  EXIST is being studied as the Black Hole Finder Probe, one of the 3 Einstein
Probe missions under NASA's Beyond Einstein program. The major science goals
for EXIST include highly sensitive full-sky hard X-ray survey in a very wide
energy band of 5-600 keV. The scientific requirements of wide energy band
(10-600 keV for the High Energy Telescope considered for EXIST) and large field
of view (approximately 130 deg x 60 deg in the current design, incorporating an
array of 18 contiguous very large area coded aperture telescopes) presents
significant imaging challenges. The requirement of achieving high imaging
sensitivity puts stringent limits on the uniformity and knowledge of
systematics for the detector plane. In order to accomplish the ambitious
scientific requirements of EXIST, it is necessary to implement many novel
techniques. Here we present the initial results of our extensive Monte-Carlo
simulations of coded mask imaging for EXIST to estimate the performance
degradation due to various factors affecting the imaging such as the non-ideal
detector plane and bright partially coded sources.
