Precise manufacturing of optical flats requires precise characterization of the surface. A scanning pentaprism system is ideal for use as an absolute and precise test for an optical flat. Such a system was built and used to test a diameter flat mirror. This system uses light from an autocollimator that is reflected from two pentaprisms to project reference beams of light onto the flat mirror. The light reflected from the mirror back through the pentaprisms provides information on low order optical aberrations in the flat mirror. We report results of the test on a flat, characterizing the errors and their sources. There is enormous potential for our system to be used to test larger flats and even curved surfaces, made of either a glass or a liquid.