Research On Secondary Mirror’s Correction Technology For The Misaligned Wave Aberration Of Large Aperture Space Remote Sensor

Space astronomical observation and space remote sensing technology have achieved rapid development,large aperture and long focal length have become one of the main development trends.Space remote sensor is subject to the combined effects of changes between ground gravity and space micro-gravity,vibration and shock during launch,temperature changes and other factors on-orbit.As a result,the optical components in the remote sensor generate slight deformation(causing the surface shape error and pose error),making the optical system to be misaligned and inducing additional wave aberration.With the aperture and focal length becomes larger,when the misalignment wave aberration caused by gravity,vibration,shock,temperature and other factors engender a great impact on the on-orbit imaging performance,in some advanced space remote sensors,active optical correction technology has been proposed and adopted.At present,the active optical correction technology for large aperture space remote sensor mainly includes the primary mirror surface shape correction technology and the secondary mirror pose adjustment technology.The surface shape correction technology is to adjust the surface shape of the mirror by the actuator distributed at the back of the mirror to apply axial force to the mirror body,so that the surface shape accuracy of the primary mirror meets the requirements.The secondary mirror adjustment technology is to adjust the pose of the secondary mirror in multiple degrees of freedom through the manipulator to correct the pose error of the secondary mirror and compensate the misalignment wave aberration caused by other mirrors in the optical system.Compared with the primary mirror surface shape correction technology,the secondary mirror pose adjustment technology can provide more adjustment freedom for the optical system.It is also a function that the large aperture lightweight remote sensor must have when using the body folding and unfolding solution.It is more concise and effective,and can ensure the adjustment consistency of the optical path of the system.The use of this technology is of great significance for reducing the fabrication difficulty,shortening the development period and reducing the cost of large aperture space remote sensor.This paper aims at the development needs of China’s future large aperture space telescope,and a large aperture lightweight space telescope design is carried out.After the discussion of the key technologies of the telescope,this research focuses on the key technology in correcting the misalignment wave aberration of the optical system through the secondary mirror pose adjustment function.According to the current development need of China’s large aperture space telescope and the key technology and manufacture level,a 2.4 meter diameter lightweight space telescope is proposed.Through analyzing the misalignment of the optical system,and then combining the aberration theory and Zernike polynomial,the aberration correction model of the optical system based on the secondary mirror pose adjustment is established.Through the finite element analysis,the misalignment of each optical element in the optical system under the action of ground gravity is obtained,and the effect of correcting the gravity misalignment of the optical system through the secondary mirror pose adjustment is simulated.The possible misalignment after the space remote sensor is launched in orbit is estimated,and the correction ability of the secondary mirror pose adjustment on-orbit to the misalignment of the optical system is further analyzed.The requirements of the misalignment correction of the optical system for the pose adjustment ability of the secondary mirror are clarified,and the detection method of the misalignment wave aberration of the large aperture optical system is briefly introduced.According to the characteristics of secondary mirror’s pose adjustment,six-degree-of-freedom(DOF)platform is chosen to achieve the adjustment function.Aiming at the requirement of high pose accuracy of secondary mirror adjustment,the kinematics of the six-DOF platform was analyzed and the control algorithm was studied.The closed-loop vector method is used to establish the inverse kinematic algorithm of the platform;the forward solution algorithm of the platform based on iterative approximation is established,and the Newton-Raphson numerical iterative method is used for solving the algorithm.The Jacobian matrix of the six-DOF platform is modeled and analyzed,and the workspace of the six-DOF platform is analyzed.Finally,the motion control algorithm was verified by the full constraint virtual prototype method.The content of this chapter can guide the six-DOF platform mechanism design and motion control implementation.To meet the special requirements of space application,the actual performance test and experiment of the secondary mirror adjustment mechanism were carried out.According to the design scheme of the secondary mirror adjustment mechanism,an engineering prototype was developed.The actual stiffness,motion resolution,stroke range and some special mechanical properties of the mechanism were tested on the engineering prototype.Pose accuracy is the most important performance requirement for the secondary mirror adjustment mechanism.In response to the high precision requirements,the kinematic calibration of the mechanism was further carried out.Through error analysis,the main error source of the mechanism is clarified.Taking the position error of the hinge points of the platform and the zero position length error of the driving rods as the calibration parameters,a calibration model of the mechanism is established,and a calibration algorithm based on the inverse kinematics solution is used for solving.In actual calibration,the actual pose output information of the mechanism is measured by laser tracker,and iterative calculation is performed through the calibration algorithm to obtain the actual parameter error of the mechanism,and the motion control parameters are corrected to achieve error compensation.After calibration,the pose of the platform is measured again and compared to the results before the calibration to evaluate the calibration effect.As a key technology for China’s future large aperture lightweight space telescope,this subject aims to explore the technical implementation plan for large aperture space remote sensor to achieve misaligned wave aberration correction by secondary mirror pose adjustment.This research can provide key technical support for the development of large aperture remote sensor in future.