Thermal compensation design of truss structure for large-scale off-axis three-mirror space telescope

摘要

The image quality of the large-scale, long-focus, large field of view, off-axis three-mirror anastigmat space telescope will decrease dramatically due to the thermal effect. This paper studies the thermal deformation of the telescope, and based on its axial deformation, it proposes a passive thermal compensation design using truss rods made of carbon fiber-reinforced plastics, utilizing its thermal conducting design, which is determined by different spread quantity and spread angle of the lamination. Meanwhile, it satisfies the system stiffness requirement. First, the truss rods are divided into several independent triangles, according to the principle of equal axial distance after deformation, the function in terms of coefficient of thermal expansion (CTE) and length of truss rods is determined. Bonded by the optical tolerance requirement, the range of the CTE is obtained. Second, according to the CTE function, coupled with the stiffness requirement of the whole instrument, the CTE of each truss rod is determined. According to the CTE and stiffness requirement, the design of truss rods lamination is carried out. Finally, the group of truss rods with unequal CTE is obtained. The results have been validated by finite element analysis, vibration experiments, and image test, which indicate that the accuracy of the space telescope studied meets the requirements of optical tolerance; in addition, the rotation angle of the front panel around the X axis is competent, implying the stiffness requirement is solved. Moreover, the modulation transfer function at Nyquist frequency is better than 0.178 under the condition of 20℃ ± 5℃.