Surface figure error and parameter testing of aspheric segments in segmented telescope primary mirrors

Abstract

The segmented primary mirror represents a major breakthrough in large-scale optical engineering, overcoming the long-standing difficulties associated with the fabrication, transportation, and installation of large monolithic primary mirrors. During manufacturing and alignment, interferometric testing based on computer-generated holograms (CGHs) is widely used for the testing of aspheric segments because of its very high accuracy. However, the central and off-axis segments require different holograms, which often leads to inconsistencies in the aspheric parameters (such as the radius of curvature) and thereby degrades the overall performance. We propose a non-null interferometric technique for the measurement of both surface figure error and aspheric parameters, enabling all segments to be measured under a unified baseline so as to ensure parameter consistency and improve the post-alignment performance. Experiments on a 450 mm aspheric mirror, together with a simulation study of a meter-class segmented telescope primary mirror (STPM), demonstrate the effectiveness of the proposed technique.