Objective In order to effectively deal with the interference of gravity error in the installation and adjustment of large aperture remote sensing camera on the surface, the installation and adjustment mode of large aperture remote sensing camera is gradually changed from the horizontal form of optical axis to the vertical form of optical axis. Correspondingly, the support structure and assembly requirements of the main mirror of the camera are also constantly innovated. In order to meet the needs of mirror support and weight reduction, the Bipod structure is a more common structural form in the form of mirror support of 1 m and above caliber. Different from the traditional frame support form, it has a great change in the optical and mechanical structure bonding and structural position calibration needs. In order to ensure that the assembly positioning accuracy and surface shape change of the mirror under the Bipod support structure meet the requirements of the system index, a high-precision assembly method for the Bipod support structure is proposed.
Methods In this paper, a method of assembly and adjustment of large-aperture mirror is proposed, which combines multi-objective spatial position conversion and Stewart structure motion inversion. This method can effectively ensure the accuracy of the optical and mechanical structure adhesion points, and realize the high-precision six-dimensional adjustment between the mirror and the main bearing plate only by relying on the mirror's own support structure. By decomposing the error index of the optical system of the remote sensing camera, the requirements of the reflector installation index are determined (Fig.4). Then the laser tracker was used to measure and construct the spatial coordinate system of the mirror and the support pad. The coordinate system restoration principle based on the nominal point was adopted to achieve the normalization of the coordinate system. According to the measured data of the nominal point on the support pad, the position of the support pad was corrected and fixed (Fig.8). Finally, based on the characteristics of the Bipod structure itself, the relationship between the length of the Bipod rod and the position of the mirror is calculated (Eq.14), and the position adjustment between the mirror and the main structure of the camera is realized by adjusting the length of the adjusting rod.
Results and Discussions The bottom-up error distribution method was adopted to sort out the tolerance of various error sources in the process of optical system installation. Error distribution follows a top-down order, with the sum of the squares of the error coefficients at the lower level equal to the square of the error at the higher level. Finally, the wave aberration RMS caused by the main mirror assembly is 0.025λ, including the influence of the position deviation and surface shape change of the main mirror assembly. The laser tracker was used to measure and locate the position of the support pad and the mirror, and the assembly of the mirror and the support pad was completed. The surface shape variation (RMS) before and after the installation was 0.008λ (Fig.9), which met the requirements of the installation error index. A laser tracker was used to measure the position deviation between the mirror and the bearing plate, and the adjusting length of the Bipod rod was calculated using the Stewart structure motion inversion algorithm. After the final debugging, the setting position error of the mirror was 0.004 1 mm, which met the setting error index requirements.
Conclusions Based on the setting process of a large aperture reflective remote sensing camera with Bipod structure, the setting error interval of the main mirror in the system is analyzed, and the shape error RMS value of the mirror assembly after setting is determined not to exceed 0.025λ. Based on this standard, the positioning, bonding, adjustment and detection scheme of the mirror Bipod structure is formulated. The practical results show that the method can effectively control the assembly positioning accuracy and surface shape error of the mirror, the positioning accuracy of the support pad can reach 0.029 mm, and the position adjustment accuracy of the mirror and the bearing plate can reach 0.041 mm. The installation results can meet the system imaging requirements of large aperture remote sensing cameras.
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