Design of space optical systems and analysis of their thermal stability

Off-axis reflective optical systems are widely employed in Earth observation and mapping owing to their advantages of wide field of view (FOV), high image quality, and stable interior orientation elements. However, conventional designs based on the point-by-point (PW) method often suffer from degraded off-axis imaging performance and thermally induced pointing drift. In this study, the third-order aberration coefficients of a three-mirror optical system are analytically derived under the condition that the aperture stop is located at the secondary mirror and that the primary and tertiary mirrors are equally spaced along the optical axis. To further enhance imaging performance, fourth-order aspheric terms are introduced on both the primary and tertiary mirrors, thereby increasing the degrees of freedom for optimization. A comprehensive image-quality evaluation function incorporating quasi-telecentric constraints is constructed, and a hybrid genetic algorithm-sequential quadratic programming (GA-SQP) approach is employed to obtain an optimized initial configuration. The resulting system achieves a focal length of 260 mm, an F-number of 10, and a 7° × 30° FOV, with a modulation transfer function (MTF) above 0.25 at 77 lp/mm, a maximum distortion of 2%, and a maximum chief-ray angle of 2.3°. Microcrystalline glass and titanium alloy are adopted as the mirror substrate and structural materials, respectively. Finite-element thermal analysis is performed under a 6.8 °C temperature gradient, and the optical axis rotation, evaluated using the TRIAD algorithm, is −0.728″ about the X-axis, 1.0816″ about the Y-axis, and 11.045″ about the Z-axis. These results confirm the excellent thermal stability of the proposed design and underscore its potential for reducing in-orbit mapping errors under uncontrolled thermal environments.