| Mirrors and their supporting structures play a pivotal role in catadioptric and off-axis reflective optical imaging systems,and they are of critical components of high-resolution aerial cameras.It has a great influence on the volume and weight of the photoelectric imaging system and the imaging quality in the complex airborne environment.Reducing the sensitivity of the mirror support structure to environmental changes and improving the stability of the support structure play an important role in ensuring image quality.Based on the development of an aviation dual-band reconnaissance camera,this paper optimized the design of the primary mirror and supporting structure in the cassegrain primary optical system through the method of integrated opto-mechanical analysis,for improving the performance of the primary mirror and its supporting structure,optical performance and environmental suitability.The main research contents and results are as follows:(1)The material,lightweight and supporting structure form of mirror blanks domestic and overseas were summarized,and the working principle of the flexible hinge was analyzed.According to the initial structural design and material selection of the primary mirror assembly studied in this paper,the aberration components of the primary mirror deformation under the action of +X,+Y,-Z direction gravity,uniform temperature change 30°C,and planarity were analyzed.The effect of these factors on the optical modulation transfer function was also analyzed.The influences of the mirror diameter-thickness ratio,the support rate and the number of mounting points on the surface error of the primary mirror under the action of the optical axis along the gravity direction and the uniform temperature change 5°C were obtained respectively,and the relationship between the maximum deformation and the surface error was obtained.(2)A method based on parameters optimization theory was proposed to improve the thermal adaptability of the primary mirror by combining the optimal design of the mirror and its supporting structure together from the structural point of view.A finite element model for parameter optimization of the primary mirror assembly was established,and the RMS value of the mirror surface error caused by uniform decrease was treated as the objective function.Based on the gradient optimization algorithm,the size parameters of the primary mirror and the flexible hinge were optimized in detail,and the size parameters configuration that meeting the design indicators was obtained.The sensitivity of dimension parameters to surface error,fundamental frequency and weight were further evaluated,and the key dimension factors that affected the response function were picked out.The results show that the size parameters exert a notable impact on the optical performance.The surface error caused under uniform temperature decrease 30°C is significantly reduced from 26.5 nm to 11.6 nm.The thermal adaptability of the primary mirror is obviously promoted,and the performance of the optical system under thermal load is improved.(3)Based on topology optimization and parameters optimization theory,a method was exhibited to improve the performance of the primary mirror assembly by combining the optimal design of the mirror and its supporting structure together under thermal load.A finite element model for topology optimization of the primary mirror assembly was established.Taking the surface error RMS caused by 30°C uniform temperature decrease as the objective function,the primary mirror and the flexible hinge were optimized based on the pseudo density method and level set function method respectively,and the topological configuration of the primary mirror and the flexible hinge were obtained.The results show that the surface error obtained by the level set method is reduced from 17.5 nm to 3.2 nm.The surface shape error obtained by the variable density method is reduced from 15.9 nm to 6.5nm.The temperature adaptability achieved by the two methods is obviously improved,but the machining process of the latter is better.According to the material distribution results,the features were extracted,and the structure model with reinforced ribs was constructed.(4)A finite element model for parameters optimization of the primary mirror assembly based on the topology optimization results was established,and the size parameters of the primary mirror and flexible hinge were optimized.The dimensional parameters were evaluated for their sensitivity to surface error,fundamental frequency and weight.The results show that the RMS value of the surface error caused by 30°C uniform temperature decrease is reduced from 54.1 nm to 7.5 nm.After optimization,the MTF value of 0°field of view of the optical system reaches 0.29,and the MTF value of the full field of view reaches 0.23.The design results of parameters optimization are better than the results of direct parameters optimization,the thermal deformation resistance of the primary mirror assembly is further reinfored,and the image quality of the camera is also significantly improved.(5)An environmental adaptability study was developed on the optimized mirror assembly.When the primary mirror moved with pitch and roll,the changes of mirror rigid body displacement,surface shape error and main aberration components were analyzed.The effects of different uniform temperature changes on the surface error of the primary mirror and on the optical modulation transfer function were exhibited.Through finite element simulation,the canonical mode,frequency response and random vibration analysis of the primary mirror assembly were implemented.The results show that the optimized primary mirror assembly has sufficient safety margin,which can withstand random vibration test conditions.(6)Based on the optimization results,the principle prototype was fabricated,and the surface interference measurement,optical transfer function test,ground temperature test,and target imaging test were carried out.Through the ground temperature test,imaging tests were performed at room temperature and low temperature,respectively.Tests shows that the image quality of the camera is degraded to a certain extent contrast to low temperature,but still meets the index requirements. |
PDF Full Text Request