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 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 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).
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 50
° 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 firstname.lastname@example.org