uses electrically driven reaction wheels to perform all slewing required for guide star acquisition and pointing control. A separate set of rate gyroscopes provides attitude information to the pointing control system (PCS). The slew rate of HST
is limited to approximately 6° per minute of time. Consequently, about one hour is needed to go full circle in pitch, yaw, or roll. After the telescope arrives at the new target, attitude updates and guide star acquisitions take an additional 13 minutes.
As a result, large maneuvers are costly in time and are generally scheduled during Earth occultation or while crossing the SAA (see Section 2.2.2
The target-to-sun angle at the time of observation must be greater that 50°. Exceptions,
however, have been made to these rules under three-gyro operations. For example, observations of Venus and a comet were cautiously obtained with the sun angle being slightly less than 50°. Significant work is required to support such observations, so very compelling scientific justification is necessary for approval. See Section 4.1.4 of the Call for Proposals
for restrictions on observations of solar system targets, as well as Section 5.3
of the Primer