2.5 Continuous Viewing Zones
The continuous viewing zones (CVZs) are regions of the sky where HST can observe without interruptions caused by target occultation by the Earth. These zones are approximately 24 degrees in size centered on the orbital poles, which are 28.5 degrees from the celestial poles. Thus, targets located in declination bands near ±61.5 degrees may be in the CVZ at some time during the 56-day HST orbital precession cycle. The CVZ interval duration depends upon the telescope orbit, target position, and constraints imposed by Sun and Earth limb avoidance. South Atlantic Anomaly crossings limit the uninterrupted visibility of any target to no more than 5-6 orbits (see Section 2.3.2 in the HST Primer).
The CVZs in two-gyro mode will be the same size as those in three-gyro mode, but the durations may be shorter because of more restrictive pointing constraints. To determine whether constrained two-gyro observations qualify for CVZ time, observers should consult the visibility tables and plots, like those discussed above. Only those observations for which the orbital visibility is 96 minutes should be considered CVZ candidates.
