| Space targets mainly refer to all orbiting spacecraft and space debris in space100 km away from the earth's surface(outer space).With the increasing frequency of space launch activities,the types and numbers of space targets have increased sharply,and the space environment has become more complex.Therefore,real-time wide-field surveillance of space targets is particularly important.Camera arrays were mainly used in the field of panoramic imaging in the early days.In recent years,they have begun to be applied in the field of space target detection.Its wide-field detection capability can provide rich movement trajectory and luminosity change information of multiple targets in real time,with short construction period and relatively low cost.Thus camera arrays and the traditional large-aperture optical detection system for space targets can form complementary advantages.The core of realizing wide-field detection lies in the design and realization of optical system arrays with large field of view,high sensitivity and high detection accuracy.This paper has carried out related key technology research around this issue,and the main research contents are summarized as follows:The thesis completed the layout design of the space target wide-field detection system and the design of the detection performance requirements of the array unit.First,based on the principle of ground-based photoelectric detection of space targets,a detection capability calculation model was constructed,which established a theoretical basis for the design and analysis of optical system arrays.Then,aiming at the problem of efficient and economical layout of array elements,a spherical arrangement method of rectangular field of view array units based on latitude and longitude lines was proposed,which realized seamless splicing and imaging of 83 array units with a field of view of 13.7°×13.7°,covering the sky area above 20 degrees of the horizon.Besides,according to the system design requirements and the sensitivity model of the influencing factors of detection capability,the lens design requirements and distortion calibration requirements of the array unit were determined.In order to solve the contradiction between the field of view of the array unit lens and the detection sensitivity,the thesis deeply studied the design and aberration compensation of the large field of view and high sensitivity array unit lens.Firstly,a reasonable initial structure was selected,and the design based on all spherical surfaces was completed with all kinds of aberrations well optimized.Besides,aberration compensation based on MTF sensitivity was carried out for the designed array unit lens,and a new computer-aided adjustment method was proposed,namely the nonlinear least square method based on MTF sensitivity.This method takes into account the nonlinear characteristics of the MTF sensitivity of the compensation element,thus can achieve better aberration compensation.Its effectiveness is verified through a simulated lens.After compensation,the MTF value of the lens edge field of view is improved from lower than 0.1@20lp/mm to better than 0.55@20lp/mm,which lays the foundation for the actual lens adjustment.The nonlinear distortion introduced by the expansion of the field of view of the array unit lens will affect the detection accuracy and stitching accuracy.In order to solve this problem,the thesis deeply studied the high-precision calibration of the nonlinear distortion.A calibration method based on photographic astrometry was proposed,and the mapping relationship between the ideal coordinates and measurement coordinates of the high-precision Gaia DR2 star catalog reference star was used to analyze the distortion information,so as to realize the ground field calibration of the lens imaging distortion.Aiming at the difficulty of astronomical observation distortion calibration in star map matching and plate model calculation for large field of view lens,a new composite calibration algorithm is proposed.The distortion is initially calibrated by the self-calibration method,and then the astrometric method is used.This composite calibration algorithm can improve the calibration accuracy of large field of view distortion and effectively expand the scope of application of high-precision distortion calibration method based on astrometry.In the aspect of self-calibration method to realize the initial calibration of large field of view distortion,a self-calibration algorithm based on the invariance of straight-line perspective projection is proposed.The algorithm only needs to take one or more images with straight line information.Under the single-parameter Division distortion model,the straight line is imaged as an arc,and the distortion coefficients can be calculated by utilizing more than three sets of arc parameters.The algorithm introduces a feature selection method based on backward sequence selection and a distortion parameter estimation method based on LLS.Experiments show that it has good robustness and calibration efficiency,thus could be used for efficient preliminary calibration of large field of view distortion.Finally,in order to verify the detection capability of the array unit and the accuracy of distortion calibration,a series of star observation experiments were carried out.First,a single lens was applied to the detection ability verification experiment,and the detection of 14.10-magnitude stars was realized under 1s integration time,which met the system requirements.Then two lenses were used to verify the principle of stitching imaging.After the distortion calibration,the mean square error of the reference star residual is better than 4.36?,and image stitching is conducted for the corrected images.Based on the comparison results of different experiments,considering the further improvement of detection ability and reduction of the difficulty of distortion calibration,a Q-type free-form surface-based optical design scheme for the array unit lens is proposed,which realizes that the encircled energy is better than 80%@17?m,the MTF is better than 0.81@20lp/mm,and the distortion is better than 0.06%. |
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