Support structure and optical alignment technology of large-aperture secondary mirror measured by back transmission method

Abstract

The diameter of secondary mirror measured by back transmission method in the on-orbit assembly space telescope validation prototype with a 1-meter aperture is Φ322 mm, and the distance between secondary mirror and mounting surface is 2.5 m. The requirements for supporting structure are as follows: surface figure error caused by supporting structure should be better than 3.88 nm, the tilt stability of secondary mirror is better than 2 ", and the displacement error in the direction of optical axis is 0.04 mm.According to the above requirements, the supporting structure and alignment method of the secondary mirror are illustrated in detail, and the physical testing is completed. Firstly, the size parameters are optimized by the method of Orthogonal Optimization and Finite Element Analysis (FEA),based on the optimized result, the supporting structure is designed, the bonding mode of the mirror body, the flexible structure to release stress and the implementation method of 4-DOF precise adjustment are emphasized, the results simulated by FEA show that the optical axis displacement of the secondary mirror is 5.2um and the error of the surface figure accuracy is 3 nm under the temperature range of 4℃ and gravity load. Finally, the alignment process and steps without the primary mirror as the benchmark are described in detail. the stability of the supporting structure is tested, and the test period is 14 days. The results and practice indicate that surface figure accuracy is 0.01 λ(λ = 632.8 nm), structure remains stable to excellent levels that the tilt is 1″and the displacement is 0.02 mm under gravity and thermal loading, all of them meet the requirements of the optical system for the supporting structure, the design is reasonable, the effect is good.