Testing of a diamond-turned off-axis parabolic mirror

Methods to test on-axis and off-axis parabolic mirrors are standard textbook fare. All we need, we are told, is a spherical wavefront and a plane mirror, or a plane wavefront and a spherical mirror. Contrasting with the implied ease of application, reports on practical experience with these tests are somewhat rare, particularly for off-axis mirrors. We have explored both variations of this testing method with a phase-shifting Fizeau interferometer, auxiliary components, and a one-inch diamond-turned 90° off-axis commercial-quality parabolic test mirror. The testing process is quick and easy only if you know how, and frustrating and time-consuming otherwise. We report on the calibration of the reference surfaces, present a detailed and systematic re-appraisal of the necessary steps for alignment and measurement validation, which have been described previously but in a less straightforward way, and present a brief characterization of the parabolic mirror that gives some insight into the diamond-turning process.