Testing of annular hyperboloid mirror for Cassegrain system

Abstract

This paper introduces the testing of annular hyperboloid mirror with 460mm diameter in the Cassegrain system. To improve the testing efficiency and meet the requirements of utilization, the study is carried out from three stages: precision grinding, precision polishing, and optical coating. In the precision grinding stage, the annular Zernike polynomial is used to fit the measured surface combining with the data measured from the coordinate measuring machine, so that it can facilitate the testing of the distribution of the surface shape deviation over the entire surface. During the precision polishing stage, a feasible Offner compensator is designed to achieve the goal of high-precision testing of the hyperboloid surface, with a measurement accuracy of RMS≤0.02λ. Also, 0.5-0.7μm and 3-5μm dual-band high reflectivity and high uniformity reflective coating is designed for the Cassegrain system requirements, the actual test reflectivity is 96.2%, which can meet the design requirements.