A variation of the Ritchey-Common configuration was applied to the subaperture testing of a 29-in diameter flat at about 65 degrees oblique incidence in the 12-in collimated beam of a Fizeau interferometer, yielding an elliptical beam footprint spanning the full diameter of the mirror under test. A set of subaperture samples was built up in a 'flower petal' pattern symmetric about the mirror center by in-plane rotation of the mirror in 30-degree increments. A key advantage of this method of sampling over raster methods is that the synthesis of the full surface map is greatly simplified by not having to keep track of individual piston and tilt terms because of the symmetry. An advantage over the Ritchey-Common configuration is that the cavity length can be made much shorter, thus greatly reducing atmospheric effects. The data reduction and surface synthesis processes simply consisted of fitting Zernike polynomial expansions to the (digitized) individual interferograms, subtracting the piston and tilt terms, then applying rotation and scaling transformations to the pupil coordinate grid to map the (circular) pupil surface data into the appropriate elliptical footprints.
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