The On-orbit Performance of the Large Area Telescope on Fermi

The Large Area Telescope (LAT) is the main instrument on the Fermi Gamma-ray Space Telescope, launched in June 2008. The LAT instrument is performing a continuous all-sky survey during the first year of the Fermi mission. It covers an energy range of 20MeV to >300GeV, with a large instantaneous field of view allowing it to sweep the entire sky every 2 orbits (approx. 190 minutes), with an energy resolution between 5% and 15%, and an angular resolution (68% energy containment) of a few degrees at 100MeV and rapidly improving to less than 1 degree at 1GeV and 0.1 degrees at 100GeV. The LAT is a pair conversion telescope built by a large international collaboration, using silicon micro-strip detectors for tracking of photon pair conversion products, a hodoscopic CsI calorimeter with large dynamic range of energy readout, and a high efficiency, segmented anti-coincidence shield for charged particle background rejection. We summarize the LAT instrument design and capabilities and its on-orbit performance, which is very close to design expectations.

This work is supported by Stanford University and the Stanford Linear Accelerator Center (SLAC) under DoE contract number DE-AC03-76SFO0515 and NASA grant NAS5-00147. Non-US sources of funding also support the efforts of GLAST LAT collaborators in France, Italy, Japan, and Sweden.