This movie provides a comparison of sky availability, over the course of one year, between 3-gyro and 2-gyro operations of the Hubble Space Telescope. The 2-gyro portion is based on current, preliminary knowledge and is meant to provide a general overview of the scheduling impacts and limitations of the mode.
BLUE indicates the portion of sky that is available for observing by Hubble, while GREY indicates the area that is NOT available.
The upper panel shows availability under normal 3-gyroscope operations. Here, the 50 degree solar avoidance zone is easily recognizable as it traverses the ecliptic. Note that with the exception of the avoidance zone, Hubble is capable of observing throughout the entire sky during any given week.
The lower panel shows availability when observing under the constraints imposed by 2-gyro operations. The drastic change in availability is due to the way various elements of the spacecraft must be used in order to achieve guided science in this mode. For more details, see the How does 2-gyro science mode work on this site. These requirements result in several easily seen effects.
First, note the large portion of sky, ahead of the solar avoidance zone, that is now unavailable.
Second, note the area of availability that tracks the trailing edge of the solar avoidance zone. You can see that HST becomes limited to observing targets only when they are on one particular side of the sun.
Third, is the periodic and alternating availability near the equatorial poles. At these high declinations, availability is tied to the precession of the HST's orbit.
Note also, the increased size of the solar avoidance zone, which is due to an anticipated increase in the pointing uncertainty following a slew to a target. Having a large solar avoidance zone is therefore necessary to insure the safety of the HST.
Over the course of a year all areas of the sky can eventually be reached in 2-gyro mode. However, the total available time for a given target is much less than when operating with 3 gyroscopes.
This concludes the movie.