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In the development of a high-precision vertical Fizeau interferometer with 300-mm aperture, uncertainty evaluation is a significant method to assess the accuracy of interferometer. In this paper, we reported an uncertainty evaluation method. Then, the uncertainty evaluation of a 300-mm-aperture vertical Fizeau interferometer was carried out based on the method. The uncertainties of interference cavity measurement and absolute testing (e.g., three-flat method) were used as indicators of uncertainty evaluation. In addition, the gravity-induced deformation of three-flat method can be compensated accurately by finite element method modeling. Finally, we obtained the uncertainties of interference cavity measurement and three-flat method. With 300-mm aperture, the expanded uncertainty of interference cavity measurement is 3.7×10-3 λ, and the expanded uncertainty of three-flat method is 1.47×10-2 λ.
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