In this paper, a refraction and diffraction mixed compensation test optical path based on computergenerated hologram (CGH) has been proposed, and the combination of the CGH and aplanatic lens is realized to measure the off-axis segment in null test with focal reducing. The total length has been reduced to 1/4 to 1/8 of that of the conventional method. And the adjustment sensitivity of the small focus ratio segment has been improved. The distortion ratio of the interferogram of the large off-axis segment has been optimized from 12. 5 to 1. 25. The segments with different off-axis distances can be measured rapidly with high precision by replacing different computer-generated holograms and keeping every elements in place. The method is used to complete the processing and testing of a Phi 330 mm off-axis trial mirror, and the root mean square (RMS) value of the test surface figure is 0.0290 lambda (lambda is wavelength). The RMS value of the test surface figure is reduced to 0. 0267 lambda, by calibrating the aplanatic lens using a standard spherical mirror.

In order to achieve high precision surface testing for the large diameter and long focal length off-axis segmented mirrors, we designed a reflective diffractive compensation null testing system. Using a computer-generated hologram and a spherical mirror to compensate for normal aberration of the off-axis mirror. The design results show that the residual wavefront error of the optical path is close to zero. For a testing system, CGH alignment optical paths corresponding to the non-axisymmetric off-axis structure are designed to ensure the feasibility of the assembly. Parameters of the optical path testing for different off-axis distance mirrors are the same. Rapid high-precision null testing of different types of segmented mirrors can be achieved simply by replacing the CGH at corresponding position and adjusting the spatial positions of the mirror to be measured. Error analysis shows that the RMS error of the mirror surface to be measured is better than lambda/40 (lambda=632.8 nm), which is caused by the manufacturing error of the compensating elements, misalignment of the optical path, repeatability of the interferometer surface measurement and standard spherical wavefront deviation of the interferometer.