Absolute spherical surface metrology by differencing rotation maps.
A simple novel method for the absolute interferometric testing of spherical surfaces is presented. This approach yields an estimate for test surface errors without changing experimental settings, such as cavity length, which may affect the apparent reference errors. The test surface is tested in three orientations: a basic position and two rotation positions. Full-surface absolute maps for each test piece are determined with the proposition that any arbitrary wavefront of a circular cross section may be expressed as a linear combination of polynomial terms and a data-processing technique based on differencing rotation maps of the rotated surface. An optimized numerical reconstruction algorithm employing the least-squares technique to determine the true azimuthal positions of part rotation is used to reconstruct the rotational data. The technique does not require any assumptions about the surfaces under test and additional measurements. The differencing rotation method provides a new approach for rotationally asymmetric removal and azimuthal errors correction in the absolute test. Experimental results are presented to compare the method proposed to the classic two-sphere method.