Bipod支撑大口径反射镜的零重力面形测试技术

双脚架结构具有静定支撑的特点，可以隔离机械附加载荷，因此成为大口径空间相机反射镜组件的常用支撑形式之一。在地面装调时，采用双脚架支撑的反射镜的面形因重力作用而下降。空间相机入轨后，随着重力变形的释放，反射镜面形会再次发生改变。有限元分析方法评估反射镜组件的重力误差，其精度难以达到高质量高分辨率成像的要求。同时，反射镜加工过程中使用的重力卸载方案也难以沿用至组件阶段。针对重力误差测试过程中装配误差与三叶像差混叠以及检测光路对球差测试精度不足的问题，提出了翻转与卸载相结合的测试方案。基于不同像差的正交性，可以进行分别测试来逐项获取各像差。通过反射镜组建的翻转测试，分离装配误差与重力误差中的三叶像差。设计一定精度的卸载装置，通过卸载前后的对比测试，得到重力造成的球差等旋转对称像差。采取上述方案可以实现对全部重力误差的实测。利用1.3 m口径高轻量化反射镜组件进行了测试验证，其重力误差面形rms和在轨面形rms分别为0.192λ （λ=0.6328 μm）和0.023λ。

0 g surface figure test of large aperture mirror supported by Bipod

Bipod structure has the characteristics of static support and can isolate the additional mechanical load. Therefore, it has become one of the common support forms of large aperture space camera mirror assembly. When installing and adjusting on the ground, the surface figure of the mirror supported by the Bipod decreases due to the action of gravity. After the space camera enters the orbit, the surface figure of the mirror will change again with the release of gravity deformation. The gravity error of the mirror assembly is evaluated by finite element analysis method, and its accuracy is difficult to meet the requirements of high-quality and high-resolution imaging. At the same time, the gravity unloading scheme used in the mirror processing process is also difficult to be used to the component stage. In order to solve the problems of aliasing of assembly error and trefoil aberration and insufficient accuracy of spherical aberration test by detection light path in the process of gravity error test, a test scheme combining turnover and unloading was proposed. Based on the orthogonality of different aberrations, individual tests could be carried out to obtain each aberration item by item. Through the gravity turnover test of the mirror, the trefoil aberration in the assembly error and gravity error was separated. The unloading device with certain accuracy was designed. Through the comparative test before and after unloading, the spherical aberration caused by gravity, was obtained. By adopting the above scheme, the measurement of all gravity errors could be realized. The 1.3 m high-lightweight mirror assembly was tested. The gravity error surface figure (rms) and on-orbit surface figure (rms) are 0.192λ (λ=0.6328 μm) and 0.023λ, respectively.