Rapid and reliable reference sphere calibration for Fizeau interferometry

I describe and demonstrate a rapid and simple technique for calibrating spherical reference surfaces in Fizeau interferometry. A flat mirror at the focus of the test wavefront and a partial occlusion of the test beam enable a double-pass measurement of one half of the surface against the other, which gives an error map for half the aperture. Subsequent rotations of the beam stop and stitching together of several measurements yields the full-aperture calibration file. The method always produces point-symmetrical surface maps and is therefore not exact. However, most actual reference surfaces are approximately point symmetric and therefore the trade-off of accuracy for expediency is reasonable in practice.