Fizeau interferometer for measuring the flatness of optical surfaces.

A Fizeau interferometer is described with which to test the flatness of optical surface up to 240-mm diam. A mercury mirror of suitable diameter is used as a flatness standard. A simple 240-mm diam lens is sufficient. For easy testing of the surfaces of wedge-shaped glass plates without adjustment difficulties the illumination and photographic arrangement, mounted in the same frame, can be tipped on an axis, going through the test surface. The influence of the aberration of the simple lens is discussed. Experimental results of the measurement of an optical flat are presented.