Influence of Fabrication and Assembly Errors on the Root Mean Square Surface Distortion of a 2 m Lightweight Mirror and Its Correction

Abstract

The influence of fabrication and assembly errors on the surface distortion due to gravity of a 2 m primary mirror and its correction method are presented. The effect of fabrication errors on the surface distortion is verified by Monte Carlo analysis. The results show that, within the 46.3% confidence interval, the surface accuracy root mean square (RMS) caused by fabrication errors is more than 5.0 nm (indicator requirement). The sensitivity of mirror surface accuracy to the matching relationship between the flexible support axial assembly position and the inherent properties (neutral surface and center of gravity) of the mirror were analyzed. Then, the correction principle of the RMS was proposed based on the analysis result. The surface accuracy RMS of surface gravity distortion is sensitive to fabrication and assembly errors, which can be effectively corrected using a flexible support mounting technique. This new flexible support mounting technique replaces the conventional method with flexible supports having shims so that adjustments can be made during testing to counteract the gravitational distortion of the mirror surface. Astigmatic aberration due to gravitational changes is effectively reduced by selecting a suitable thickness of shim, and the relationship between the astigmatism and the thickness of shim was investigated using the finite element analysis method. Finally, the finite element analysis results showed that the optimal surface gravity accuracy of the mirror assembly could be obtained by adjusting the shim, while the other performance of the assembly was not affected.