Cooled infrared off-axis freeform three-mirror system design with convenience in testing and assembly

A cooled infrared off-axis freeform three-mirror system, especially with a small f -number and 100% cold stop efficiency, continues to present challenges in design, testing, and assembly. A primary mirror and tertiary mirror integrated structural model is proposed to design a cooled infrared off-axis three-mirror system that is convenient to test and assemble. In the proposed model, the f -number is used to calculate the parameters of the mirrors, and a freeform surface is adopted to achieve high imaging quality, which means the spot diagram radius (RMS) is smaller than the Airy radius. To evaluate the performance of the proposed method, two design examples with different f -numbers (2, 1.4) were constructed. The results showed that the cooled infrared off-axis three-mirror system can be designed with a small f -number, 100% cold stop efficiency, and high convenience in testing and assembly.