Due to the all-day and all-weather information acquisition ability of infrared space remote sensing, it has application significance in the fields of environmental protection, land and resources investigation, forest fire monitoring, emergency disaster reduction, and military target detection. In recent years, with the continuous application of infrared space remote sensing data, higher requirements for the spatial resolution and imaging width of infrared space imaging remote sensors have been put forward in various application fields. At the same time, to improve the quantitative application level, the requirements of distortion and image illumination uniformity are becoming increasingly higher. Therefore, the infrared space imaging remote sensor urgently needs the optical system to realize the design of long focal length, large field of view (FOV), low distortion, and high illumination uniformity at the same time.

To effectively suppress the internal stray radiation, the traditional infrared optical system requires that the exit pupil should match with the cold stop perfectly. The large incident angle on the image plane increases the difficulty of aberration correction such as coma, astigmatism, and distortion, which results in the decline of image quality of the optical system. A novel re-imaging telecentric optical system configuration is proposed and the imaging principle is analyzed to solve the problem of traditional infrared optical systems. Then the coaxial catadioptric re-imaging telecentric optical system is constructed and the initial structural parameters are analyzed. Finally, based on the coaxial catadioptric re-imaging telecentric optical system, the off-axis catadioptric re-imaging telecentric optical system can be obtained by field bias. This optical system has the advantages of imaging with large aperture and long focal length of off-axis three-mirror optical system and imaging with large FOV of the refractive optical system. Additionally, the transmittance is improved by the elimination of obstruction.

Based on the performance requirements of the thermal infrared space imaging remote sensor with high resolution and wide imaging width, the optical system design specifications are decomposed. The working spectral band is 8?10 μm, the aperture is 435 mm, the focal length is 1038 mm, and the FOV is 8.84°×1.10°, with the image length reaching 160.4 mm. According to the imaging principle and initial structure solution method of the off-axis catadioptric re-imaging telecentric optical system, the initial structural parameters with a better structural layout and imaging quality are obtained. Due to the large aperture of the primary optical system, the field bias angle is higher at 9.54°. Therefore, large off-axis asymmetric aberrations will be produced at the intermediate image and it is difficult to conduct correction by the coaxial relay optical system. The free-form surface characterized by the XY polynomial is employed on the rear surface of relay lens 2 to achieve the correction of off-axis asymmetric aberration and further improve the imaging quality. The optimized off-axis catadioptric infrared space optical system consists of three mirrors and six relay lenses (Fig. 4). The modulation transfer function (MTF) of each FOV is greater than 0.316@25 lp/mm (Fig. 5), the relative distortion is less than 0.25% (Fig. 6), and the image illumination uniformity is better than 99% (Fig. 7). The manufacture and assembly tolerances of the optical system are analyzed, with the average MTF greater than 0.2732 at 90% probability and greater than 0.2816 at 80% probability. We carry out the manufacture and inspection verification of the free-form relay lens 2 to ensure the engineering feasibility of the optical system. The CGH compensator is designed and adopted to guide the manufacture and inspection of the free-form surface of relay lens 2 (Fig. 9). The surface shape accuracy of the free-form surface reaches 0.033λ (λ=632.8 nm). The manufacture and inspection results indicate that the free-form relay lens 2 has sound engineering feasibility.

We propose an off-axis catadioptric re-imaging telecentric optical system, which breaks through the match limitation of the exit pupil and cold stop in traditional infrared optical systems to achieve a perfect design with the long focal length, large FOV, low distortion, and high illumination uniformity simultaneously. Meanwhile, we analyze the imaging principle and initial structure solution method of off-axis catadioptric re-imaging telecentric optical systems. By taking the example of designing an off-axis catadioptric infrared space optical system with a working spectral band of 8?10 μm, an aperture of 435 mm, a focal length of 1038 mm, and a FOV of 8.84°×1.10°, we validate the effectiveness of the proposed method. The MTF values of each FOV at the Nyquist frequency (25 lp/mm) are all greater than 0.316, approaching the diffraction limit of the optical system. The relative distortion within the full FOV is less than 0.25%, and the image illumination uniformity is better than 0.99. The manufacture and assembly tolerances of the optical system are analyzed and the free-form relay lens 2 is manufactured to ensure the engineering feasibility. The design, manufacture, and inspection results indicate that the off-axis catadioptric infrared space optical system with a long image surface and low distortion has excellent imaging quality and sound engineering application significance, which can meet the application requirements of infrared space imaging remote sensors for high spatial resolution, wide imaging width, and high quantitative application level.