| As the key component of large-aperture space telescope with long focus,the surface accuray and the position precision of the large-aperture mirror have a direct influence on the imaging quality of the optical system.With the improvement of the imaging quality and detection sensitivity of space telescope,larger aperture mirror are required to develop.In general,the manufacturing,adjustment and detection of space telescopes are completed on the ground,then the telescpes are sent into space on carrier rockets and work in microgravity space environment.The complex thermodynamic environment such as vibration,impact,gravity released and temperature variation puts forward higher design requirements for the development of the space mirrors.Despite of the increase of the space telescope’s weight and the launching cost,the increase of the space mirror’s aperture heightens the sensitivity of the mirrors to gravity,temperature and other environmental factors which will reduce the surface accuracy of the mirror.In addition,the special conditions for the adjustment of the mirror on the ground,the launching into orbit and the on-orbit operation require its supports with good structural stiffness and adaptation to temperature stress.Therefore,it’s one of the key technologies for the successful development of space telescopes to design and optimize the large-aperture space mirror and its supports so as to minimize the launching weight while meeting the requirements of the surface accuracy.In this paper,based on the 1.2 m mirror and its supports of a space telescope,the research work are as follows:(1)A lightweight design method for large-aperture mirror is proposed.According to the requirements of the optical performance,weight and working environment for the 1.2 m mirror,the initial lightweight structure of the mirror is designed with the traditional experience and theoretical formulas.Based on the ground structure method,the topology optimization design of the initial structure is carried out.According to the contributions to the mirror’s mechanical properties,the stiffeners and the semi-open back plate of the mirror are grouped respectively,and a kind of light-weight structure of the semi-open back mirror is obtained.Based on the NSGA-II algorithm,the structure sizes of the mirror are optimized by using the Isight integrated software aiming at the weight of the mirror,the surface accuracy of the mirror generated by the self-weight and the surface accuracy of the mirror’s optical manufacture.In the end,the weight of the optimized mirror is only 68.4 kg and the areal density is 58.5 kg/m~2.The surface accuracy RMS in the condition of radial-oriented gravity is 3.3 nm,which is superior toλ/50(λ=632.8 nm).(2)Based on the compliance matrix of bipod flexure,its mechanical and functional characteristics are analyzed.The compliance matrix of bipod flexure is established based on screw theory.Then the freedom degrees and parasitic motion of bipod flexure are studied based on the matrix:the freedom degrees of bipod flexure are studied through decomposing the characteristic motion screw and characteristic wrench of bipod flexure’s compliance matrix,and the key parameters of bipod flexure on the sensitivity of the flexure’s freedom degrees are analyzed.The the bipod flexure’s parasitic motion is studied based on the flexure’s rotation center,and the parasitic motion under temperature variation and gravity are analyzed respectively.Finally,the compensation method for the parasitic motion of bipod flexure is obtained.(3)An optimization method of bipod flexure is proposed.Based on the compensation method of bipod flexure’s parasitic motion,the multi-objective optimization design model of bipod flexure is established.With the model,the key parameters of bipod flexure are optimized by NSGA-II algorithm.Then the finite element model of the mirror assembly is established,and the static analysis,modal analysis and dynamic response analysis are carried out.The surface accuracy of the assembly in the condition of radial-oriented gravity or under the condition of 2°C change in temperature is mostly superior toλ/50(λ=632.8 nm),and the assembly’s fundamental frequency is 222.65 Hz.(4)A kind of adjustable bipod flexure for large-aperture mirror of space telescope is proposed.The proposed flexure support can decrease the surface distortions caused by the machining error and the assembly error of the mirror assembly in a horizontal optical testing layout.Through the analysis of the compliance matrix of conventional bipod flexure,the positional relationship between the rotation center and the nominal rotation center of the flexure is investigated.Then,the principle of the adjustable flexure,known as the trapezoidal switching principle,is proposed based on the analysis result.In addition,the inverse motion of the adjustable flexure is analyzed and its motion characteristics are analyzed.Finally,the finite element model of the mirror assembly with adjustable bipod flexure is established,the optical characteristics of the mirror are analyzed,and the working principle of the adjustable flexure is verified.(5)In order to verify the support principle of bipod flexure and the practicability of optimization design method,the test mirror component with the same support scheme is designed with the same design method as in the paper,and static mechanical test of the mirror assemble is carried out. |
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