We describe a variation of the liquid-flat technique fordetermining the absolute flatness of a 240-mm-diameter optical surfaceto an accuracy better than 1/100λ in both its horizontal(three-point support) and vertical orientations. Using theappropriate mathematics to calculate the surface deformation of a diskdue to gravity, we achieved verification of the method by comparingmeasurements carried out on a pair of optical flats and a liquidreference surface.
© 1998 Optical Society of America
Ian Powell and Emmanuelle Goulet, "Absolute figure measurements with a liquid-flat reference," Appl. Opt. 37, 2579-2588 (1998)