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, attitude updates and guide star acquisitions take an additional 21 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 2.3.2).

During two-gyro operations, HST's scientific instruments do not observe targets that are

• within 60 degrees of the Sun;
• within 20 degrees of any illuminated portion of the Earth;
• within 6 degrees of the dark limb of the Earth; or
• within 9.5 degrees of the Moon.

Exceptions have been made to equivalent rules under three-gyro operations. Significant work is required to support such observations, so very compelling scientific justification is necessary for approval.