2.6 Scheduling Efficiency and Visit Orbit Limits

Calendar time for the scheduling and execution of long visits has become severely oversubscribed. To assure a greater degree of scheduling efficiency and avoid such heavy oversubscription, a 5-contiguous-orbit limit was imposed for Cycle 8 and beyond for both CCD and MAMA visits. Observers with programs using more than 5 orbits per target should take note of the following advice when splitting their observations into multiple visits:

• Target acquisition sequences will need to be repeated at the start of each visit when using small entrance apertures with STIS. Imaging and slitless spectroscopy do not normally require a target acquisition.
• There exists the potential for some image rotation between visits unless the SAME ORIENT AS <first visits #> special requirement is used. When SAME ORIENT AS is used, the accuracy is a few milliarcseconds.
• Users are strongly encouraged to structure observations of 2-3 orbits duration each, unless longer visits are explicitly required to reach the stated science goals.
• Moving targets and spatial scanning: As of Cycle 28, limitations due to the gyro configuration of HST and its pointing control system restrict visits on moving targets to no more than two contiguous orbits. For similar reasons, for spatial scanning programs, each visibility period must have at least 6 minutes of time under FGS control (i.e., 6 minutes without scanning).

HST pointing is generally very stable within a visit (~5 milliarcseconds rms). Small inaccuracies (~20 milliarcseconds rms) can occur between visits, due to the way guide-star acquisitions are handled at the start of a new visit. Although these offsets cannot be avoided, there are many situations where they are unimportant, or where their impact can be greatly reduced. For example, if the visit makes observations of several different targets by moving the telescope, and if there is no need for exact offsets (few milliarcsecond accuracy) between the different targets, then the visit could simply be split into separate visits with one target per visit. In cases where a target acquisition is performed for slit spectroscopy, one should follow the same sequence for the target acquisition in the second visit as in the first.

Visits can be scheduled close in time by specifying the GROUP WITHIN visit requirement. Timing constraints of this type, however, should be used only if required by the science, since overconstraining visits affects the ability to schedule efficiently.