Absolute calibration of optical flats

A method for measuring the phase error of an optical flat, comprising:

• providing a measurement beam and a reference beam from a phase shifting diffraction interferometer (PSDI), wherein said measurement beam and said reference beam comprise a relative phase shift;
• directing said measurement beam along an optical path that comprises an optical flat under test, an auxiliary focusing optic, the reflective exit face of the reference fiber of said PSDI, an imaging optic and a camera;
• directing said reference beam along an optical path comprising said imaging optic and said camera, wherein said measurement beam and said reference beam combine to produce a first interference pattern on said camera;
• determining from said first interference pattern the combined phase errors from said optical flat and said auxiliary optic;
• removing said optical flat under test from said optical path to produce a modified optical path that comprises said auxiliary focusing optic, said reflective exit face of said reference fiber of said PSDI, said imaging optic and said camera;
• redirecting said measurement beam along said modified optical path, wherein said measurement beam and said reference beam combine to produce a second interference pattern on said camera;
• determining from said second interference pattern the phase error of said auxiliary optic;
• and
• substracting said phase error of said auxiliary optic from said combined phase errors from said optical flat and said auxiliary optic to provide the phase error from said optical flat.