Cooled mid-infrared unobscured three-mirror system with an intermediate image plane using freeform surfaces

Unobscured reflective systems can also be applied in infrared region for higher image quality and optical efficiency. For infrared reflective systems, the signal noise ratio (SNR) is a critical parameter, which would be influenced by the F-number and the external thermal radiation. To suppress these stray lights, a common method is that the cold stop of the infrared detector matches the entrance pupil of the optical system. But the cooled mid-infrared unobscured system is difficult to design due to the asymmetrical structure. Freeform surfaces provide designers more flexibility with respect to the aberration correction in the optimal design process, which have been widely applied to reduce the optical system volume and improve the imaging performance. Within this article, a cooled mid-infrared three-mirror (CMT) system with an intermediate image plane using freeform surfaces is presented. In Section 2, the optical and mechanical designs are thoroughly discussed, including the selection of initial structure and the optimization of freeform surfaces. The imaging performance and stray light analysis would be conducted to verify the cold stop efficiency of this system. In Section 3, this system assembly and performance test are implemented, which will verify machining precision of these reflectors and this design system feasibility at the infrared application.