Objective Vertical assembly and adjustment is one of the key technologies of long focal length and large aperture space camera. In order to overcome the inconsistency between the on-orbit and the ground caused by gravity change, material deformation and other factors, the secondary mirror adjustment has become one of the key technologies to correct the defocus of the optical remote sensor and the relative position change of the primary mirror and the secondary mirror. Precision secondary mirror adjustment technology has been widely used in high-resolution space optical remote sensors. For example, Stewart platform 6-DOF parallel mechanism, which has been successfully applied in Hubble telescope and reconnaissance camera, has the advantages of high accuracy, high bearing capacity and high rigidity, and has the ability to precisely adjust the secondary mirror components in the optical system. Many theoretical analysis and engineering research have been done on the 6-DOF adjustment mechanism in China, but the 6-DOF adjustment mechanism also has the disadvantages of complex structure and control system, high cost and relatively large weight. Therefore, it is necessary to develop a single-degree-of-freedom secondary mirror adjustment mechanism with high accuracy, high integration and high reliability to solve the inconsistency between heaven and earth faced by the current high-resolution space optical remote sensor.
Methods In order to meet the secondary mirror adjustment requirements of a high-resolution camera, a high-precision and high-stability secondary mirror adjustment mechanism combining precision linear transmission, flexible transmission and flexible support technology is built in this paper (Fig.1). The linear transmission device （Fig.4） adopts the redundancy design of one main and one standby, and has precision position telemetry capability. The flexible transmission hinge with transmission ratio of 10∶1 is used for motion transmission. Compared with the traditional gear reducer, it has the advantages of small impact, no wear, stable transmission, and high reliability. At the same time, the flexible hinge has the advantages of high-precision transmission in the range of small displacement. The secondary mirror uses bipod flexible support to design unloading force thermal deformation, and ensures its flexibility along the optical axis direction (focusing direction) through three pairs of 120° flexible guide hinges.
Results and Discussions This set of precision adjustment mechanism has carried out adjustment precision test after completing the mechanical environment assessment. The test is carried out according to 0.088° rotation of step motor (corresponding theoretical step distance of secondary mirror 8.858 μm). The initial position of the secondary mirror is zero. The secondary mirror is drived to complete the whole adjustment cycle of "zero position→positive limit position→zero position→negative limit position→zero position". The adjustment accuracy test results after the mechanical environment assessment of the optical and mechanical products of the adjustment mechanism show that the mechanism has the ability to achieve high-precision adjustment in a large range (Fig.8-11), and meets the requirements of the on-orbit application of space optical remote sensor.
Conclusions In this paper, a set of high-precision secondary mirror adjustment mechanism is designed by combining the flexible support, precision linear drive and flexible hinge transmission technology of the second mirror. This paper first introduces the optical and mechanical structure, working principle and transmission link of the mechanism, then describes the design of ultra-light secondary mirror assembly, high-precision linear actuation and high-precision focusing transmission, and finally introduces the adjustment accuracy test after the vibration test. The test results show that the measured adjustment stroke of the set of precision adjustment mechanism is more than ±120 μm, the axial adjustment step precision is 0.18 μm, the maximum translation error of the secondary mirror within the adjustment stroke is 1.3 μm, and the maximum tilt error is 1.9″. It has the characteristics of wide adjustment range and high adjustment accuracy, and meets the requirements of the precision secondary mirror adjustment of the space optical remote sensor.
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