[论文解读] 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 sensing被引用 4

一句话总结

本文研究了下一代巨型拼接镜面望远镜（GSMTs）中的偏振像差，利用Zemax光线追迹和hcipy波前传播模拟其对高对比度成像日食仪的影响。研究发现，主镜和次镜引起的延迟率离焦限制了可见光波段1λ/D处的原始对比度为10⁻⁵–10⁻⁴，红外波段为10⁻⁵–10⁻⁶，且镀膜显著影响像差强度，因此在仪器设计中必须专门采取缓解措施。

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.

研究动机与目标

• 评估GSMT光学元件引起的偏振像差对高对比度成像性能的退化影响。
• 量化镜面镀膜和望远镜几何结构对偏振诱导对比度退化的影响。
• 在真实偏振像差条件下，评估理想化日食仪可实现的原始对比度。
• 识别主要像差类型（尤其是延迟率离焦），这些像差在可见光和红外波段限制了对比度。
• 通过推荐补偿、鲁棒日食仪设计或偏振校准策略，为未来仪器设计提供指导。

提出的方法

• 在Zemax®中使用琼斯矩阵形式进行偏振光线追迹，以计算波前在光瞳面上的相位和振幅变化。
• 从ELT、TMT和GMT的望远镜光学模型出发，生成I、R和J波段的琼斯光瞳图。
• 利用hcipy（一种物理光学仿真框架）将琼斯光瞳图通过理想化日食仪进行传播。
• 在多个波长和望远镜配置下模拟偏振像差，以评估对比度极限。
• 通过比较其二向色性和延迟率响应，评估镜面镀膜（如铝与Gemini型）的作用。
• 开发基于ZOS-API和Python的偏振光线追迹工具，以实现对光学系统的可推广分析。

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

实验结果

研究问题

• RQ1 下一代GSMT中的偏振像差的幅度和空间结构如何？
• RQ2 主镜和次镜引起的偏振像差如何影响1λ/D处日食仪的对比度？
• RQ3 哪些像差类型（如延迟率离焦、倾斜）对高对比度成像最具破坏性？
• RQ4 镜面镀膜如何影响偏振像差的强度和可实现的对比度？
• RQ5 偏振像差在多大程度上限制了可见光和红外波段日食仪系统的原始对比度？

主要发现

• 由陡峭镜面倾角引起的延迟率离焦是主导的对比度限制像差，导致可见光波段（I、R、J）1λ/D处的原始对比度限制在10⁻⁵–10⁻⁴之间。
• 红外波段（L、M、N）可实现更好的对比度，第二和第四阶日食仪的峰值对比度可达10⁻⁶，但仅在理想条件下。
• 蓝光区域（I波段）的对比度水平超过10⁻⁴，未达到地球类系外行星探测所需的10⁻⁷–10⁻⁸水平。
• 尽管反射率较低，铝镀膜产生的偏振像差弱于Gemini型镀膜，表明需在延迟率和二向色性之间进行协同优化。
• 折叠镜面引起的延迟率倾斜会导致光束偏移，必须在高对比度仪器中加入补偿光学元件，因为自适应光学系统无法校正此类像差。
• 仅由望远镜光学元件引起的偏振像差已接近或超过在730 nm处探测氧气A带所需的对比度水平，因此必须在仪器设计中采取缓解措施。

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