Optical design for off-Axis three-mirror two-channel imaging system with freeform surfaces

This paper presents an optical design for the all-reflective dual-channel imaging system based on freeform surfaces. This system may be useful in remote sensing where coarse searching and fine observation are both needed. For this system, an off-Axis three-mirror system with a middle image is chosen to design and the uniform stop is placed before the first optical surface. Meanwhile, beam splitter can be placed between secondary mirror and the location of the middle image to obtain multiple paths and the different curvatures of the tertiary mirrors can be used to differentiate the focal lengths of two channels and then get a zoom ratio of this system. One channel with a wide FOV of 3×1.5° but a small focal length of 700 mm can be used for searching, while the other one with a long focal length of 1480 mm but a narrow FOV of 0.5×0.5° can be used for fine reconnaissance. Furthermore, An XY polynomial, established as an even function of x, was employed to improve imaging quality, so we obtained a system of the symmetry about the YOZ plane, which can bring considerable convenience to alignment and testing for the system. The modulation transfer function curves of both channels are above 0.3 at 50 line pairs per millimeter, which indicates a good imaging quality.