  We report our progress on the development of pixellated imaging CZT detector
arrays for our first-generation balloon-borne wide-field hard X-ray (20 - 600
keV) telescope, ProtoEXIST1. Our ProtoEXIST program is a pathfinder for the
High Energy Telescope (HET) on the Energetic X-ray Imaging Survey telescope
(EXIST), a proposed implementation of the Black Hole Finder Probe. ProtoEXIST1
consists of four independent coded-aperture telescopes with close-tiled (~0.4
mm gaps) CZT detectors that preserve their 2.5mm pixel pitch. Multiple
shielding/field-of-view configurations are planned to identify optimal geometry
for the HET in EXIST. The primary technical challenge in ProtoEXIST is the
development of large area, close-tiled modules of imaging CZT detectors (1000
cm2 for ProtoEXIST1), with all readout and control systems for the ASIC readout
vertically stacked. We describe the overall telescope configuration of
ProtoEXIST1 and review the current development status of the CZT detectors,
from individual detector crystal units (DCUs) to a full detector module (DM).
We have built the first units of each component for the detector plane and have
completed a few Rev2 DCUs (2x2 cm2), which are under a series of tests. Bare
DCUs (pre-crystal bonding) show high, uniform ASIC yield (~70%) and ~30%
reduction in electronics noise compared to the Rev1 equivalent. A Rev1 DCU
already achieved ~1.2% FWHM at 662 keV, and preliminary analysis of the initial
radiation tests on a Rev2 DCU shows ~ 4 keV FWHM at 60 keV (vs. 4.7 keV for
Rev1). We therefore expect about <~1% FWHM at 662 keV with the Rev2 detectors.
