# Precision Analysis and Error Compensation of a Telescope Truss Structure Based on Robotics
## 4 Secondary Mirror Adjustment Experiment  
### Construction of the Experimental Platform  
An adjustment experiment platform was built to verify the adjustment ability of the SM with respect to the system as well as the accuracy of the sensitivity matrix calculation. In this study, the experimental object is the primary and secondary mirror system of the RC telescope. The experimental system is composed of a PM system, a SM system, a compensator, an interferometer, and a plane mirror. Since the Stewart platform has the same adjustment capabilities as the secondary mirror truss based on robotics, the SM is connected to the SM truss through the Stewart platform, and it is then adjusted through the Stewart platform after the error is calculated. The positioning accuracy of the Stewart platform can reach 30 \(\upmu\)m, which is the same as the robotic arm selected in the article. The schematic diagram of the experiment setup is shown in Figure 14.  
Since the Stewart platform has the six degrees of freedom adjustment capability, we have proved that the robotic secondary mirror truss also has the same six degrees of freedom adjustment capability as the Stewart platform in Section 2.3. The secondary mirror needs to have the adjustment function of five degrees of freedom in addition to the rotation along the optical axis, so both the robotic secondary mirror truss and the Stewart platform can meet the system requirements. In addition, since the experiment focuses on the correctness of the secondary mirror adjustment strategy, the Stewart platform can be used in the experiment.  
Figure 14: Schematic diagram of the experiment setup.  
Figure 13: Post-compensation Z4–Z9 change curves in the gravity field.  
When establishing the experimental system, first, place the primary and secondary mirror system in a horizontal position, fix the compensator behind the PM, and adjust the plane mirror and the interferometer simultaneously via the five-dimensional adjustment platform. The convergent spherical wave emitted by the interferometer is subsequently emitted from the simulated focal point after passing through the compensator, before returning to its original path after being reflected by the primary and secondary mirror system, thus forming a self-collimating loop. It should be noted that covering the detection light path can reduce the influence of air flow disturbance to a certain extent.  
Due to the offset error of the PM and the SM, the surface error of the PM, and other uncontrollable factors in the experiment, the adjustments obtained in Sections 3.2 and 3.3 cannot be directly applied to compensate the system wavefront aberration. Therefore, this article uses an interferometer to measure the wavefront aberration of the system. All the error factors can be considered comprehensively, and then the adjustment of the secondary mirror can be solved by the high-order sensitivity matrix method.  
When determining the on-axis field of view, the rotational symmetry axis of the PM and SM coincides with the optical axis of the system, and therefore the rotational symmetry axis of the PM must be calibrated to serve as the detection reference for the on-axis field of view wave aberration. For a single parabolic mirror, only the on-axis field of view can achieve aberration-free imaging. Off-axis field of view imaging is affected by astigmatism and coma. Based on this principle, the relative positions of the interferometer and compensator are adjusted to minimize the detected aberration. Then, the detection field of view is the on-axis field of view. The experimental diagram is shown in Figure 15.