In-situ absolute measurement method for reference surface error of large aperture interferometer based on oblique incidence

Existing absolute measurement methods are difficult to avoid the replacement and rotation of the large aperture interferometer's reference flat, and it is impossible to achieve in-situ absolute measurement on the basis of keeping the reference flat of the interferometer not rotating and replacing each other. In this paper, a test method based on oblique incidence is practically implemented in the interferometric measurement process, and an auxiliary rotating device for large aperture flat is designed. Three sets of wavefront data are achieved through cavity interference measurement with a Fizeau interferometer and one oblique incidence measurement. An iterative algorithm is applied to retrieve the absolute surface shape of the test flat. The absolute measurement experiment is carried out on the 600-mm aperture interferometer produced by Zygo. The experimental results show that the absolute surface error of the reference flat measured according to the proposed method is 2.0 nm (RMS), differing from the absolute measurement results based on the Zygo approach.