HST Primer: Pointing, Orientation, and Roll Constraints
An overview of HST positional 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° 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 HST Primer: Orbital Constraints).
The target-to-sun angle at the time of observation must be greater than 55°. Exceptions, however, have been made to these rules under three-gyro operations. For example, observations of 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 HST Observation Types and Special Requirements for restrictions on observations of solar system targets, as well as HST Primer: Observing Considerations.
Orientation and Roll Constraints
The orientation (ORIENT) of the telescope is defined as the position angle of the U3 axis on the sky measured from north through east (see HST Primer: System Overview).
In principle, HST is free to roll about the U1 optical axis. However, this freedom is limited by the need to keep sunlight shining on the solar arrays and by a thermal design that assumes that the Sun always heats the same side of the telescope.
For a particular pointing, the orientation of the telescope that optimizes solar array positioning with respect to the Sun is called the nominal roll. At this orientation, the Sun is in the half-plane defined by the U1 axis and the negative U3 axis. Consequently, the nominal roll required for a particular observation depends on the location of the target and date of the observation. The same target observed at different times will, in general, be made at different orientations. Some departures from nominal roll are permitted during HST observing (e.g., if a specific orientation is required on a particular date or if the same orientation is required for observations made at different times).
Off-nominal roll is defined as the angle about the U1 axis between a given orientation and the nominal roll. Off-nominal rolls are restricted to less than approximately 5° when the U1-to-sun angle is between 55° and 90°, less than 30° when the angle is between 90° and 178°, and it is unlimited between 178° and 180°. (To achieve an anti-sun pointing of 178° to 180°, the target must lie in or near the plane of the Earth’s orbit.)
Observations requiring a certain ORIENT for a target at a particular time may not be feasible because the required off-nominal roll angle may be outside the allowed limits. The Visit Planner in the Phase II mode of the Astronomer’s Proposal Tool (APT) software can be used in such cases to assess the feasibility of the observations. Please contact the HST Help Desk at http://hsthelp.stsci.edu for assistance.