As an important internal parameter of the camera, the measurement accuracy of distortion directly affects the image processing accuracy and the geometric positioning accuracy of the camera on orbit. The traditional high-precision laboratory calibration method relying on three-axis turntable has strict requirements for test equipment and test environment. With the increase of camera focus, aperture and volume, this method has increasingly high requirements for equipment and site. The idea of achieving high-precision geometric distortion calibration simply by improving the volume and accuracy of equipment is not applicable. On the basis of the traditional precision angle measurement method, this paper proposes a geometric distortion calibration technology of large aperture and long focal length optical system based on the interference principle. Compared with the traditional precision angle measurement method, this method does not require a high-precision experimental turntable, has good robustness and high accuracy. This paper introduces the basic principle, test method and error link of the distortion test method. The test results of this method are compared with the traditional distortion test method, which shows that the test accuracy of this method meets the requirements of remote sensing camera development and has a wider application range. It can be used for reference for the distortion test of aerospace long focal length and large aperture remote sensing camera.
Objective As an important internal parameter of the camera, the measurement accuracy of distortion directly affects the image processing accuracy and the geometric positioning accuracy of the camera on orbit. At present, the distortion test methods for optical cameras are generally divided into three categories, the high-resolution spaceflight remote sensing camera is more suitable for the test method based on the precision angle measurement theory. However, the precision angle measurement method has strict requirements for the test equipment and the test environment. With the increase of the focal length, aperture and volume of the spatial high-resolution optical system, higher requirements are also put forward for the size and test accuracy of the turntable. It is difficult to realize in engineering and cannot meet the development requirements of various types of space high-resolution remote sensing cameras. Meanwhile, for the space high-resolution optical systems with ultra-large aperture and ultra-long focal length, in order to reduce the influence of gravity in the process of alignment, the vertical method is usually used for alignment. The visual axis of the lens is always perpendicular to the earth in this case, and it is impossible to use the traditional precision angle measurement method to calibrate the distortion of the optical lens in the laboratory. In order to solve this problem, a geometric distortion calibration technology of large aperture and long focal length optical system based on the interference principle is proposed.
Methods The whole test system includes laser tracker, special target ball, 4D interferometer, a measured lens, high-precision angle measuring system and plane reflector (Fig.2, Fig.5). In the measurement space coordinate system, the target ball of the laser tracker is placed at the image point (Fig.3). When the center of the target ball coincides with the focus position of the interferometer, the interference self-collimation fringe can be formed (Fig.4). When the target ball is precisely positioned at the image point, the laser tracker can be used to test the image point coordinates to obtain the image height data. The field angle corresponding to the image height of the lens can be obtained by using the high-precision angle measurement system (Fig.6), and the lens distortion value can be calculated by the image height and its corresponding field angle.
Results and Discussions Comparative experiments were carried out on optical lens with a focal length of 2 000 mm and a field angle of 2.8° using the traditional angle measurement method and the distortion measurement method based on the interference principle. The calibration results show that the results of the two test methods are highly consistent, and the maximum relative distortion of the two methods are 1.48% and 1.49% (Tab.1). This shows that the method based on interference principle can meet the development requirements of remote sensing camera. A long focal length and large aperture optical lens is tested with the new method. During the test, a total of 21 test points were taken from the full field of view, and three effective tests were conducted (Tab.1). The root-mean-square distortion of the three tests is less than 3 microns, and the maximum relative distortion value is 1.43%. The maximum relative distortion design value of the lens linear array direction is 1.5% (Fig.7), and the test results are in good agreement with the theoretical design value.
Conclusions Based on the traditional angle measurement method, a distortion measurement method for aerospace large aperture and long focal length optical system is proposed. This method can meet the distortion test requirements of various types of optical systems. It is used to calibrate the distortion of the traditional optical lens and the large-aperture high-resolution optical lens with vertical adjustment in the laboratory. The results are consistent with the design values, which provides a reference for the development and test of space high-resolution optical remote sensor.
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