Design of off-axis three-mirror systems with ultrawide field of view based on an expansion process of surface freeform and field of view

Unobscured reflective optical systems with a wide field of view (FOV) have significant application values. However, the aberration increases with the increase of the system FOV, so a wide FOV system is difficult to design. In this paper, a design method that is effective in achieving off-axis three-mirror systems with ultrawide FOV is proposed. In this method, the system FOV is expanded stepwise in the design process, and the surface optical freeform polynomial terms are extended based on the judgment of image quality and some constraint conditions, and to obtain a prospective ultrawide FOV system. A freeform off-axis three-mirror imaging system with a focal length of 1000 mm, an F-number of 10, and an ultrawide FOV of 80°×4° is designed as an example. This design result shows that the system has a high imaging quality of RMS wavefront error value of 0.040λ(λ=0.633  μm), and it demonstrates that the method is effective in achieving off-axis three-mirror systems with an ultrawide FOV.