  Imaging at hard X-ray energies (~10-600 keV) over very large fields of view
(~60 deg per telescope) is required to conduct a high sensitivity all-sky and
all-time survey for black holes. The proposed Energetic X-ray Imaging Survey
Telescope (EXIST) could achieve the high sensitivity required for the mission
science objectives by scanning an array of widefield coded aperture telescopes
with aperture mask holes radially aligned to minimize auto-collimation by the
thick (~7mm) masks required for high energy imaging. Simulation results from a
preliminary design study are reported which quantify the improvement in
off-axis imaging sensitivity vs. the conventional case with mask holes all
perpendicular to the mask. Such masks can be readily constructed from a stacked
laminate of thin (1mm) laser-etched W sheets. An even more dramatic increase in
coded aperture imaging sensitivity, and dynamic range, for a realistic
telescope and imaging detector with typical systematic errors can be achieved
by continuously scanning the field of view of the telescope over the source
region to be imaged. Simulation results are reported for detectors with
systematic errors 1-10%, randomly distributed but unknown in each detector
pixel. For the simplified case of a 1-D coded aperture telescope scanning along
its pattern, the systematics are removed identically. Results are also
presented for the 2-D case with both 1-D and partial 2-D scanning which
demonstrate the feasibility of a coded aperture scanning telescope with
systematic errors achieving nearly Poisson-limited sensitivity for
signal/background ratios S/B ~ 10^-4, in constrast to limits typically ~10-
100X worse that have been actually achieved by pointed or dithered coded
aperture telescopes flown (or proposed) previously.
