Fermi

The Large Area Telescope, a pair-conversionpair-conversion telescopeAbove about 20 MeV a gamma ray converts into an electron–positron pair in a heavy foil; tracking the pair and measuring its energy in a calorimeter gives the photon's direction and energy. The design of the Fermi LAT. detector with a 2.4 sr field of viewfield of viewThe patch of sky an instrument images at once, in square degrees for wide-field cameras or in steradians (sr) for all-sky monitors; 4π sr is the whole sky., surveys the entire sky every three hours from 20 MeVelectronvoltThe energy an electron gains across 1 volt, 1.602 × 10⁻¹⁹ J. High-energy astronomy labels photons by energy: optical light is about 1.7–3.1 eV, X-rays 0.1–100 keV, gamma rays from about 100 keV to beyond 100 TeV. A 1 keV photon has wavelength 1.24 nm. to over 300 GeV; its 4FGL-DR4 catalogue lists 7,194 sources, mostly blazars and pulsarspulsarA rotating, magnetised neutron star whose beamed emission sweeps past Earth once per turn. Millisecond pulsars, spun up by accretion, keep time to tens of nanoseconds over decades.. The Gamma-ray Burst Monitor's 14 scintillatorsscintillatorA crystal (NaI, BGO, CsI) that emits a flash of visible light when a gamma ray deposits energy in it, read by photomultipliers or photodiodes. Simple, large and omnidirectional, so used for burst monitors. watch all the unocculted sky from below 10 keV to above 25 MeV, with about 200 bursts a year predicted at design.

Operator

NASA

Orbit

Low Earth orbit

Aperture

LAT: about 2.4 sr field of view

LAT

20 MeV – 300 GeV

GBM

10 keV – 25 MeV

Launch

11 June 2008