Following is an example of a Zygo Interferogram that is supplied with all OTAs manufactured by Optical Guidance Systems.
(An interferogram is a CCD camera recording of an optical interference pattern caused from laser light beam shearing between the incoming and outgoing beam of the interferometer. The straighter the lines, the better the optics.)
Surface Contour Map
This map shows high and low areas on 20″ f/8 Ritchey-Chretien test mirror with a total wavefront deviation of .18 waves peak-to-valley and .031 waves R.M.S.