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.
There are numerous reflecting optical system designs that call for large-aperture convex surfaces, such as secondary mirror in on-axis three mirror anastigmatic (TMA). Several methods to test high accuracy hyperboloid convex surfaces are introduced separately in this paper. A kind of arrangement is chosen to test a surface with diameter of 420mm, radius of 1371mm, and conic K -2.1229. The CGH compensator for testing is designed, which is made up of illumination lens and hologram test plate with designed residual wavefront aberration less than 0.001λ (RMS). The second transmitted method that is equipped with a technical flat surface coating by Ag film in the bottom of surface mirror under test, which form an auto-collimation optical system to eliminate the aberration. The Hindle-Simpson test that requires a larger meniscus lens to compensate the optical aberration, and the designed result of optical test system is less than 0.0016λ. Contrasting the CGH compensator and the second transmitted method, the Hindle-Simpson testing method has the advantage of it is easily to manufacture and adjust; meanwhile the test result is stable and has been less affected by the environment. It has been found that the method is rational and reliable, and it can fulfill the requirement of manufacturing and testing process for hyperboloid convex mirrors.
A zerodur mirror whose aperture is 900mm is chosen to be the primary mirror of an optical system. The mirror is polished by rapid polishing and precision polishing methods relatively. The final surface figures of the mirror are as follows: the peak-to-valley value (P-V value) is 0.204λ (λ=632.8nm), and the root-mean-square value (RMS value) is 0.016λ, which meet the requirement of the optical system. The results show that the polishing process is feasible.
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