Off-axis zoom optical systems based on mirror rotation and their design method

This paper proposes a type of off-axis zoom optical system, achieving zoom through the rotation of one mirror. These systems have two different operating states, where one state has a shorter focal length and a larger field of view, and the other with the opposite. Switching between the two states is accomplished by simply rotating one mirror by a small angle. In contrast to the traditional method of simultaneously controlling the axial translation and rotation of the mirrors to achieve zoom, these systems only require a small rotation of the third mirror to achieve zoom, which is advantageous for reducing the system's control difficulty. In addition, a method for the design of this type of system is presented and used to design an off-axis zoom imaging optical system incorporating three freeform mirrors. The system has an entrance pupil diameter of 20 mm, a focal length of 200 mm in the long focal state, a focal length of 100 mm in the short focal state, and a zoom ratio of 2 times. The system requires only a 2.93° rotation of the third mirror to switch between the two states. The final system illustrates excellent imaging performance close to the diffraction limit in the visible light wavelength range.