Subaperture stitching test of convex aspheres by using the reconfigurable optical null

Subaperture stitching test in combination of the reconfigurable optical null we proposed recently provides flexible solutions to various surfaces including convex aspheres and even aspheres of large aperture. However it is challenging for the stitching optimization to get the real surface error because the surface error is strongly coupled with misalignment-induced aberrations in near-null subaperture measurements. Aiming at this challenge, we first figure out the property of aberrations induced by misalignment of optical null or test surface. It shows that identical misalignment of the optical null introduces nearly identical aberrations to subapertures with different off-axis distances, while misalignment of the test surface introduces little aberrations to the central subaperture. The stitching algorithm is then proposed with focus on decoupling surface error and induced aberrations. The major step is to calibrate out the effect of misaligned near-null optics before stitching optimization by using the central subaperture measurement. We also present the through-the-back null test for the purpose of cross test. The axial distance is precisely monitored by a low coherence interferometer, which enables accurate determination of the spherical aberration component of surface error. Final experimental results show consistent spherical aberration obtained by stitching test and by the through-the-back null test. It is a big step towards instrumentation of subaperture stitching test for aspheres with rather big amount of misalignments in surface metrology practice.