[Paper Review] Polarization aberrations in next-generation giant segmented mirror telescopes (GSMTs) I. Effect on the coronagraphic performance

Ramya M. Anche, Jaren N. Ashcraft+9 more|arXiv (Cornell University)|Apr 4, 2023

Adaptive optics and wavefront sensing4 citations

TL;DR

This paper investigates polarization aberrations in next-generation giant segmented mirror telescopes (GSMTs), simulating their impact on high-contrast imaging coronagraphs using Zemax ray-tracing and hcipy wavefront propagation. It finds that retardance defocus from primary and secondary mirrors limits raw contrast to 10⁻⁵–10⁻⁴ at 1λ/D in visible bands and 10⁻⁵–10⁻⁶ in infrared, with coatings significantly influencing aberration strength, necessitating dedicated mitigation in instrument design.

ABSTRACT

Next-generation large segmented mirror telescopes are expected to perform direct imaging and characterization of Earth-like rocky planets, which requires contrast limits of $10^{-7}$ to $10^{-8}$ at wavelengths from I to J band. One critical aspect affecting the raw on-sky contrast are polarization aberrations arising from the reflection from the telescope's mirror surfaces and instrument optics. We simulate the polarization aberrations and estimate their effect on the achievable contrast for three next-generation ground-based large segmented mirror telescopes. We performed ray-tracing in Zemax and computed the polarization aberrations and Jones pupil maps using the polarization ray-tracing algorithm. The impact of these aberrations on the contrast is estimated by propagating the Jones pupil maps through a set of idealized coronagraphs using hcipy, a physical optics-based simulation framework. The optical modeling of the giant segmented mirror telescopes (GSMTs) shows that polarization aberrations create significant leakage through a coronagraphic system. The dominant aberration is retardance defocus, which originates from the steep angles on the primary and secondary mirrors. The retardance defocus limits the contrast to $10^{-5}$ to $10^{-4}$ at 1 $λ/D$ at visible wavelengths, and $10^{-5}$ to $10^{-6}$ at infrared wavelengths. The simulations also show that the coating plays a major role in determining the strength of the aberrations. Polarization aberrations will need to be considered during the design of high-contrast imaging instruments for the next generation of extremely large telescopes. This can be achieved either through compensation optics, robust coronagraphs, specialized coatings, calibration, and data analysis approaches or by incorporating polarimetry with high-contrast imaging to measure these effects.

Motivation & Objective

• To assess how polarization aberrations from GSMT optics degrade high-contrast imaging performance.
• To quantify the impact of mirror coatings and telescope geometry on polarization-induced contrast degradation.
• To evaluate the achievable raw contrast of idealized coronagraphs under realistic polarization aberration conditions.
• To identify dominant aberration types—especially retardance defocus—that limit contrast in visible and infrared bands.
• To guide future instrument design by recommending compensation, robust coronagraphy, or polarimetric calibration strategies.

Proposed method

• Performed polarization ray-tracing in Zemax® using a Jones matrix formalism to compute pupil-level phase and amplitude variations.
• Generated Jones pupil maps from telescope optical models of the ELT, TMT, and GMT across I, R, and J bands.
• Propagated Jones pupil maps through idealized coronagraphs using hcipy, a physical optics simulation framework.
• Simulated polarization aberrations across multiple wavelengths and telescope configurations to assess contrast limits.
• Evaluated the role of mirror coatings (e.g., aluminum vs. Gemini-like) by comparing their diattenuation and retardance responses.
• Developed ZOS-API and Python-based polarization ray-tracing tools for generalizable analysis of optical systems.

Figure 1: Optical layout of the telescopes from Zemax ® for the three telescopes

Experimental results

Research questions

• RQ1 What is the magnitude and spatial structure of polarization aberrations in next-generation GSMTs?
• RQ2 How do polarization aberrations from primary and secondary mirrors affect coronagraphic contrast at 1λ/D?
• RQ3 Which aberration types (e.g., retardance defocus, tilt) are most detrimental to high-contrast imaging?
• RQ4 How do mirror coatings influence the strength of polarization aberrations and achievable contrast?
• RQ5 To what extent do polarization aberrations limit the raw contrast of coronagraphic systems in visible and infrared bands?

Key findings

• Retardance defocus, originating from steep mirror angles, is the dominant contrast-limiting aberration, restricting raw contrast to 10⁻⁵–10⁻⁴ at 1λ/D in visible bands (I, R, J).
• Infrared bands (L, M, N) achieve better contrast, with peak values of 10⁻⁶ for second- and fourth-order coronagraphs, but only under optimal conditions.
• The blue region (I band) shows contrast levels >10⁻⁴, which falls short of the 10⁻⁷–10⁻⁸ requirements for Earth-like exoplanet detection.
• Aluminum coatings produce weaker polarization aberrations than Gemini-like coatings, despite the latter’s higher reflectivity, indicating a need to co-optimize for retardance and diattenuation.
• Retardance tilt from the fold mirror induces beam shifts that require compensation optics in high-contrast instruments, as they cannot be corrected by AO systems.
• Polarization aberrations from telescope optics alone already approach or exceed the required contrast levels for detecting the oxygen A-band at 730 nm, necessitating mitigation in instrument design.

Figure 2: Diagram illustrating a typical polarization ray tracing interaction in reflection with the $q$ -th surface in the optical system. The wave vector before the surface $\mathbf{k}_{q-1}$ is incident on the surface with a linear polarization shown in purple. The angle of reflection is determin