In‐Flight Calibration of EGRET on the Compton Gamma‐Ray Observatory

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory has been operating for over 7 yr since its launch in 1991 April. This span of time far exceeds the design lifetime of 2 yr. As the instrument has aged, several changes have occurred owing to spark chamber gas exchanges as well as some hardware degradation and failures, all of which have an influence on the instrument sensitivity. This paper describes postlaunch measurements and analysis that are done to calibrate the instrument response functions. The updated instrument characteristics are incorporated into the analysis software.