Objective The large aperture mobile optical system has bad working conditions, and its support structure needs to ensure the surface shape accuracy, pose accuracy and support stiffness of the mirror after or when experiencing the turbulence and impact of vehicle transportation. Therefore, it is necessary to analyze and design the support structure of the large aperture mobile mirror in detail. Domestic researches on the support structure of mobile mirror mainly focus on mirrors with a diameter of no more than 500 mm and a small mass, and the support structure is simple, which is not suitable for the support of mobile meter-level mirrors. However, the support mode of traditional non-mobile meter-level large aperture mirrors and foreign mobile meter-level mirrors such as Stratospheric Observatory for Infrared Astronomy (SOFIA) usually adopts a set of tangential support mechanism to provide tangential support and tangential positioning at the same time, and the positioning mechanism in the device has a certain deformation due to the force, resulting in reduced positioning accuracy. For a system with a complex optical path, adding an adjusting mechanism to each link of the optical path to compensate for the pose error of the primary mirror will lead to complex system structure, increased cost and tedious installation. Therefore, it is necessary to develop a new tangential support mode of the mirror to improve the pose accuracy of the mobile meter-level mirror.

Methods Aiming at the 1.57 m primary mirror of the vehicle optical detection system, this paper proposes the tangential support mode of 6 groups of flexible tangential rod positioning mechanism combined with 16 groups of lever counterweight supporting mechanism. The lever counterweight supporting mechanism carries the weight of the primary mirror, and the flexible tangential rod positioning mechanism realizes the primary mirror positioning, decoupling tangential support and tangential positioning, improving positioning accuracy and enhancing support stiffness. Combined with the axial Whiffletree support mode, the support and positioning of the primary mirror are realized. At the same time, the lever in the lever counterweight support mechanism can rotate around the support point, and there is no displacement constraint on the primary mirror, so there is no displacement coupling between the lever counterweight support mechanism and the flexible tangential rod positioning mechanism, that is, the lateral support force exerted by the lever counterweight support mechanism does not affect the pose accuracy of the primary mirror.

Results and Discussions The analysis results show that the new lateral support mode can greatly reduce the sinking of the primary mirror, and prove that the new lateral support mode can effectively improve the pose accuracy of the primary mirror. The maximum thermal deformation Root Meam Square (RMS) of the primary mirror at working ambient temperature is 7.7 nm (Tab.2), and the maximum thermal stress at storage ambient temperature is 10.5 MPa (Tab.3). The thermal analysis results show that the support mode of axial flexible rod and lateral flexible hinge has good pyrolytic coupling ability and can meet the working and storage requirements. The main mirror shape error is 5.7 nm (Tab.4) RMS under gravity load. It can be seen that the support mode can effectively carry the weight of the primary mirror, restrain the generation of additional load, and prevent large surface shape errors. The first 5 order natural frequency of the support system indicates that the support system has good support stiffness. The results of vibration analysis show that the new support mode can be well adapted to road transportation.

Conclusions The analysis results show that the lateral support method proposed in this paper can reduce the subsidence of the primary mirror from 24.3 μm to 3.6 μm. The maximum integrated surface error of the primary mirror is about 9.6 nm RMS. The lowest natural frequency of the whole mode reaches 54.93 Hz; In random vibration analysis, the maximum displacement response of the support system in three directions of X, Y, Z axis is 0.11 mm, 0.10 mm, 0.36 mm, and the maximum stress response is 12.1 MPa, respectively. The maximum displacement response of the main mirror in the three directions of the X, Y, Z axis is 1.6 μm, 9.7 μm, 12.3 μm respectively, and the maximum stress response is 0.44 MPa. The evaluation results show that the support scheme can provide good surface shape accuracy and pose accuracy for the primary mirror, and can be effectively adapted to vehicle transportation. After vehicle transportation, the measured primary mirror shape accuracy RMS is 0.0237λ (λ=632.8 nm), which verifies that the support scheme has a good effect and proves the accuracy of finite element analysis.