The Low-frequency Surface Error Study For Thin Meniscus Mirror Based On Active Support Technique

The active support technology has been widely used in large-ground telescopesystem. The appearance of the technology revolutionarily reduces the need ofprimary mirror’s stiffness. This paper discusses the correction capacity oflow-frequency surface error for thin meniscus mirror based on active supporttechnique. The aim of discussion is to analyze the compensation ability of differentlow-frequency Zernike modes deformation by EFA. Then we can ignore some mirrorresidue errors in the mirror process. Finally the residue errors can be correct whenthe mirror produces a deformation by active support technique.The active support system design for the1.2m meniscus mirror can bedecomposed into three parts: the mechanical structure; control system and thecorrection forces algorithm. This paper discusses an active support system during thefabrication process for a1.2m primary mirror. An air flotation support device and theworking principle of the support system are expounded. This paper mainly analyzesthe mathematical model of low-frequency errors correction for meniscus mirror andobtains some mathematical models including with the low-frequency Zernikemodels deformation, the computation of correction forces, the computation ofinfluence matrix and the influence of fitting errors. A method about balancingcorrection forces is introduced.After getting the influence functions by37basis deformation, the conclusionwhat the correlation terms make a great infection on the correction forcescomputation is obtained. It proves that the faint fitting errors have a relationship withcorrection forces. The residue errors PV and RMS values are used as the evaluationindex. We know that the Z coordinate values method is better than the formermethods.The limitations of37basis deformation show that the basis deformation couldaffect the correction forces. The basis deformations which compute the influencematrix enhance to61,73and97groups from37. The increasing of basisdeformation can increase the nonlinear parts of the influence functions. It makes theZernike coefficient method computes a better result.Finally the forces coupling problem for the4m mirror is discussed in the fifth chapter. The main content, innovation and conclusion are expressed in the lastchapter.