Radiation Belt Passages High particle-radiation levels are encountered as the Observatory approaches perigee. Data acquisition ceases whenever certain particle-radiation thresholds are exceeded. A working number for the altitude at which this takes place is about 60,000 km. Moon avoidance - viewing is restricted to angles larger than 6 degrees from the limb of the Moon. Bright Earth avoidance - viewing is restricted to angles larger than 20 degrees from the limb of the bright Earth.
North is up, and East is to the left.
Roll is measured positive, west of north. The roll angle shown is . The exact value depends on the relative positions of the Sun, Earth, and Moon as seen from Chandra . Figure 3.3 illustrates the point that the Earth avoidance region is nearly stationary in equatorial coordinates. The Chandra visibility showing contours of fractional visibility averaged over the 12-month interval of Cycle 3.