Subaperture stitching interferometry of high-departure aspheres by incorporating a variable optical null

Originally, subaperture stitching interferometry enabled full-aperture measurement of large-aperture spheres and flats using 4″ or 6″ interferometers and transmission elements. Later, mild aspheric surfaces could be measured using the same fundamental principles. In both cases, stitching algorithms automatically compensate for systematic reference wavefront and distortion errors. Aspheres with up to 1000 waves of departure from best-fit sphere can be measured without dedicated null lenses using a variable optical null (VON™) device which generates an optical wavefront that closely matches the surface of the asphere within a local subaperture. This paper presents the principles of subaperture stitching interferometry incorporating VON technology.