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Fizeau interferometry is widely used to measure the surface of optical components and the wavefront aberration of optical systems due to common-path configuration, high accuracy and easy implementation. However, due to the existence of wavefront slope, when the transmission light is not perfectly perpendicular to the reflection flat (RF), the gap between the RF and the test piece will cause the reflected beam to be slightly misaligned from the transmission beam in the transmission test, which is also called beam walk-off error. If the beam walk-off error is less than 1/4 of the pixel spacing, its effects can be ignored. In this work, we will analyze the wavefront slope tolerance of 800mm aperture Fizeau interferometry and suppress it by optimizing the design of the beam expansion system, which is the key component that affects the overall wavefront slope. For the camera with 2048×2048 pixel array, the wavefront slope should be less than 9.77μrad to neglect the influence of beam walk-off error within a cavity length of 5000mm. To obtain the 800mm aperture test light, a Galilean beam expander is designed to expand and collimate the 100mm aperture light into an 800mm aperture test light. Compared with the Keplerian design, the Galilean design can decrease spherical aberration by the combination of positive and negative lenses. Tested by interferometry, the slope of the transmitted wavefront of the Galilean beam expander is 3.83μrad, which ensures the high-precision measurement.
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