Research on the calibration method of reference flat based on gravity deformation

In high-precision vertical large-caliber flat interferometry, the gravity deformation of the reference flat will lead to a large error of the surface shape measured. Based on the analysis of the finite element method, this paper combines the three flats test method with Zernike polynomial fitting to compensate and calibrate the error of the reference flat. For the diameter of 300 mm and a thickness of 90 mm of the fused silica reference flat, the results of analysis show that the PV deformation under the T-bracing is 0.021 λ. (λ = 632.8nm). The simulation test result of the influence of deformation on the three-flats test shows that the reference flat the gravity deformation not only affects the detection of self surface, but also has a great influence on the surface detection results of the undeformed flat. And the PV of the surface shape detection results residual is close to 0.011λ. In the end, the new calibration method base on compensate reference deformation for three flats test was proposed, and the validity was verified by simulation.