Liang Rong, Zhou Xiaojun, Xu Huangrong, Wu Dengshan, Li Chenxi, Yu Weixing
Appl Opt. 2024 Mar 10;63(8):1995-2003. doi: 10.1364/AO.502610.
For gravitational wave detection, the telescope is required to have an ultra-low wavefront error and ultra-high signal-to-noise ratio, where the power of the stray light should be controlled on the order of less than 10. In this work, we propose an alternative stray light suppression method for the optical design of an off-axis telescope with four mirrors by carefully considering the optimal optical paths. The method includes three steps. First, in the period of the optical design, the stray light caused by the tertiary mirror and the quaternary mirror is suppressed by increasing the angle formed by the optical axes of the tertiary mirror and the quaternary mirror and reducing the radius of curvature of the quaternary mirror as much as possible to make sure the optical system provides a beam quality with a wavefront error less than /80. Next, the stray light could satisfy the requirement of the order of 10 when the level of roughness reaches 0.2 nm, and the pollution of mirrors is controlled at the level of CL100. Finally, traditional stray light suppression methods should also be applied to mechanics, including the use of the optical barrier, baffle tube, and black paint. It can be seen that the field stop can efficiently reduce stray light caused by the secondary mirror by more than 55% in the full field of view. The baffle tube mounted on the position of the exit pupil can reduce the overall stray light energy by 5%, and the difference between the ideal absorber (absorption coefficient is 100%) and the actual black paint (absorption coefficient is 90%) is 3.2%. These simulation results are confirmed by the Monte Carlo method for a stray light analysis. Based on the above results, one can conclude that the geometry structure of the optical design, the quality of mirrors, and the light barrier can greatly improve the stray light suppression ability of the optical system, which is vital when developing a gravitational wave telescope with ultra-low stray light energy.
对于引力波探测，要求望远镜具有超低的波前误差和超高的信噪比，其中杂散光的功率应控制在小于10的量级。在这项工作中，我们通过仔细考虑最佳光路，为四镜离轴望远镜的光学设计提出了一种替代的杂散光抑制方法。该方法包括三个步骤。首先，在光学设计阶段，通过增大第三镜和第四镜光轴之间的夹角，并尽可能减小第四镜的曲率半径，以抑制由第三镜和第四镜引起的杂散光，确保光学系统提供波前误差小于λ/80的光束质量。其次，当粗糙度达到0.2nm且镜面污染控制在CL100水平时，杂散光可满足10量级的要求。最后，传统的杂散光抑制方法也应应用于机械结构，包括使用光阑、遮光筒和黑漆。可以看出，视场光阑在全视场中可有效降低由第二镜引起的杂散光超过55%。安装在出瞳位置的遮光筒可使整体杂散光能量降低5%，理想吸收体（吸收系数为100%）与实际黑漆（吸收系数为90%）之间的差异为3.2%。这些模拟结果通过杂散光分析的蒙特卡罗方法得到了证实。基于上述结果，可以得出结论，光学设计的几何结构、镜面质量和光阑可以大大提高光学系统的杂散光抑制能力，这在开发具有超低杂散光能量的引力波望远镜时至关重要。
