For scheduling purposes, the Compton Gamma Ray Observatory mission has been divided into phases corresponding to proposal cycles. Phase 1 lasted from 1991 April, through 1992 November, and constituted the first complete survey of the gamma-ray sky. Phase 2, between 1992 November and 1993 September, revisited many regions of the sky for deeper or repeated exposures. Viewing periods have lasted from a few days to three weeks.
Table 1 lists the times of each viewing period and the number designation used to identify it. In most cases, the gaps between viewing periods are a few hours, the time needed to re-point and stabilize the Compton Observatory. During Phase 2, several longer gaps represent the times used to prepare to re-boost the observatory to a higher orbit, and the gap between viewing periods 229.5 and 232.0 is caused by the scheduling of the first viewing period of Phase 3 between these two. The coverage of the sky has not been uniform, although there has been some coverage of all parts of the sky. Figure 1 displays the EGRET exposure in galactic coordinates in terms of cm
s for the sum of Phase 1 and 2.