In this paper, a homogeneous off-axis reflective infrared imaging system with an aperture of Φ300mm is designed according to the requirements. The calculation and optimization of structural parameters are completed in the optical software Code V. The system obscuration is eliminated by combining aperture off-axis with off-axis field of view. By introducing complex surface type to correct the residual aberrations, the off-axis optical system with good capability and structure size meeting the overall layout requirements is obtained. The system is composed by main reflector (F-1), secondary mirror (F-2), plane mirror (M) and the third mirror (F-3). The mirror M is a folding reflector; even aspheric surface is used for both F-1, F-2 and F-3 mirror. The uncooled thermal imager is used as the imaging device. The target plane of the detector is 10.88mm × 8.70mm, and the corresponding field of view is 0.6 °× 0.45 °. At 15lp / mm, MTF > 0.2 is close to the diffraction limit of the system, and the wavefront aberration of the full field of view is less than 0.01λ. The distortion is less than 1.2%.
