| Off-axis aspheric mirrors have the advantages of improving the imaging quality of the system and reducing the complexity of the structure.It mainly adopts subaperture processing technologies such as small tool manufacturing,magnetorheological and stress lap,and can obtain excellent low-frequency errors.Stressed mirror polishing(SMP)is a full-aperture polishing technology developed for the construction of largeaperture astronomical telescopes.It can reduce the difficulty of polishing off-axis aspheric surfaces to the level of polishing spherical surfaces.This method has obvious advantages in suppressing mid-and high-frequency errors and is beneficial to improve the processing efficiency and product consistency.So far,this technology has been adopted in the mass splicing sub-mirror processing of the primary mirror of optical astronomical telescopes with apertures of more than 8m in the world.The material of the mirror blanks is low-expansion glass,and the mirror body is not lightweighted.Silicon carbide(SiC)is an optical mirror material with the advantages of stable performance,high thermal conductivity,low thermal expansion coefficient,and high specific rigidity.The technology of sintering,weight reduction and modification of the mirror blanks has been mature,and the single off-axis aspheric SiC mirror has been widely used in ground equipment and space loads.To this end,this thesis focuses on how to transplant the stressed mirror polishing technology of the splicing sub-mirrors of large-aperture telescopes into the processing of SiC mirrors,and launches systematic and in-depth basic research work.It aims to provide theoretical and methodological basis for the specific engineering implementation of off-axis aspherical SiC thin-plate mirror and lightweight mirror full-aperture polishing.Contents include:1.Starting from the approximation of the elastic thin plate of the mirror body in the stressed mirror polishing,the reason why the stressed mirror polishing of the aspheric thin plate mirror is based on the small deflection deformation theory of the elastic thin plate is analyzed.The physical basis of the theory about stressed polishing technology is explained,including the deflection,the spherical surface aberration,the bending plate and the off-axis aspheric surface,and the method of solving the parameters of related equations.The macroscopical frame of finite element analysis methodis analyzed,which is the main mechanical simulation method used in the research work.In addition,the method of solving the coefficient of Zernike polynomials and the relation between the coefficient of Zernike and Seidel aberration are summarized.For the dissertation work to do professional theory and methodology preparation.2.Zerodur materials of off-axis aspheric mirror plate as scale down model of implementing the stressed mirror polishing technology,launched a polishing model,discrete points stress load calculation,material removal simulation,surface residual control,mirror plate shape result determination,and a series of complex operation method of whole process simulation analysis.By combining the theoretical basis of stressed mirror polishing,modeling method and the techniques of co-simulation of various finite element software,the fast switching and calling of Matlab program and finite element analysis software was realized.At the same time,in order to design and develop different caliber off-axis aspheric prestressed loading device,the bonding module,load lever,at the elastic cushion,support module,support columns,mechanical sensor,a mechanical analysis has been done for the buffer rod and other components or layout planning,completed the denotative loading and built-in load such as the concept of two kinds of load of the structure of the prestressed load equipment design.3.The simulation process,the design of the mirror body,the selection of the prestress loading parameters and the processing technology of the off-axis aspherical mirror stressed polishing with SiC thin plate as the mirror blank were firstly studied.An offaxis aspheric SiC thin plate submirror prestress loading model is established,and the frequency doubling method for selecting prestress loading points is proposed.The simulation analysis of full aperture polishing of off-axis aspheric SiC thin plate mirror was carried out by finite element method.The parameters,shape error and prestress of SiC thin plate submirror are determined In terms of the manufacturing process,considering that the hardness of SiC material is large,the removal rate is low,and the surface shape convergence is slow,the time to remove the material of unit thickness through polishing is about 5 times that of glass,and it is enough to offset the advantages of stressed polishing.For the first time,the concept of "initial aspheric surface method" for fast prestressing of SiC thin plate mirror with off-axis aspheric surface was proposed.The corresponding realization steps are designed,which can reduce the removal amount and improve the polishing efficiency compared with the traditional technical route.4.For the first time,the stressed mirror polishing technology and fast machining of lightweight SiC mirror blank off-axis aspheric reflector were studied.Stressed mirror polishing technology is the physical basis of elastic and small deflection plate theory,yet lightweight mirror back is mesh reinforcement structure,not directly use the same as the mirror plate can be solved by the analytic expression of applying prestress distribution,so it will soon be active optical concept and method is introduced into the process of stressed lightweight mirror polishing.By applying unit load on the actuating position of the edge of the lightweight mirror with the finite element software extract the surface shape response matrix of the mirror body of the actuating point,Comparing the light-weight model actuation surface response matrix and the thin plate mirror body actuation surface response matrix,completed the selection of the equivalent thin plate model,and obtained the corresponding loading parameters.An equivalent thin plate method for stress loading of lightweight mirror was proposed,and a face shape removal model was established.The connection between the finite element analysis of the embedded actuated hybrid mirror and the simulation of mirror shape polishing is realized.The concept of "iterative initial aspheric surface method" is proposed for the off-axis aspheric surface SiC lightweight mirror prestressing polishing,and the corresponding mirror blank manufacture and polishing process are designed.Furthermore,the feasibility of the application of stressing mirror polishing technology in the rapid production of off-axis aspheric lightweight mirror is demonstrated. |
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