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

**Authors:** Ramya M. Anche, Jaren N. Ashcraft, Sebastiaan Y. Haffert, Maxwell A. Millar-Blanchaer, Ewan S. Douglas, Frans Snik, Grant Williams, Rob G. van Holstein, David Doelman, Kyle Van Gorkom, Warren Skidmore  
**arXiv:** [2304.02079](https://arxiv.org/abs/2304.02079)  
**Date:** 2023-04-04  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.02079v1/

## Key points

- Next-generation large segmented mirror telescopes are being developed to directly image and characterize Earth-like rocky planets
- High contrast limits of $10^{-7}$ to $10^{-8}$ are required at wavelengths ranging from I to J band
- Polarization aberrations arising from the reflection of light from the telescope's mirror surfaces and instrument optics affect the raw on-sky contrast
- Researchers simulated these aberrations and estimated their effect on achievable contrast for three next-generation ground-based large segmented mirror telescopes using ray-tracing in Zemax and a polarization ray-tracing algorithm
- The impact of these aberrations on contrast was estimated by propagating Jones pupil maps through idealized coronagraphs using hcipy, a physical optics-based simulation framework
- Polarization aberrations create significant leakage through a coronagraphic system, with retardance defocus being the dominant aberration originating from steep angles on primary and secondary mirrors, limiting contrast to $10^{-5}$ to $10^{-4}$ at 1 $\lambda/D$ at visible wavelengths and $10^{-5}$ to $10^{-6}$ at infrared wavelengths.
- Coatings play an important role in determining the strength of these aberrations.
- It is essential to consider polarization aberrations during high-contrast imaging instrument design for next generation extremely large telescopes.
- The paper provides a detailed description of each telescope's optical layout and proposed coronagraphic instruments in Section 2.
- Mitigation strategies such as polarimetry with high contrast imaging or incorporating compensation optics, robust coronagraphs, specialized coatings, calibration techniques, and data analysis approaches can be used to reduce or eliminate these effects.
- Section 8 emphasizes that considering polarization aberrations during high contrast imaging instrument design is crucial for next generation extremely large telescopes.

## Summary

Next-generation large segmented mirror telescopes are being developed to directly image and characterize Earth-like rocky planets, which requires high contrast limits of $10^{-7}$ to $10^{-8}$ at wavelengths ranging from I to J band. However, one critical aspect that affects the raw on-sky contrast is polarization aberrations arising from the reflection of light from the telescope's mirror surfaces and instrument optics. In a recent study, researchers simulated these aberrations and estimated their effect on achievable contrast for three next-generation ground-based large segmented mirror telescopes. The study involved performing ray-tracing in Zemax and computing the polarization aberrations and Jones pupil maps using a polarization ray-tracing algorithm. The impact of these aberrations on contrast was then 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 giant segmented mirror telescopes (GSMTs) showed that polarization aberrations create significant leakage through a coronagraphic system. The study found that retardance defocus is the dominant aberration originating from steep angles on primary and secondary mirrors, limiting contrast to $10^{-5}$ to $10^{-4}$ at 1 $\lambda/D$ at visible wavelengths and $10^{-5}$ to $10^{-6}$ at infrared wavelengths. The simulations also revealed that coatings play an important role in determining the strength of these aberrations. Therefore, it is essential to consider polarization aberrations during high-contrast imaging instrument design for next generation extremely large telescopes. The paper provides a detailed description of each telescope's optical layout and proposed coronagraphic instruments in Section 2; for instance, it describes ELT with a 39m diameter segmented primary mirror currently under construction on Cerro Armazones in northern Chile expected to have its first light in 2027. It has five mirrors with an intermediate focus, as shown in Figure 1. The Multi AO Imaging Camera for Deep Observations (MICADO) is one of the proposed coronagraphic instruments. Section 3 provides a summary of the polarization ray tracing algorithm and estimation of polarization aberrations while Section 4 presents Jones pupil maps evaluated at the exit pupil of each telescope. Section 5 shows diattenuation and retardance variation on all mirror surfaces for coatings used in these telescopes. The description of coronagraphs and simulations to estimate achievable contrast is given in Section 6; moreover, this section discusses mitigation strategies such as polarimetry with high contrast imaging or incorporating compensation optics, robust coronagraphs, specialized coatings, calibration techniques, and data analysis approaches that can be used to reduce or eliminate these effects. Finally, Section 8 provides a summary emphasizing that considering polarization aberrations during high contrast imaging instrument design is crucial for next generation extremely large telescopes.