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Nowadays, more and more large aperture aspherical mirrors are wildly used in the optical system, many of them with fast ratio and large asphericity. When manufacturing these aspherical mirrors, we can only choose the null testing method, so the null compensators for these surfaces are required. For reduce the residual error and easy to process, the compensator used in the null testing usually accompany by the existence of the ghost image, customarily the ghost image is considered to be useless. In this research we analyze the relationship between the ghost image and the optical axis of the system by study the optical properties of the null testing, find out that adjusting the misalignment of the optical axis not only should eliminate the coma term, but also should rectify the field, the center field also the correct optical axis means that the ghost image should be at the center of the interference image. Then we discuss the auxiliary role of the ghost image in the measurement and adjustment, and apply that to the experiment: null testing a 1.6 meter aspherical mirror by the compensator, which has obvious ghost image in the interferogram, by combined adjusting the compensator and the interferometer during the operation we finally get the optimized method. The result shows that according to the state of the ghost image and with the help of the scientific adjusting method, we can quickly and accurately realize the correction of the optical axis when null testing the aspherical surface by the null compensator, that has important scientific guiding significance for modern manufacturing and testing of the large aperture aspherical mirrors.
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