EXIST would include 3 large area (2.7 m2 each) coded aperture telescopes each with field of view 60° x 75°, for a combined total detection area of 8 m2 and field of view 180° x 75°. The 180° "fan beam" is oriented perpendicular to the orbital direction so that the full sky is imaged each orbit. Any given source is observed for at least 20% of the time, with sources closer to the orbital poles having correspondingly longer exposures. The EXIST survey sensitivity is a factor of ~1000 greater than the only previous full-sky, but non-imaging survey (HEAO-A4; 1979).