Parabolic mirrors can be tested by means of diffraction-generated test beam technique. The parabolic mirror under test
focuses the collimated laser beam from Fizeau digital interferometer. When a very small diffraction element is placed at
the focal point, it diffracts the focused light and generates a spherical wave in the test arm. This spherical wave from the
focus is reflected back to the interferometer by the parabolic mirror under test and services as a test beam. The optical
path difference (OPD) between the test wavefront and a plane reference wavefront indicates the surface irregularity of
the parabolic mirror in terms of interference fringes. There is no central hole in on-axis parabolic mirror testing.
Experiments have been carried out to test high-precision parabolic mirror.