Research On Lightweight Design And Support Technology Of Space Mirror

Nowadays,space telescopes,as one of the important means of cosmic exploration,earth observation and civil information,have become one of the key research fields of various countries.With the increasing demand of resolution and detection distance of the space telescope,the aperture of the space telescope also increases.As the most important optical element in the space telescope,its performance directly determines the imaging quality of the space telescope.The increase in the diameter of the mirror will lead to an increase in the weight of the space telescope system.In order to reduce the launch cost,it is necessary to carry out a lightweight design for the large-aperture mirror.However,due to the reduced structural rigidity of the lightweight mirror,the sensitivity to the support structure increases.To ensure the good surface shape accuracy and position accuracy of the mirror,a high-performance support structure must be reasonably designed.Therefore,the lightweight design and high-performance support technology of large-aperture mirror is the focus and difficulty of space mirror research,which is also of great significance to the technological development of large-scale space telescopes.The space mirror is an important optical component of an off-axis Three Mirror Anastigmatism telescope under development.How to ensure the high surface shape accuracy of the mirror and improve the lightweight rate of the mirror is the key difficulty.Based on the research status and development trend of spatial mirror and support structure,the optimal design method of mirror components is expounded.Traditional experience design method according to the mirror diameter,design index and support form to obtain the mirror lightweight form,and use the empirical formula to obtain the mirror structure size,the method cannot obtain high performance mirror lightweight structure,this paper puts forward a combination of topology optimization design and integration optimization design of mirror lightweight method.The initial lightweight structure of the mirror is obtained according to the traditional empirical design method,and a comprehensive topology optimization objective function based on static compliance and dynamic stiffness is constructed by the compromise optimization method.After reasonable thickness grouping,a lightweight structure of a mirror with a closed hexagonal hole is finally obtained.Combined with the Isight integration optimization method,with the surface error RMS_L generated by radial gravity and mass Mass as the optimization target,with the first order natural frequency as the constraint,using the NSGA-II algorithm to optimize the 800mm aperture mirror structure,the mirror lightweight rate reaches 72.1%,the surface accuracy RMS_L is 3.42nm,better thanλ/70(λ=632.8nm).The design of the off-axis Three Mirror Anastigmatism telescope system is quite complicated,the feasibility of the scheme is verified by evaluating the optimized performance of different aperture mirrors in the evaluation stage of the scheme.Therefore,the research on the rapid optimization method of space mirrors can realize the rapid optimization design of lightweight mirrors with the same configuration,ensure the optimal structural performance of the mirrors,and effectively shorten the design cycle.Given the limitations and low efficiency of integrated-based optimization methods applied to multi-aperture mirror optimization,this paper constructs a rapid optimization method for the mirror structure with diameters ofФ700mm～Ф1000mm based on the IPSO-IAGA-BPNN neural network surrogate model.In order to overcome the shortcomings of traditional Back-Propagation Neural Network(BPNN),an improved Particle Swarm Optimization(IPSO)and Adaptive Genetic Algorithm(IAGA)are used to optimize the connection parameters of BPNN,and a high-precision surrogate model is constructed.Compared with the prediction accuracy of other six different surrogate models,the mean absolute error MAPE of the IPSO-IAGA-BPNN surrogate model for the prediction value of the test set is less than 3%,and the linear coefficient of determination R~2 is greater than 0.99.By using the mirror fast optimization method proposed in this paper to perform multi-objective optimization with RMS_L and Mass objectives for four different aperture mirrors,the total optimization running time is only 0.18%of the time based on the integrated optimization method,and Optimization results for the 800mm-aperture mirror are similar to the integrated optimization results,which respectively verify the efficiency and feasibility of the mirror structure optimization method based on the surrogate model.In the ground test and on-orbit imaging stage,the space mirror support technology is the key technology to ensure that the mirror has good surface shape accuracy and position accuracy.The study of the flexible support structure with high stiffness,high stability and large rotation angle,is conducive to improving the surface shape accuracy and structural stability of the spatial mirror,and meeting the requirements of high-resolution imaging.The paper determines the side three-point support as the mirror support scheme according to the mirror exact positioning theory.A two-axis Bipod flexure(TBF)with large rotational capacity,compact structure and high axial stiffness is proposed.We derive the closed compliance matrix of the TBF based on screw theory and Castigliano’s second theorem and verify the closed compliance equation by comparing finite element analysis with theoretical calculations.Based on the compliance equation,the parasitic motion under the gravity,temperature change and the axial support position were studied,and the sensitivity of the size parameters of the TBF to various performance indicators was analyzed,and the optimal design interval and optimization target of the support structure were determined.The Isight optimization link combined with the NSGA-II to optimize the mirror components yields the optimal TBF size parameters.After static analysis,the RMS_T under 4℃thermal change is 4.44nm,the surface shape accuracy RMS_L and rigid body displacement under1G horizontal gravity are 2.26nm and 4.22μm respectively,the natural frequency is250Hz,and the design structure meets the requirements of optical design indicators,which verifies the effectiveness and feasibility of the design method and optimization model.There are many test and inspection links involved in the processing and adjustment of the space mirror assembly.Correctly verifying each link is the basic requirement for the successful development of the space mirror assembly.In this paper,the experimental verification of the 800mm-aperture mirror prototype and key technologies is carried out.The influence of mirror bonding process on bonding strength was explored by means of experimental design.The three-direction translational compliance and rotational compliance of the TBF are measured,which verifies the correctness of the theoretical derivation and simulation analysis of the TBF compliance matrix.The rationality of the bonding process and the reliability of the structural design of the mirror assembly were verified by combining the test results of the mirror shape consistency before and after gluing and the surface shape test results supported by the mirror.The feasibility and stability of the design method of the mirror support structure are effectively verified through the dynamic test of the mirror simulation assembly.