# Precision Analysis and Error Compensation of a Telescope Truss Structure Based on Robotics
## 1 Introduction  
Owing to the versatility offered by their mobility, vehicle-mobile telescopes are a useful observation tool for astronomers, the military, and in other fields. However, owing to the transport-related restrictions on the height and volume of the vehicle-mobile telescope, traditional vehicle-mobile telescopes have a meter-scale aperture. At present, the maximum aperture of the vehicle-mobile telescope system is 1.2 m. To a certain extent, this limits light-gathering ability, resolution, and observation ability of vehicle-mobile telescopes [1; 2; 3; 4; 5; 6].  
The secondary mirror (SM) of the telescope is fixed principally to the Serrurier truss through the Stewart platform for position error adjustment and correction. The Serrurier truss occupies half of the height space of the telescope system, representing the main obstacle preventing caliber improvements for vehicle-mobile telescopes [7; 8; 9; 10]. To solve this issue, this paper proposes an SM truss structure based on parallel robotics to replace the traditional Serrurier truss, enabling the telescope to be adapted depending on whether the observation state or the transport state is required. This structure can provide space for a larger-caliber telescope system, thus improving the observation performance of the vehicle-mobile telescope while satisfying transportation requirements [11; 12; 13; 14; 15; 16].  
As the aperture of the telescope increases, misalignment errors of the primary and secondary mirrors caused by structural gravity, rigid displacement of the supporting structure, thermal deformation, and other factors also gradually increase, which reduces the imaging quality; therefore, correction of these misalignment errors is necessary. In terms of offset error correction, the European Southern Observatory used correction technology based on active optics for the first time on the New Technology Telescope (NTT) [17], but the active support technology of the primary mirror is difficult to apply in vehicle-mobile telescopes. Elsewhere, [PERSON] et al. analyzed the sensitivity of the third-order coma of the two-reflection system to off-center [18]. In addition, [PERSON] used the inverse optimization method to adjust a Cassegrain telescope, thereby adjusting the wavefront aberration from 0.283\(\lambda\) to 0.194\(\lambda\)[19]. Using nodal aberration theory, [PERSON] studied the aberration type corresponding to the free-form surface type located at any position in the system [20]. However, the adjustment range in these methods is small, and the adjusted wavefront aberration cannot meet the requirements in this case.  
In order to achieve a larger aperture in a vehicle-mobile telescope under the conditions required for road transportation, this paper proposes a novel robotic SM support structure, which optimizes the posture of the robotics via the stiffness performance index to reduce the misalignment of the SM. Then, using high-order sensitivity matrices, the mapping relationship between the misalignment of the SM and the Zernike coefficient is established. In addition, the wavefront aberration caused by the shape of the primary mirror (PM) under the influence of gravity and the compensation ability of the SM are analyzed quantitatively. Finally, the feasibility of the method is verified experimentally.