Study On Cross Test Of Aspheres With Combined Multiple Techniques

With the urgent demands and fast development of large optical projects such as ICF, deep space exploration, and high resolution earth observing systems, large complex aspheres have been widely used for their predominant optical performance. The reliability of the surface figure testing result which plays an important role in determining the quality of optical imaging is crucial to the success of the projects. Methods of aspheric surface figure metrology become rich diversity with distinctive features, and with respective limitations at the same time. It is not enough to get the measurement result with a single method. Cross tests with multiple techniques in accordance with the characteristic of the surface under test to strengthen the complementary advantages of methods may be an effective solution. The thesis focuses on optical measurement of large convex high-order aspheres with widely accepted methods including null test, back-through test, and subaperture stitching interferometry. Matching analysis and quantitative evaluation of the testing results obtained with different methods are studied to finally improve the reliability.This thesis comprises four parts of contents:1. Optical design and implementation of back-through test solution with null lens. According to the back-through transmission imaging characteristic of the aspheric surface, a singlet null lens is designed. Machining tolerances of the null lens are analyzed. Then the interferometer and spherometer are used to measure the surface figure and central thickness after machining, respectively. During null test, precise alignment of the optics involved in the test system is achieved through a thickness measurement instrument based on low coherence interferometry. After that, impacts of instrument measurement error and glass homogeneity are estimated, and manufacturing errors are also separated by Zernike polynomial fitting method.2. Improvement of adjusting device of the variable null optics of subaperture stitching interferometry. The study starts from aligning freedom requirement analysis of the optics, which consists of two counter-rotating CGH plates. A novel device of three concentric rigid rings is designed to meet the need of four-degree freedom alignment of each CGH plate, with important features of fairly compact size along the optical axis and large center through-hole. Variable compensation of aberrations is realized through mounting the CGH mounts on rotatory tables. Adjusting mechanisms of the tables and the test mirror are also designed to complete the aligning freedoms. The alignment efficiency is verified by experiments.3. Design and implementation of front null test of the mirror. A large aspheric null lens is designed to test the convex aspheric surface as a mirror. The aspheric null lens itself is also calibrated with Offner null lens. This method is applicable to convex aspheric reflectors of non- transparent materials. Light path through the aspheric null lens is identical in the testing and calibration layouts, which avoids the influence of optical homogeneity of the large aspheric null. The measuremnt errors are then analyzed.4. Study on cross test with multiple techniques and quantitative evaluation of the results. Seven parameters are optimized including relative rotation, translation and scaling, to best match two results in the form of error maps. It is realized with Matlab optimization tool. Quantitative evaluation of the results is achieved with SSIM index algorithm after image matching. Bayesian analysis method is also tried to reduce the measurement uncertainty.