A METHOD FOR MEASURING THE ABSOLUTE SURFACE SHAPE OF LARGEAPERTURE OPTICAL FLAT

The absolute shape measurement of large aperture optical flats usually requires a standard mirror with the same or larger aperture as the element to be measured as the reference surface for calibration. However, the large aperture standard mirror is difficult to process, and the measurement resolution is limited. To solve this problem, a method of measuring the absolute surface shape of a large aperture optical flat is proposed. The aperture optical flat is divided into several sub-apertures using the sub-aperture stitching method. The reference mirror shape is obtained by the odd and even function four-step absolute measurement method and separated to obtain the absolute surface shape of the sub-aperture. The immune algorithm is optimized and applied to the adjacent sub-aperture stitching to obtain the complete absolute surface shape. The 210 mm long flat crystal was measured, and the closed-loop self-test was designed to verify the correctness of the obtained reference mirror shape. The peak and valley value (PV) of reference mirror B is 53.64 nm, and the root mean square (RMS) value is 9.96 nm. The results of the closed-loop self-test are 58.63 nm and 10.86 nm, and the data are basically consistent with the surface shape diagram. The surface shape test results pass the closed-loop self-test. The absolute surface shape results after stitches are compared with those measured by a large aperture interferometer. The deviation of the PV value is less than lambda/135 (lambda=632.8 nm), and the deviation of the RMS value is less than lambda/75. The measurement resolution is 11 times that measured by a large aperture interferometer, which fully proves the accuracy and high resolution of the method.