A computer-generated hologram(CGH) was designed and produced for precisely testing the off-axis aspheric mirror with departure of 1200um in a three mirror astigmatism optical system.To solve the problem of selection of incident light position in CGH design Depended on engineering experience was likely to increase spatial frequency of CGH which reduced testing accuracy of aspheric mirror. A CGH design method was proposed based on quantitative calculation of incident light position which reduced the reliance on experience. The testing result by CGH was compared with that of non-contact profile testing method when the off-axis aspheric mirror surface was polished to 0.05λ(RMS).Two testing results show a good agreement. By CGH testing method,the surface error is 0.016λ RMS now.
Smart robot polishing experiments of a reflective mirror are carried on in the paper to verify the pre-filling method. First, pre-filling method is used to simulate the surface figure of a zerodur mirror whose edge is out of the measuring scope of the Zygo interferometer partly in contrast to that of the original one. Second, the polished surface figures with different polishing depths are simulated in the paper to get the relatively right polishing depth of the mirror whose surface figure is processed with the pre-filling method. And at the same time, the polished surface figures with different polishing depths with the unfilled surface figure are also simulated by contrast. Third, smart robot polishing experiment is carried on to verify both the chosen polishing depth and the pre-filling method. Forth, the real surface figure of the mirror after polishing is calibrated. The measuring results show that the surface figure of the mirror after polishing is changed greatly in comparison of that of the mirror before polishing: the surface figure of the mirror before polishing is that the root-meansquare value (RMS value) is about0.374 λ (λ=632.8nm), and the edge is invisible partly, while the RMS value of the surface figure of the mirror after polishing is about0.231 λ, and the edge is visible. The convergence rate of RMS value is 38.2%. The experimental results show that pre-filling method can simulate rightly the surface figure of the aspheric mirror whose edge is out of the measuring scope of the Zygo interferometer.
Edge-trimming experiment of a reflective mirror is carried on in the paper to study the influence of edge-trimming on the surface figure of a reflective mirror. First, the influences of both self-gravity and the supporting method in measuring on the surface figure of the same mirror before and after edge-trimming are simulated. The simulation results show that both self-gravity and the supporting method in measuring have little influence on the surface figure. Second, the real surface figure of the mirror before edge-trimming is calibrated. Third, edge-trimming experiment of the mirror is carried on the ultrasonic grinding machine. Forth, the real surface figure of the mirror with the same effective aperture after edge-trimming is calibrated. The measuring results show that the surface figure of the mirror after edge-trimming is changed greatly in comparison of that of the mirror before edge-trimming: the surface figure of the mirror before edge-trimming is that the root-mean-square value (RMS value) is about λ/40 (λ=632.8nm), while the RMS value of the surface figure of the mirror after edge-trimming is about λ/15. At last, the mirror is polished again. The final RMS value of the surface figure of the mirror is about λ/40. The experimental results show that edge-trimming can bring stress to the mirror. Therefore, edge-trimming influences the surface figure of the mirror considerably. The consequent polishing experiment can erase the influence. The experiment shows that edge-trimming can be carried on with the reflective mirror with lower surface precision. The whole experiment will provide some helpful reference on the choice of edge-trimming occasion of the reflective mirror if it is necessary.
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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