Interferometric testing of large optical components with circular computer holograms.

In testing a large aspheric mirror such as a paraboloid, one uses as interferometric reference a small spherical mirror together with a test glass. The combination of mirror and test glass produces a parabolic wavefront for comparison. Recently holograms have been used as synthetic test glasses. The average fringe distance of the grating-like holograms has to be fine enough if separation of the diffraction orders is required. We propose to use holograms with circular fringes. Now the different diffraction orders come to focus at different locations on the optical axis, where all but one order can be eliminated. This is easily possible, since the waves coming from a mirror with a central hole are hollow cones. Several ways for making these computer-generated circular holograms are suggested.