Optical System Design of Offner Compensator with Infinity Object Distance Applied on Aspheric Mirror Testing

This paper aims to make a meter-scale aspheric mirror testing on a large aperture. Third-order aberration theory of Offner null compensator with infinity object distance is introduced. And the normalized optical parameters of the aspheric mirrors are calculated in different aperture ratios of the compensators to the mirrors under test. Then the relative optical designs corresponding to different aperture ratios and six kinds of curvature radii are scaled using optical software, and the maximum diameter and relative aperture of the aspheric mirror are obtained. The results show that, with the reduction of the aperture ratio, it is more difficult to test the relative aperture of the aspheric surface tested with plane wave compensation. An appropriate minimum aperture ratio can be selected for the aspheric surfaces with different relative apertures. Considering engineering feasibility, the Offner compensator with an aperture ratio alpha(1) =0.2 is used to test a concave aspheric mirror with an aperture of 720 mm. The wavefront error (RMS) of the design is 0.0033 lambda, and in the actual test, the RMS is 0.019 lambda, which meets the requirement of the aspheric surface test.