The Fermi Gamma-ray Space Telescope (formerly called GLAST) is an international and multi-agency space observatory that will study the cosmos in the photon energy range of 8,000 electronvolts (8 keV) to greater than 300 billion electronvolts (300 GeV). An electronvolt is a unit of energy close to that of visible light, so Fermi will catch photons with energies thousands to hundreds of billions of times greater than those we see with our eyes (1 keV = 1,000 eV, 1 MeV = 1,000,000 eV, 1 GeV = 1,000,000,000 eV).
Fermi carries two instruments: the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM). The LAT is Fermi’s primary instrument.
Based on our knowledge of the gamma-ray sky from previous missions, scientists defined the following requirements for the Fermi instruments:
The Large Area Telescope (LAT)
Because the sky at gamma-ray energies has so many variable sources, the LAT must have a large field of view, over 2 steradians (one-fifth of the entire sky).
To identify and study sources accurately, the LAT must be able to measure the locations of bright sources to within 1 arcminute (about 1/30 of the diameter of the full Moon).
The study of gamma rays covers a broad energy range, so the LAT must catch photons with energies from 30 MeV to greater than 300 GeV. In particular, the LAT will have high sensitivity above 10 GeV, because almost nothing is known about cosmic objects at these energies.
Since gamma-ray bursts can release a torrent of gamma rays within a fraction of a second, the LAT must be able to measure gamma rays over short time intervals.
Because scientists need long observations to understand many types of sources, the LAT should be able to operate for many years without degradation.
Because of the high flux of cosmic rays, which can mask the much smaller flux of gamma rays, the LAT must be able to reject 99.999% of signals generated by cosmic rays.
