source of the Hubble null corrector failure). With the advent of computer-generated holograms, it is now possible to create a hologram with the phase response of an arbitrary mirror. Such a hologram can be made to analytically duplicate the phase response of the desired mirror, then be tested with the null corrector just as the real mirror would be tested. If the combination looks like a spherical mirror to the interferometer, then both the null corrector and the hologram are correct with high probability, since the null corrector and the hologram are constructed independently by different procedures. This procedure was used to test (and find an error in) the null corrector used for the MMT Observatory single-mirror retrofit. References Mirrors Optical devices Microscopy