Abstract: Techniques for absolute interferometric measurement ofsurface figure are often used for qualification ofspherical surfaces specified as $lambda@/20 PV orbetter. As the radius of curvature increases, thesetechniques become impractical due to the length ofcavity required, and environmental effects. Ourapproach has been to certify a reference surface with ashorter, i.e., manageable radius of curvature, and thento proceed through a series of measurements withsurfaces of increasing radius, while maintaining amanageable cavity length. At each stage, the`reference' data from the previous stage is subtractedfrom the new measurement in order to better approximatethe actual surface. The requirements of spatialregistration and image magnification are stringent ifone is to use the phase measured data directly. Thisprocess can be simplified if one uses a polynomial fitto the surface, e.g., Zernikes. The technique isdescribed for the qualification of a surface with an 8meter radius of curvature and is examined for sourcesof error. !6
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