We describe a variation of the liquid-flat technique for determining the absolute flatness of a 240-mm-diameter optical surface to an accuracy better than 1/100# in beth its horizontal (three-point support) and vertical orientations. Using the appropriate mathematics to calculate the surface deformation of a disk due to gravity, we achieved verification of the method by comparing measurements carried out on a pair of optical fiats and a liquid reference surface.#1998 Optical Society of America OCIS codes: 220.4840, 120.3180.
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