  The EXIST active shields, planned for the main detectors of the coded
aperture telescope, will have approximately 15 times the area of the BATSE
detectors, and they will have a good geometry on the spacecraft for viewing
both the leading and trailing Earth's limb for occultation observations. These
occultation observations will complement the imaging observations of EXIST and
can extend them to higher energies. Earth occultation observations of the hard
X-ray sky with BATSE on the Compton Gamma Ray Observatory developed and
demonstrated the capabilities of large, flat, uncollimated detectors for
applying this observation method. With BATSE, a catalog of 179 X-ray sources
was monitored twice every spacecraft orbit for 9 years at energies above about
25 keV, resulting in 83 definite detections and 36 possible detections with 5
sigma detection sensitivities of 3.5-20 mcrab (20-430 keV) depending on the sky
location. This catalog included four transients discovered with this technique
and many variable objects (galactic and extragalactic.) This poster describes
the Earth occultation technique, summarizes the BATSE occultation observations,
and compares the basic observational parameters of the occultation detector
elements of BATSE and EXIST.
