Interferometric cross test of a convex even asphere

Abstract

Large convex aspheres are difficult to test since larger null optics are required. We propose to test a convex even asphere by near-null subaperture stitching based on the reconfigurable optical null. This paper starts with an introduction to the principle of reconfigurable optical null and the near-null stitching algorithm. For the purpose of cross test, alternative two measurements of the surface are obtained and quantitatively compared with the stitching test. The first one is null test with a single asphereic lens which is in situ calibrated by an ordinary null. The test beam travels the same path when used for measuring the convex asphere and calibrated by the ordinary null, which relaxes the tolerance on transmissive quality and fabrication error of the large aspheric lens. The other alternative is back-through null test with a null lens. A singlet is designed to balance the positive spherical aberration existing in the back-through test of the even asphere. A low coherence interferometer is employed to precisely measure or monitor the surface thickness or air clearance in the null test. The three measurements are finally registered to each other to calculate the difference or similarity quantitatively.