Design of a low F-number freeform off-axis three-mirror system with rectangular field-of-view

In this paper, a low F-number freeform off-axis three-mirror system with rectangular field-of-view (FOV) is presented, and the system has an integrated configuration that can be aligned with ordinary methods. The initial structure is solved analytically, and the optimization process is described in detail. This F/1.38 freeform system with a 4 degrees x 5 degrees FOV provides good imaging performance, especially in middle-wave and long-wave infrared bands. The primary mirror and the tertiary mirror are approximately tangent, so these two freeform mirrors can be fabricated on a single substrate. This configuration provides a solution to the practical application of freeform off-axis three-mirror systems. Additionally, a prototype of this system is demonstrated, and the imaging performance of the prototype is tested experimentally. The actual performance of the prototype is discussed and analyzed.