Results of a design study of a wavefront folding interferometer were applied to the fabrication of a prototype interferometer which was experimentally employed to make angus diameter measurements of missile re-entries. The interferometer, based on a modification to Lloyd's mirror, displays the space frequency Fourier transform, including both am tude and phase, of an incoherently illuminated object. Because the wavefront folding interferometer provides a direct positional reference, this instrument was used in field to provide a measure of both apparent angular diameter and tracking error. The field instrument consists of a cm collector telescope, a wavefront folding interferometer and an 8 mm camera to record the fringe pattern. During the experimental program carried out on the Pacific Miss Range, the interferometer was installed on a Roti Optical tracking mount. Because of the extreme pointing sensitivity of the instrument and the relatively crude tracking provided difficulties were encountered which would be absent with more accurate tracking. Use of the folding interferometer to measure the apparent diameter of a time varying source
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