Objective The star sensor system exhibits high measurement accuracy, determining spacecraft attitude by observing stars at different spatial positions. It plays an increasingly critical role in both military and civilian aerospace applications. Modern star sensor systems are evolving toward larger apertures, broader spectral ranges, and wider operational temperature ranges. In extreme temperature environments, thermal variations alter the optical system parameters, degrading overall performance. To ensure imaging quality for star sensor systems with large apertures and broad spectral ranges under harsh temperature conditions, athermal design must be employed to eliminate focal shift caused by temperature fluctuations.

Methods This paper proposes a design approach for a catadioptric star sensor system with a long focal length and a wide operational temperature range. Based on the influence of temperature variations on the parameters of the primary mirror, secondary mirror, and lens elements, the athermalization equations for the catadioptric optical system are derived. According to the specified optical system requirements (Tab.1) and the stringent volume constraints of the star sensor system, the optical parameters of an on-axis two-mirror system were designed and determined (Tab.2). Both the primary and secondary mirrors, along with their supporting structures, were made of the same material—AlSi40. Subsequently, guided by the athermalization equations, the compensating lens group was selected from the same glass family, with further optimization applied to the high-power lenses. The optimization process involved iterative glass substitution, power distribution, and aberration balancing until a well-performing catadioptric star sensor optical system was achieved, demonstrating stable optical performance across a wide temperature range.

Results and Discussions The optimized optical system consists of a primary mirror, a secondary mirror, and three corrective lenses (Fig.3). At room temperature, the imaging performance is as follows: the maximum RMS radius in the spot diagram is 12.86 μm, with 80% of the energy of the imaging spot across the entire field of view concentrated within 2 pixels. The modulation transfer function (MTF) across the full field of view exceeds 0.4@17 lp/mm (Fig.3). The imaging performance under high and low temperatures (−40 ℃ to 60 ℃) (Fig.4) and the results of the opto-thermo-mechanical integrated analysis (Fig.6, Tab.4) indicate that the variation of the spot RMS radius is less than 5 μm, the decline in the MTF value is less than 0.1 and for 80% of the energy-encircle, the radius change is within one pixel. Further, it was demonstrated AlSi40 exhibits superior MTF as mirrors and support structure material system compared to AlSi30 and AlSi50. At various temperatures, the change in the spot position is less than a single pixel compared to the simulation. (Fig.7-Fig.9, Tab.5).

Conclusions This paper presents the design of a catadioptric star sensor system with an operational wavelength range of 0.9 μm to 1.7 μm, a focal length of 1500 mm, an entrance pupil diameter of 100 mm, and an operating temperature range of −40 ℃ to 60 ℃. Both the primary and secondary mirrors, as well as their supporting structures, are made of AlSi40, and an optical passive athermalization approach is employed to achieve athermal design. Opto-thermo-mechanical integrated analysis demonstrates that the optical system maintains a MTF value greater than 0.3@17 lp/mm across a wide temperature range and offset from the design value by less than 0.1, with 80% of the imaging spot energy concentrated within 3 pixels over the entire field of view, indicating excellent imaging performance under varying thermal conditions. The high- and low-temperature test results show that the imaging spot diameter is less than 3 pixels across a wide temperature range, with variations in spot size under different temperatures being less than 1 pixels. Meanwhile, the star spots exhibit roundness without significant trailing, and the imaging remains clear, validating the effectiveness of the star system design.