Null test of an off-axis parabolic mirror. I. Configuration with spherical reference wave and flat return surface.

We demonstrate the precise figure measurement of a one-inch (25.4 mm) diamond-turned 90 masculine off-axis commercial-quality parabolic mirror. The test is carried out with a phase-shifting Fizeau interferometer fitted with a spherical reference surface, auxiliary components and a flat return mirror. We present a detailed and systematic appraisal of the necessary steps for alignment and calibration of the instrument and the alignment of the parabolic mirror. Alignment errors and interferometric sensitivity variations are characterised and corrected, and the results give some insight into the diamond-turning process.