Subaperture stitching interferometer for large optics

Abstract

Ultra-high precision measurement of the form is critical in deterministic machining of optical components. It is generally accomplished with an interferometer. However, a standard interferometer is incompetent for large convex aspherical surfaces or those with large relative apertures. Hence subaperture stitching interferometry was proposed to extend the vertical range of measurement and enhance the lateral resolution, based on the idea of "stitching pieces together". This paper presents a prototype design for subaperture stitching interferometer (SASI) for large optics. It applies to large flats (clear aperture<700mm), large concave spherical or aspherical surfaces (clear aperture<350mm, 1:2<relative aperture<1:1). Firstly, three designs are compared to choose an appropriate layout of the optical path for testing large optics. Then the multi-axis mechanical design is introduced in detail. The kinematical model of SASI is built according to the kinematics of open-loop robots. It helps to determine the stroke, resolution and accuracy of each axis. The advantages of the iterative subaperture stitching algorithm, e.g., no requirements of precise prior knowledge of subaperture alignment error, considerably facilitate the mechanical design.