2.5 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 Figure 2.2).

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 the 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 (see Figure 2.1). Consequently, the nominal roll required for a particular observation depends on the location of the target and the date of the observation. Observations of the same target made 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 degrees when the U1-to-sun angle is between 60 and 90 degrees, less than 30 degrees when the angle is between 90 and 179 degrees, and it is unlimited between 179 and 180 degrees. (Note that in order to achieve an anti-sun pointing of 179-180 degrees 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 help@stsci.edu if you need assistance.