FEM analysis of ultra thin mirror supporting structure effect on surface deformation in gravity field

摘要

With the development of science and technology, a primary mirror, whose diameter is over 2m ,will be used. Thus , the conceive of ultra thin segmented mirror has emerged ,and developing such a mirror has come to the forefront. Ultra thin mirror with thickness about 2-4mm is an important component of segmented optical system. In order to measure and adjust the mirror surface, it is necessarily important to unload gravity and support the ultra thin mirror in gravity field effectively. This paper describes the effect of supporting project on the surface of ultra thin mirror with FEM. The micro-shift of any supporting point will greatly effect the mirror surface for the ultra thin mirror. Thus it is necessary to choose appropriate form of FEM element which can be used to create the FEM model in high precision and to regularly arrange the node. The scheme makes it possible that the supporting points in the model are accurately shifted in correct direction. The precision of FEM model of the mirror is confirmed by computing and building the model with 20 nodes hexahedron elements. Its unload supporting structure is simulated with simple fixation model, and the effect of the supporting structure on two 500mm-diameter ZORODUR ultra thin mirrors ,whose thickness is 2mm and 4mm,is analyzed. In their horizontal position in gravity field simulation, the RMS of surface can be limited in λ/30 (λ=632.8nm) with proper supporting structure. This aim can be achieved by using less supporting points in vertical position. As a conclusion, the difficulty of designing supporting structure and adjusting the ultra thin mirror surface can be minished in vertical position.