Zone-Plate Interferometer for Measuring the shape Error of Mirrors with Large Numerical Aperture and Step Asphericity

Abstract

A compact zone-plate interferometer is developed to measure the shape error of concave mirrors. A diffracted light of (0, 0) and that of (-1, -1) or (+1, +1) are used in the interferometer. The interferometer enables us to measure the shap error of mirrors with a large numerical aperture and a steep asphericity. the shape errors of a spherical mirror and a parabolic mirror are measured by the interferometer, and that of the former agrees well with that measured by Fizeau interferometer. Methods for adjustments of zone plates and the mirrors being tested are proposed to perform precise measurements of mirror shapes.