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    • The Measurement of PSF Ellipticity of an Unobstructed Off-Axis Space Telescope: Error Analysis

    Kuo Fan,Jing Luo, Xu He,Chenghao Li,Xiaohui Zhang

    IEEE Access（2021）SCI 3区SCI 4区

    Chinese Acad Sci

    Cited 3|Views7
    Abstract
    It is an effective method to detect weak gravitational lensing (WL) in the universe by measuring ellipticities of galaxies via astronomical telescopes. Optical properties of telescopes are critical to WL detections. To ensure the used telescopes to be competent, it is necessary to measure point spread function (PSF) ellipticities of telescopes in labs. In this paper, a way based on simulated star target imaging is proposed to measure PSF ellipticity of an unobstructed off-axis space telescope. Related errors are identified and modeled carefully for the first time. Effects of detector noises, micro-vibration of optical platforms, defocusing of simulated star target, wavefront errors (WFEs) and central obstructions of collimators on PSF ellipticity measurements of the telescope are analyzed. Results show that the measurement error of PSF ellipticity decreases from 0.0105 to 0.0043 by adopting 10 iterations of the iterative weighted centroiding algorithm when SNRs are under 24 dB. To ensure PSF ellipticity measurement errors are not larger than 0.01, the micro-vibration angle of the optical platform should be less than $0.05''$ . When focal length of the collimator is twice that of the telescope, the measurement errors of PSF ellipticity are under 0.01 if the defocusing of simulated star target is controlled to be not larger than 0.1 mm. In addition, WFEs and central obstructions of collimators change PSF ellipticity measurement errors to different degrees at different fields of view (FOVs). Due to 20 nm RMS WFE of the collimator, the maximum value of PSF ellipticity measurement errors over full FOVs is 0.1 and the average value is 0.0269. If the radius of central obscuration of the collimator is 150 mm, the maximum measurement error of PSF ellipticity over full FOVs is 0.0091. According to the results shown in this paper, significant references for high accuracy measurements of PSF ellipticity of telescopes can be provided.
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    Key words
    Weak gravitational lensing,unobstructed off-axis space telescope,PSF ellipticity measurements,error analysis
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    【要点】：本文提出了一种基于模拟星目标成像的方法来测量一个非遮挡离轴空间望远镜的点扩散函数（PSF）椭圆度。首次对相关误差进行了仔细的辨认和建模。分析了探测器噪声、光学平台微振动、模拟星目标离焦、波前误差（WFE）和准直器的中心遮挡对望远镜PSF椭圆度测量的影响。结果显示，在信噪比低于24 dB时，通过采用迭代加权质心法的10次迭代，PSF椭圆度的测量误差可从0.0105减小到0.0043。为确保PSF椭圆度的测量误差不大于0.01，光学平台的微振动角度应小于0.05 "。当准直器的焦距是望远镜焦距的两倍时，如果模拟星目标的离焦控制在0.1 mm以下，PSF椭圆度的测量误差将保持在0.01以下。此外，不同视场下，波前误差和准直器的中心遮挡将以不同程度影响PSF椭圆度的测量误差。由于准直器的20 nm RMS波前误差，全视场的PSF椭圆度测量误差最大值为0.1，平均值为0.0269。如果准直器的中心遮挡半径为150 mm，全视场的PSF椭圆度测量误差最大值为0.0091。根据本文的结果，可以为望远镜PSF椭圆度的高精度测量提供重要参考。
    

    【方法】：本文采用模拟星目标成像的方法来测量非遮挡离轴空间望远镜的PSF椭圆度，并对相关误差进行了建模。
    

    【实验】：通过采用迭代加权质心法的10次迭代，PSF椭圆度的测量误差可从0.0105减小到0.0043。为确保PSF椭圆度的测量误差不大于0.01，光学平台的微振动角度应小于0.05 "。当准直器的焦距是望远镜焦距的两倍时，如果模拟星目标的离焦控制在0.1 mm以下，PSF椭圆度的测量误差将保持在0.01以下。同时，波前误差和准直器的中心遮挡对PSF椭圆度的测量误差产生不同程度的影响。准直器的20 nm RMS波前误差导致全视场的PSF椭圆度测量误差最大值为0.1，平均值为0.0269。如果准直器的中心遮挡半径为150 mm，全视场的PSF椭圆度测量误差最大值为0.0091。根据本文的结果，可以为望远镜PSF椭圆度的高精度测量提供重要参考。使用的数据集名称未提及。