The Scanning Modulation Collimator, HEAO A-3. The A-3 experiment consisted of two scanning modulation collimators with proportional counters, aspect sensors, and electronics. The major purpose of this experiment was to determine accurately the position of selected cosmic X-ray sources and to investigate their size and structure. Principal Investigators were Herbert Gursky and Daniel Schwartz of the Smithsonian Astrophysical Observatory and Hale Brad t of MI T. Hard ware was provided by American Science and Engineering, Inc.