A multiple-beam interferometer that permits evaluation of autostigmatic systems is described. A small reference sphere is compared with a spherical wavefront in a manner analogous to the comparison of a plane wavefront with the reference flat in a Fizeau interferometer. To prevent walk-off of the fringes, a field lens is used at the center of curvature of the reference sphere. The instrument described has been used for evaluating spherical mirrors, concentric windows, and lenses.
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