A shop method for testing large mirrors using a square-array sampling screen has been developed. The main features of this method are simple test setup independent of random air turbulence, high surface sampling rate, reliability and repeatability of results, detection of surface departures ±0.1λ (at 5000 Å) or greater from the ideal surface, and self-calibration. Only large-area departures can be accurately detected by this method.
I. Ghozeil and J. E. Simmons, "Screen Test for Large Mirrors," Appl. Opt. 13, 1773-1777 (1974)