Interferometric measurement of convex aspheres using holographic test plates

A new method of measuring large, highly aspheric convex secondary mirrors is being pursued at the Steward Observatory Mirror Lab. This new interferometric test uses a full-aperture test plate with a computer-generated hologram (CGH) fabricated onto a concave spherical reference surface (See Fig. 1). Fringes of interference are viewed through the test plate, which is supported several millimeters from the secondary. The hologram consists of annular rings of metal spaced at intervals as small as 80 μm and as large as 500 μm. The accuracy of the surface measurement using this technique is expected to be better than 50 nm P-V and 8 nm rms for mirrors with up to 331 μm departure from the best-fit sphere. Considerably higher accuracy can be attained for less aspheric surfaces.