Mirror Compensator for Testing Convex Secondary

The testing of convex secondary mirror of large telescope is known as a difficault thing, because it needs for a large mirror or lens. In USA, they used to use Hindle sphere for testing convex hyperboloids, and in West Germany as I know, they often use concave matrix probe for testing convex mirrors. In the case of Hindle method, the diameter of testing sphere always is about 2 or 3 times as the convex mirror under testing, in the later case, the matrix probe must has the same size as the convex mirror, further more, the probe itself needs another testing facilities. Some other methods had been suggested by R. Wilson, E, Gaviola and others but they have rarely been used in practice.