Optical design of a null test of a synthetic reflective mirror based on the subaperture test method

Machining multiple mirror surfaces on one common substrate during the fabrication of off-axis three-mirror or four-mirror optical systems can take less time and drastically improve the alignment efficiency. However, the difficulty of the surface test remains the same. We theoretically propose a subaperture test method to carry out the null test of two mirrors on the synthetic reflective mirror. Specifically, we design a special zoom null lens and selectively use its subaperture wavefront aberrations of different configurations to nullify the surface normal wavefront aberrations of the according mirrors on the synthetic reflective mirror. The proposed method is verified by simulating the null test process of a synthetic reflective mirror integrating an off-axis high-order primary mirror (PM) and a coaxial high-order tertiary mirror (TM) of one off-axis three-mirror system, with the consideration of the fabrication and alignment errors. Simulation results show that, within a limited range of feasible tolerances, the residual wavefront aberration is 0.033λ root mean square (RMS) for the PM and 0.025λ RMS for the TM, at a wavelength of 1064 nm.