With the rapid development of science and technology, large-aperture optical systems are more widely used in space optics, astronomy optics and other fields. Large optical flats are often used as reference surfaces in autocollimation optic tests. Thus, a high-quality surface and a suitable and highly precise test method is necessary. Currently the largest commercial digital interferometer of 1 meter aperture is not available to cover the large mirror which diameter is more than 1m during tests. This paper analyzes some common method which widely used in large aperture flat mirror test, and select Ritchey-Common test method as the main content. According to the theory of Ritchey-Common test, this paper analyzes the relationship between the system wavefront and flat mirror surface. To solve the difficulties in the Ritchey-Common test that a single measurement could not obtain the test flat mirror surface, this paper change the main optical path to derive the surface from two different test angles. And we utilize the relationship between system pupil coordinate and flat mirror coordinate to derive the surface. The details and accuracy of the data processing method is well presented. The accuracy can reach to 0.01 ( =0.6328μm), a more actual flat surface error could be obtain for the alignment error of optical path separated. Compared the methods of using the influence matrix with our method of using relationship between system pupil coordinate and flat mirror coordinate to figure out the flat surface by simulation.The result shows that our method is more accuracy and suitable for Ritchey-Common test. A suitable range of the Ritchey angle is identified, and its error allowance is determined by simulation. The ratio of image size to the pupil plane is used to calculate the Ritchey angle in test, the accuracy of this method for calculating is high enough. The distance between the lens focus and the flat mirror center is analyzed. We test a flat mirror with a small diameter in three different Ritchey angles. The results of the Ritchey-Common test and the direct measurement with Zygo are very close; the experimental results confirm that this Ritchey-Common method is effective and accurate.And this provided analysis for actual tests. This paper also introduces the composition and principle of the scanning pentaprism test system which is very important for large aperture optical test. The method could be used to test large flat mirror which diameter is larger than 1 m. This paper focuses on analyzing scanning accuracy of the pentaprism, and programming software that used to calculate the manufacturing errors of the pentaprism and the angle error when moving in three-dimensional direction. The results are used in the subsequent data processing. We also analyze the relationship between the scan lines and the primary aberrations, and select the polar coordinate scan sampling method for testing which is more accurate and efficiency. We build the Ritchey-Common test path for a 1.5 m flat mirror. A well sphere with the diameter of 1.8 m and the radius of curvature is 15 m was used as a reference. We derive the surface from two different test angles. In the results, PV and RMS can reach 0.391 ( =0.6328μm) and 0.0181 ( =0.6328μm) respectively. The test of 1.5m flat mirror is finished by Ritchey-Common method. The experimental results confirm that this Ritchey-Common test method is effective and accurate.
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