Optical design of a null test for off-axis three-mirror system based on refractive-diffractive zoom hybrid compensator

In general, the reflectors of off-axis three-mirror system have large apertures and large differences in surface shape, which means it is difficult to use one set of null compensators to test all the aspheric mirrors in the off-axis three-mirror system because of the limitation of the dynamic range of the testing equipment. In this paper, we propose a surface shape test method that combines a refractive-zoom lens group and a Spatial Light Modulator (SLM) as a null compensator to test all the aspheric surfaces of one off-axis three-mirror system. The proposed method is verified by simulating the null test process of three aspheric mirrors of one off-axis three-mirror system, with the consideration of the fabrication and alignment errors. Simulation results show that the probability that residual wave aberration is within 0.1λ RMS is 50% for the primary mirror and 40% for the secondary mirror and 80% for the tertiary mirror.