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Abstract

Apparatus for indicating with precision relative rotation in terms of integral numbers of relatively large basic angular intervals and small angular deviations from such intervals. A portion of the apparatus is typically mounted to a rotatable support where it is desired to measure and calibrate, with great precision, the angular position of the support every fixed number of degrees, such as one or ten degrees, and to measure offsets from integral numbers of such basic angular intervals. The apparatus comprises two slightly non-parallel mirror surfaces rotatable with the support and illuminated by an autocollimator. The mirrors provide multiple reflections of the illumination. The angle between the mirrors define the basic angular interval. The number of reflections varies with the number of basic angular intervals of support rotation. At each precise integral number of basic intervals, the illumination will return to a fixed location on the autocollimator viewing screen. The screen is mounted to indicate deviations from the exact multiple of basic intervals.

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First Claim