| The use of aspherical elements, which not only can significantly improve the performance of the optical system, but also can simplify the system structure, can reduce the weight and cost of the optical system. Therefore, aspheric mirrors are widely used in the civil, military, astronomy, aerospace and other scientific fields. However it is hard to find the right tools, standard and unified examination methods for the aspheric machining and manufacturing. The aspheric machining and testing method is far complex and difficult than the spherical mirror. Especially the testing of large diameter and high precision aspherical mirrors in the process of manufacturing is particularly difficult.At present, the manufacturing of large diameter aspheric optical mirror mainly includes milling, grinding and polishing stages. Surface testing techniques mainly include profile measurement and optical interferometry. Profilometers generally used in grinding stage and interferometers generally used in the polishing stage. However, in the transition stage from grinding to polishing(fine grinding and coarse polishing), the accuracy of the common profilometers is not high enough, the sampling density is low and the time of testing is long, which will influences the processing efficiency. At the same time, the dynamic range of the interferometer is small, it is difficult to test the full aperture surface error in the transition stage when surface error is usually large. So both profilometer and interferometer can’t guide the optical processing effectively in that stage.Fringe reflection method, as one kind of effective optical detection means, has the advantages of simple structure, large dynamic range, high measurement precision, high detection speed, good anti-jamming performance, low cost and easy operation. And it can realize the aspheric surface shape measurement with large diameter and large deviation without other auxiliary components. It not only solved the problem of profilometer and interferometer failing to link up the measurement range effectively, but also reduced the cost, shorten the processing period. In view of these, this article puts forward a method using fringe reflection method to test aspheric mirror surface shape in the fine grinding stage and in the early polishing stage. The research works of this paper mainly include the following four parts:1) The study on related theories of software configurable optical test system(SCOTS). First, we introduce the principle of SCOTS to detect surface slope and shape. Then we select an axis symmetric spherical mirror, whose diameter is 100 mm and vertex curvature radius is-1000 mm, as the testing mirror in simulation test system. We verify its detection principle by ray tracing function found in CODE V to track light trajectory in the working process of the fringe reflection detection system. Finally, we study on the experiment of SCOTS and the result validates that the accuracy of the testing system can meet the need of aspherical mirror testing in the processing of fine grinding stage and early polishing stage.2) The study on SCOTS calibration technology. The calibration precision of reflection ray in SCOTS will reduce because of the existence of aperture imaging aberration and image noise. So we proposes a new reflection ray calibration method by moving LCD and using phase tracking technology to find common phase point to improve the reflection ray calibration precision in fringe reflection optical 3D surface shape measuring. The effectiveness and superiority of the proposed method are verified by experiment.3) The study on fringe reflection testing based on PMD. Camera calibration error will reduce the testing accuracy of the fringe reflection testing system greatly, so we proposes a simple, reliable, high precision and new method of 3D surface shape detection. This method measures the absolute height and gradient of the large diameter reflection mirror under test with incident light which is got by moving the LCD display, coordinates of the camera holes and virtual auxiliary surface. This method can reduce the requirement of laboratory equipment location, it doesn’t need to calibrate the reflection rays into the camera and it can avoid the influence of the calibration error of the reflection ray in detection process. The novel testing method has high detection accuracy without complex camera calibration and it is verified by simulation and initial experiment.4) Experiment on testing large aperture aspheric mirrors by fringe reflection method has been done. An off-axis parabolic mirror, whose diameter is 821.65 mm and off-axis magnitude is 550.57 mm, has been tested by fringe reflection method. The testing result is coincide well with the result got by interferometry, which proves the feasibility of the method of fringe reflection method used to test large aperture aspheric surface, and it also proved the effectiveness of the calibration method and fringe reflection method based on PMD used to test large aperture aspheric surface with full aperture in the transition stage. |
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