Annular subaperture interferometry for high-departure aspheres using paraboloidal parameterization

A low-cost technique is presented for constructing stitched Fizeau interferometric measurements of high-spherical-departure concave aspheres of arbitrary conic constant without the use of null optics. The optical test configuration assembles the surface figure of the asphere using subapertures parameterized as variances from best-fit paraboloids. Subtracting the optical path difference between each idealized paraboloid and the corresponding annulus of measured data removes the annular wavefront aberrations without the need to fit Zernike polynomials. The proof-of-concept measurement and reconstruction of a 250 mm diameter diamond-turned ellipsoidal mirror with more than 3000 waves of spherical departure are reported. The presented technique is an inexpensive addition to the array of tools used to measure large-aperture aspheres and high-departure freeform optics.