Iterative deformation calibration of a transmission flat via the ring-point support on a 300-mm-aperture vertical Fizeau interferometer.

In the development of a high-precision vertical Fizeau interferometer with a 300-mm aperture, the deformation of the transmission flat because of clamping and gravity must be considered. In this paper, we proposed a ring-point support scheme for the deformation calibration of a large-diameter transmission flat. The calibration theory of the ring-point support system with elastic deformation was derived. The changes in the surface and stress field of the transmission flat were analyzed quantitatively by finite element method modeling, leading to the optimization of the support structure. To validate the proposed calibration approach, we performed an absolute test of the transmission flat using a liquid reference. The test result was compared to a measurement of the Zygo interferometer demonstrating the effectiveness of the proposed ring-point support design. Finally, with the iterative deformation calibration, the accuracy of the transmission flat reached λ/25 (Peak Valley, PV) for a 300-mm aperture.