A large high quality optical flat can be used to test parabolic mirrors. Often the flat is called a Cassigrain flat since it has a small hole in the center. Light from a small laser goes through the hole in the Cassigrain flat and shines on the mirror under test. The laser light source must be defocused to illuminate the entire mirror under test. If the mirror under test is parabolic and the light source is at the focus then the light from the parabolic mirror strikes the flat and returns to the mirror under test as a collimated beam and is focused back through the hole in the Cassigrain flat to the knife edge or ronchi grating. Using this method errors in the figure of the parabola under test are doubled since the light strikes the same point on the parabola twice. It is also possible to generate interference patters of the parabolic mirror surface deviation.
See diagmak.htm for more information.