Described is a technique for accurately measuring the radius of curvature of very long radius concave optical surfaces in a relatively short working length. Presented is the basic theory of the technique and the detailed information necessary to provide for its practical application. Using the technique, radii of up to 175 meters can be measured to an accuracy of better than 0.01 percent with a one-meter long scale and slide, and in an overall working length of less than 5 meters. Generalizations of the technique allow for the measurement of convex surfaces and even longer radius concave surfaces. Photographs, interferograms, calculations, and an error analysis illustrate the technique.