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.

For the MAMA detectors, this limit is due to the number of South Atlantic Anomaly (SAA) free contiguous orbits. It is possible to have 6 of these orbits, however they are rare. Anything longer will never be schedulable. If a 6-orbit constraint is combined with any phase, timing and/or orient constraint restrictions then the visit will likely be unschedulable. STIS MAMA visits with 6 orbits require a compelling science justification and proposers must take into account the additional limitations that these longer visits place on observations and schedulability.

STIS CCD observations are possible to schedule in SAA impacted orbits. Therefore, there is slightly more flexibility with those observations being longer than 5 orbits with proper science justification. Scheduling longer orbit visits can be challenging, particularly when coupled with phase, time and/or orient constraints. Visits with 7 or more contiguous orbits can have very few scheduling opportunities per cycle (between zero and three).

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).

In structuring their programs, observers should carefully consider the positional accuracies that will be required. Acquisitions of point-source targets are generally accurate to ~10 milliarcseconds (see Section 8.2), and somewhat greater accuracy can be achieved by using a peakup in a small aperture (Section 8.3; see also the summary in Table 16.5). Within a given visit, HST pointing is generally very stable (~5 milliarcseconds rms), and small angle maneuvers can be similarly accurate (Section 8.2.3). For most single, non-variable point-source targets, a long series of exposures could, in principle, be obtained either in a single long (but difficult to schedule) visit or in two or more shorter visits, with somewhat shorter total exposure time but greater schedulability. If there are several different, closely-spaced targets to be observed (requiring slight movements of the telescope but using the same guide stars), and if very precise offsets (few milliarcsecond accuracy) between the different targets are required, then it might be desirable to observe all the targets in a single long visit.  If such precise offsets are not required; however, then the observations could be obtained in separate shorter visits (e.g., with one target per visit). In cases where a target acquisition is performed for slit spectroscopy, one should follow the same procedure for the target acquisition in the subsequent visits 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.