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Tolerance study of the sub-mirror's surface shape and misadjustment for segmented mirror synthetic aperture optics (SAO)

An experimental demonstration system of segmented SAO is built, which is a Cassegrin-type telescope, to demonstrate the segmented mirror technology, and the aperture is 300mm, F/number is 4.5. The primary mirror consists of three sub-mirrors, and each sub-mirror has five degrees of freedom. In order to get the diffraction limit in the wavelength of 632.8nm, ZYGO interferometer is used to sample the system's wavefront, and evaluate the performance. Although each sub-mirror of the experimental demonstration system has high precision surface figure, the difference of machining and the edge effects caused by change of environment and release of stress, makes the surface figure inconsistent, causes wavefront root-mean-square (RMS) value to become large, and badly affects the imaging. Based on the measurement and tolerance budget, other systems with 4, 6, 8, 24 and 36 sub-mirrors are simulated to analyze the requirements of the surface figure and misadjustment tolerance. Inverse limit sensitive tolerance evaluation is used to define the tolerance requirements and tendency. And the result shows that the sub-mirrors' number, configuration, location, precision, consistency and assembly should be considered and treated as a whole . And at the beginning of the system assembly, the sub-mirrors' surfaces figure must be measured to reassign the tolerance budget of the whole system.