Originally designed to be attached to the International Space Station, EXIST was redesigned as a free-flying mission in 2001. A low X-ray energy telescope was added in 2004 to improve source position determinations and extend the energy coverage. Changing scientific objectives led to the replacement of the low energy telescope by an optical-IR telescope (IRT) in 2007; this is the current baseline.
The primary instrument on EXIST is the High Energy Telescope (HET), complemented by the Optical-Infrared Telescope (IRT). The HET uses coded-aperture imaging, with a tungsten mask and Cadmium-Zinc-Telluride (CZT) detectors. The IRT is at the center of the HET; the two telescopes are co-aligned. The large volume of CZT in the HET is also an effective detector of high-energy particles (cosmic rays) so "active" shielding in the form of panels of cesium iodide (CsI) scintillator is used as an HET Anti-COincidence System (HACOS). Likewise, the HACOS can also detect photons, and provides useful energy resolution and effective area up to roughly 20 MeV.
EXIST will be launched into Low Earth Orbit (LEO) at an altitude of 500 km with 5° inclination and a 95-minute period. The low inclination orbit largely avoids the South Atlantic Anomaly (SAA), a region of high particle background where the EXIST detectors would be swamped with noise. Both HET and IRT will usually point away from Earth (zenith pointing). Whether the spacecraft will nod around the zenith is being studied.
Because of the very high data rates--event rates in the HET will be over 10000 counts per second--there will be three data downloads per day, using Ku-band communications via the Tracking and Data Relay Satellite System (TDRSS). The TDRSS S-band system will be used for real-time control and alerts, such as Gamma-Ray Burst (GRB) notifications. This telemetry plan is similar to that of Fermi.
The nominal mission lifetime is 5 years with a goal of 10 years. The first year or two will be dedicated to the primary mission, a full-sky deep survey. The spacecraft will slew autonomously when a gamma-ray burst is detected onboard to point the IRT at the location of the burst. In the second phase of the mission, the IRT will be pointed at sources detected in the first phase to help identify the sources.