like exoplanets at small angular separations, coronagraphs are limited by telescope stability and polarization aberrations. The state of the art handles the diffraction model, ray aberration model, and polarization model separately across multiple research and development centers which complicates the design process. This research adds new propagation physics to the open source to capture the geometrical and physical regimes of the electric field simultaneously. To do so, we’ve built an open-source platform which contains two new physics modules. Gaussian beamlet decomposition is a ray-based diffraction calculation which permits the modeling of ray aberration and diffraction simultaneously. We outline a formal algorithm for its general implementation in ray trace models of optical systems, and propose a novel approach that increases its computational efficiency for more accessible use. Polarization ray tracing is a ray-based method of vector field propagation that enables the simulation of polarization aberrations, a known performance limiter for high-contrast imaging instruments. We characterize the polarization aberrations of ground and space observatories to quantify the influence of polarization on coronagraphy, and propose a novel method to mitigate polarization aberrations. These integrated methods of physical optics calculation unite the geometric regime of light with the physical regime, providing powerful insights into the performance of high-contrast imaging instrumentation. Using these insights from optical propagation physics we can simplify the design process and create a more comprehensive model of high-contrast imaging instruments to aid in the design of future astronomical instrumentation.