HST 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 degrees 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 for periods of Earth occultation or crossing of the South Atlantic Anomaly (see
Section 2.2.2). The target-to-sun angle at the time of observation must be greater that 50 degrees.
Exceptions have been made to these rules under three-gyro operations in the past. For example, observations have been made of Venus and a comet despite the sun angle being slightly less than 50 degrees. 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, along with
Section 5.3 of the
Primer.
A large backlog of approved HST orbits in Cycle 20 is already allocated. This severely limits the possibility of scheduling further observations in certain RA ranges in Cycle 20 without significantly delaying either these pending observations or any newly approved observations. As a result, RA restrictions will be imposed for GO observations in this cycle.
