2.4 Pointing Constraints
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, the FGSs will take up to 8 additional minutes to acquire a pair of guide stars. 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 2.3.2).
During normal operations, the telescope does not observe targets that are
- within 50 degrees of the Sun;
- within 15.5 degrees of any illuminated portion of the Earth;
- within 7.6 degrees of the dark limb of the Earth; or
- within 9 degrees of the Moon.
Some rare exceptions have been made to these rules. For example, the moon has been observed, and 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 these observations, so very compelling scientific justification is necessary for approval. Observations of the Moon, in particular, have required so much distortion of normal operating procedures that success rates have been low.
