Research On Stitching Errors Analysis And Control Technology For Large Synthetic Aperture Telescope

Wherever on the ground or space, the developing trend of astronomical telescope is to enlarge the aperture size for the high-resolution observation.In a sense, the observing ability for one telescope is always defined according its aperture size, however, the design, manufacturing, machining and testing suffer unprecedented challenge with the augmenting diameter of telescope aperture. Segmentation has enabled astronomers to build larger and larger telescopes. However,new problems arise with segmentation such as the misalignments of the segments in piston, tip and tilt, the inter-segment gaps, the aberrations of individual segments,and the variations in reflectivity between the segments,all these effects will affect the image quality of a segmented mirror. Thereby these segments need to be properly co-phased to form a primary mirror.In this paper,in response to the “2m segmented infrared telescope” proposal by Shanghai Institute of Technical Physics, Chinese Academy of Sciences for researching on the next generation of astronomical physical mission, the stitching errors of the large segmented primary mirror are investigated from both theory and experiment.The optical design for a 2m infrared telescope with segmented primary mirror is presented in this paper,and the lightweight design and support of the 2m-SiC primary mirror is given, the stress and deformation analysis for the lightweight mirror according to its operating modes are carried out with the finite element software.The misalignment errors of the segmented primary mirror are analyzed by the theory of geometrical aberration. Wavefront aberration representation is derived through aspheric surface equation, the relation between wavefront aberration and misalignment error is established.Due to some conditions to rotational symmetry,the wavefront aberration polynomial expansions of the segmented primary mirror are presented herein.The names of the wavefront aberration coefficients and their relationship to the Seidel aberration terms are given.The five monochromatic aberrations coefficients of Spherical aberration,Coma,Astigmatism, Field curvature and Distortion can be obtained from the Zernike polynomials coefficients.The simulation model is set up to interferometric test the segmented primary mirror based on optical design software ZEMAX, and the relation between stitching errors and aberrations in terms of fringe Zernike coefficients is simulated and analyzed by using this model. A Python-based program is compiled in order to establish communication with ZEMAX, the program can automatic obtain Zernike fringe coefficient data from ZEMAX by Dynamic Data Exchange(DDE), and thus can help tremendously to improve the efficiency of simulation.Based on aberration theory and optical software simulation, a conclusion that effects of all stitching errors of a parabolic reflector on wavefront aberration are linear superposition is presented in this paper. According to this conclusion, a linear retrieval algorithm of stitching errors base on sensitivity matrix is proposed.The stitching principle prototype and its experiment platform are completed. By Offner null test, the single aspheric sector-shaped segment stitching experiment has implemented using computer to control stepper motor. The data of stitched experiment have analyzed by the optical data analysis software SAGUARO, the result of data analysis verifies the validity of the Zernike coefficients.