Polarization aberrations in next-generation giant segmented mirror telescopes (GSMTs)

I. Effect on the coronagraphic performance

¹ Steward Observatory, University of Arizona, 933N Cherry Avenue, Tucson, AZ 85721, USA
e-mail: ramyaanche@arizona.edu
² James C. Wyant College of Optical Sciences, University of Arizona, 933N Cherry Avenue, Tucson, AZ 85721, USA
³ Department of Physics, University of California, Santa Barbara, CA 93106, USA
⁴ Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁵ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
⁶ TMT International Observatory LLC, 100 W. Walnut St., Suite 300, Pasadena, CA 91124, USA

Received: 7 December 2022
Accepted: 5 February 2023

Abstract

Context. 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 (i.e., polarization-dependent phase and amplitude patterns in the pupil) arising from the reflection from the telescope’s mirror surfaces and instrument optics. These polarization aberrations induce false signals for polarimetry that can be calibrated to a certain degree, but they can also fundamentally limit the achievable contrast of coronagraphic systems.

Aims. We simulate the polarization aberrations and estimate their effect on the achievable contrast for three next-generation ground-based large segmented mirror telescopes.

Methods. 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.

Results. 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.

Conclusions. 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.