Design of small f-number off-axis three-mirror optical system

Small F-number optical system often used in night vision devices. However, in some extreme cases, the common transmission optical system has a certain energy loss and the signal-to-noise ratio of the system is insufficient. In this paper, we design an off-axis three-mirror optical system, which F-number is 1.5, the focal length is 250mm, the field of view is 6°. Considering to reduce the difficulty of processing and adjusting, the primary mirror and third mirror are put on the same lens substrate. The result shows that in a relatively loose tolerance, the MTF of the system is higher than 0.6 at 40 line pairs, which has good imaging quality and meets the design requirements.