Compact Optical Test Facility for Evaluating Very Long Focal Length Mirrors

Various techniques have been used to test the optical figure and radius of curvature of optical flats and long focal length optics1. If optical flats become too large to be handled manually, they are often measured using a variant of the Ritchie-Common test, although Fizeau and Twyman-Green interferometers have also been used. The familiar knife-edge test is an excellent means for measuring the optical figure of a flat qualitatively using a well-corrected large parabola on an optical bench. It is also useful for measuring the figure and radius of curvature of concave spherical mirrors. If the radius of curvature is in the 10- to 100-m range, however, as is common for laser optics, air turbulence reduces the accuracy of the measurement. A more quantitative technique for recording the optical figure of long focal length optics and determining their radius of curvature is to use a "transmission sphere," basically a Fizeau interferometer modified for converging or diverging light. Parallel light incident on the transmission sphere is focussed by the lens, whose surface on the sample side is accurately normal to the exiting light beams. It thus is the reference surface in the modified Fizeau interferometer. Transmission spheres can be obtained in a variety of focal lengths and f numbers, but they are quite expensive and are specific for a relatively short range of f numbers in the mirrors tested. Air turbulence is a problem for long radius of curvature mirrors just as it is in the knife-edge test. The Zygo Corporation, which manufactures transmission spheres, also manufactures large beam expanders for testing optical flats of 30 cm (12 in.) in diameter or more. However, such large beam expanders are quite expensive. This paper describes a relatively inexpensive technique using the Zygo interferometer and one or more transmission spheres together with a large parabolic mirror or well-corrected lens and a fringe analysis system for testing both large optical flats and large, long focal length concave or convex mirrors. A range of focal lengths extending to infinity can be measured without utilizing long path lengths, thus minimizing air turbulence problems. Both concave and convex mirrors can be measured using the same transmission sphere, and unlike most other techniques for measuring long focal length optics, the longer the focal length the better the system operates.