Spectral confocal displacement sensor is a kind of non-contact displacement sensor based on wavelength shift modulation. The principle of spectral confocal measurement transforms the displacement information into the wavelength information, and obtains the measured displacement through the spectrum measurement. Large aperture aspheric surface is the core component of optical system, and its measurement accuracy is directly related to the imaging accuracy of optical camera. The non-contact differential confocal method has the characteristics of non-contact and highprecision. Through the integration of dual frequency interferometer to accurately measure the distance and real-time compensate the measurement distance, the nano level high-precision surface measurement results are obtained, and the measurement accuracy of optical components is improved. In this paper, the non-contact differential confocal method is used to detect the concave aspheric surface with a diameter of 300 mm. Through error file compensation, the surface shape detection accuracy is improved to 12 nm rms, and the scanning detection accuracy is greatly improved to the interference detection accuracy, which meets the imaging requirements of optical system.
Optical fabrication and metrology technologies are studied in the paper to improve the accuracy of surface figure of a convex aspheric mirror. First, the main specifications of a convex aspheric mirror which is chosen to be the secondary mirror of an optical system are presented. The aperture of the mirror is 400mm. The mirror is made of ultra-low expansion (ULE) glass with honeycomb sandwich structure to get the ideal lightweight requirement. Then the mirror is surfaced by ultrasonic grinding, smart robot lapping and smart robot polishing processes relatively. Large-apertured tool is applied to reduce the mid-frequency surface error. Both the contour measuring method in the grinding and lapping stage and the measuring method with meniscus lens and its calibration mirror in the polishing stage are studied. The final surface figure of the mirror is that the root mean-square value (RMS value) is 0.016λ (λ=632.8nm), which meets the requirement of the optical system. The results show that the forging surfacing processes and measuring methods are accurate and efficient to fabricate the convex aspheric mirror and can be applied in optical fabrication for larger-apertured convex aspheric mirrors.
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