Interferometer-induced wavefront errors when testing in a nonnull configuration

Sub-Nyquist interferometry and other extended range techniques have the potential to allow measurement of aspheric surfaces with large departures from a reference sphere. The optic is tested in a non-null configuration, and aberrations are introduced into the wavefront by the interferometer optics. Consequently, the wavefront measured at the sensor is different from the wavefront initially produced by the test surface, and the interferometer must be calibrated if useful measurements of aspheres are to be made. The aberrations produced by a Twyman-Green interferometer for this application were examined. To study the severity of these interferometer induced errors, a defocused spherical surface was used to generate a non-null configuration. With wavefront departures up to 400 waves, errors up to 12 waves rms were found to be introduced by the non-null test setup.