Establishing a flatness standard.

A further-developed three-plate rotation method [Appl. Opt. 31, 3767 (1992)] for the absolute testing of flats is used as a measuring method for establishing flatness standards. A special phase-stepping Fizeau interferometer was built that permits flats of diameters up to 200 mm to be tested. First results show a mean error between 0.002 and 0.003 λ in the determination of the absolute flatness deviations (λ = 632.8 nm). These deviations are obtained at 500-600 points on each plate. A number of experimental conditions connected with the thermal and mechanical stability of the plates must be fulfilled.