  The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed next
generation multi-wavelength survey mission. The primary instrument is a High
Energy telescope (HET) that conducts the deepest survey for Gamma-ray Bursts
(GRBs), obscured-accreting and dormant Supermassive Black Holes and Transients
of all varieties for immediate followup studies by the two secondary
instruments: a Soft X-ray Imager (SXI) and an Optical/Infrared Telescope (IRT).
EXIST will explore the early Universe using high redshift GRBs as cosmic probes
and survey black holes on all scales. The HET is a coded aperture telescope
employing a large array of imaging CZT detectors (4.5 m^2, 0.6 mm pixel) and a
hybrid Tungsten mask. We review the current HET concept which follows an
intensive design revision by the HET imaging working group and the recent
engineering studies in the Instrument and Mission Design Lab at the Goddard
Space Flight Center. The HET will locate GRBs and transients quickly (<10-30
sec) and accurately (< 20") for rapid (< 1-3 min) onboard followup soft X-ray
and optical/IR (0.3-2.2 micron) imaging and spectroscopy. The broad energy band
(5-600 keV) and the wide field of view (~90 deg x 70 deg at 10% coding
fraction) are optimal for capturing GRBs, obscured AGNs and rare transients.
The continuous scan of the entire sky every 3 hours will establish a
finely-sampled long-term history of many X-ray sources, opening up new
possibilities for variability studies.
