During thermal infrared hyperspectral imaging detection, the thermal background radiation caused by the optical-mechanical structure of the detection load itself will directly cause the annihilation of the detection signal, making it impossible to obtain an excellent signal-to-noise ratio. Designing deep cryogenic environment for the optical-mechanical structure is the only way to achieve an excellent detection effect. However, in order to ensure that the single components do not need to be adjusted during the transition from room temperature assembly to deep cryogenic operation of the lens group, the impacts of the thermal forces on the lens assembly under large temperature differences must be reduced, and the offset of the displacement caused by the thermal deformation of the single component must be further reduced to enhance the reliability and stability of the system. Hence, these issues pose huge challenges in the design of optical mirror support structures. Therefore, we aim to design a flexible support structure that can not only adapt from normal operations at normal temperature to operations at deep hypothermia of 70 K but also withstand rocket launch vibration tests. We also aim to reduce the time required to seal the single mirror and assembly as well as adjust the components.

According to the “zero” expansion design principle, the structural design of the optical system is carried out by individually sealing each lens and forming a lens group with multiple such single lenses. In designing and selecting materials, we consider using materials with similar thermal expansion coefficients for structures that directly match the mirror. For the lens support base and spectrometer base plate, “zero” thermal expansion coefficient materials are used (Table 1). After the materials are determined, a finite element simulation is used to obtain the test sample simulation results, and the thermal deformation and thermal stress of the single component are controlled through parameter adjustments. These data are combined with a range analysis of the test results to quickly identify changing trends of the effect curves of influencing factors, improve the design efficiency of single components, and ultimately form the global optimal design (Fig. 3).

For the design of the flexible support ring, it is identified that the length L, thickness a, width b, and gap c of the flexible block are the key influencing factors on the design of the flexible block (Table 2). According to the results of the range analysis of the orthogonal test, the corresponding ranges of b, a, L, and c decrease in turn (Table 3). According to the finite element analysis results, the maximum thermal deformation of the optical mirror of the long-wave spectrometer during the transition from room temperature assembly to deep cryogenic operation of the lens group occurs on lens 1, which is 79 nm and thereby meets the requirement of the surface shape being less than λ/6. The maximum thermal stress occurs at the connection between the grating and the adhesive layer with a value of 12.8 MPa (Fig. 8), which meets the requirement of a safety margin greater than 0.25. The mechanical vibration results show that the inherent fundamental frequency of the thermal infrared long-wave spectrometer component is 376 Hz, the fundamental frequency drift before and after the test is less than 1%, and the system modulation transfer function (MTF) change before and after mechanics is 0.01 (Table 4), which meets the usage requirements. A system test platform is built for performance testing. The results show that the vacuum degree is better than 8.6×10^-5 Pa, the final temperature is approximately 70 K, the temperature uniformity is better than 2.9 K (Fig. 11), and the system MTF is better than 0.4 (Table 5), thereby meeting the requirement of a product index MTF value greater than 0.15.

This paper studies and solves the problem that the flexible support structure of the lens can easily cause damage to the lens and cause the surface shape of the lens to degrade under large temperature differences. The results show that the optimal design parameters of a single flexible block are a=1 mm, b=3.5 mm, c=0.3 mm, and L=11 mm. Additionally, the minimum bonding area of the glue layer is 207.96 mm², and the safety margin coefficient is 1.39. In a vacuum environment with an operating temperature of approximately 70 K, the MTF of the thermal infrared long-wave spectrometer imaging system is greater than 0.4, which meets the usage requirements. The designed flexible support structure also considers the role of optical reference transmission. It is a thermal infrared multi-lens structure in the context of large temperature differences. The lens support structure form provides the design basis.