Mission Design
Concept History
Originally designed to be attached to the International Space Station, EXIST was redesigned as a free-flying mission in 2001. The design was further refined in Fall 2004, notably by adding a low-energy telescope to improve source position determinations and extend the energy coverage. The current mission design is described below.
Current Design
The primary instrument on EXIST is the High Energy Telescope (HET), complemented by the Low Energy Telescope (LET). Both use coded-aperture imaging. The HET uses Cadmium-Zinc-Telluride (CZT) detectors and the LET uses Silicon strip detectors. 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.
Full Spacecraft Schematic
The HE and LE telescopes point roughly at the zenith, away from Earth, but the whole spacecrafts nods by ±15 - 20° about the velocity vector (see FOV graphic below).
Instrument Packaging
The total combined field of view of the HETs and LETs are almost identical. Ideally the HET and LETs can be arranged along the field of view of each telescope as shown below (left). In order to package both HETs and LETs inside the shroud of launching vehicle (possibly Delta IV-H or Altas V), the instruments are rearranged as shown below (right) and the fully coded field of view of each telescope is not interrupted.
EXIST Field of View (FOV) and Sky Coverage
Fully Coded HET FOV is 154° x 65° and the LET FoV is 160° x 64°. FOVs are co-aligned and cover roughly 20% of the sky at any time. The spacecraft nods back and forth about the orbital velocity vector 5 times per orbit to cover the poles. The following figures show the instantaneous sky coverage of the HET (left) and the coverage after one, two and 15th orbits (right).
 Fully Coded FoV of HET |
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Orbit and Sky Coverage
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 LET will point away from Earth (zenith pointing) and view the sky in a "fan beam" pattern, scanning in a full 360° circle as EXIST orbits the Earth. Because the HET and LET fields of view do not extend 180° in the direction perpendicular to the orbit vector, EXIST will nod "up and down" by 15 - 20° to cover the sky around the orbital poles. The net result is remarkably uniform coverage of the full sky during each orbit.
Telemetry
Because of the very high data rates--event rates in both the HET and LET will be over 10000 counts per second, with a combined data rate of 2.5 million bits 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.
Operational Plan
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. In the second phase of the mission, the normal scans can be interrupted by pointed observations of GRBs (either detected by EXIST or other satellites) or by planned observations of particularly interesting sources discovered during the survey phase. These observations might be made to obtain better images and spectra, or to monitor temporal variability without interruption.