Recommendations for HST Observations
When constructing the visits of your Phase II program, please consider the guidelines and rules given in the HST Primer: Orbit Calculation Overview, to determine when to create new visits. The following is an explanation of the rationale for the new visit recommendations and requirements outlined in the Primer.
Maximum Duration of a Visit
Because the SAA reduces the full orbital visibility period of many targets for over half of the HST orbits each day (see the HST Primer: Orbital Constraints South Atlantic Anomaly (SAA)), visits that require more than 5 consecutive orbits are less likely to have scheduling opportunities on most days of the HST observing cycle. Consequently, any visit that exceeds 5 orbits should be broken into separate, smaller visits if there is no compelling science reason not to. Likewise, such smaller visits should not be linked to enforce contiguity that would exceed 5 orbits in duration. Proposers should consult with STScI if they feel that their science goals require the use of more consecutive orbits.
Limiting the Separation Between Target/Aperture Combinations
In order to ensure guide star (GS) availability for all targets in the visit, targets must be grouped using the 2 arc minutes limit. Groups of exposures consisting of target and aperture separations of less than 2 arc minutes are more likely to share a common guide star acquisition (GS Acq). Each set of target/apertures that cannot be combined requires a full GS Acq and the start of a new orbit. These multi-orbit, multi-GS Acq visits have less scheduling flexibility, and consequently fewer scheduling opportunities, than visits with one GS Acq. If a new full GS Acq is not required, a GS Reacquisition (GS Reacq) can be done for the second and subsequent orbits in the visit. The ability to use GS Reacqs in multi-orbit visits provides more scheduling flexibility and therefore more scheduling opportunities.
Additionally, changes in target pointing and aperture selection of less than 2 arc minutes introduce overheads which consume time in the orbital visibility period in the form of small angle maneuvers (SAMs). SAM durations are roughly proportional to the magnitude of the separation. Therefore, keeping target separations to a minimum within a visit also helps to maximize the time available for science exposures. Here again, target/aperture groupings that require large SAMs may be better served by creating a new visit if the science is not adversely affected.
Exposure Ordering For Efficient Use of the Orbital Visibility Period
Changes in instrument and detector usage between observations require time to complete. Whenever possible, visits should be constructed and exposures ordered such that instrument configuration transitions occur between orbital visibility periods. A new visit at such transitions ensures maximum scheduling flexibility.
Target Pointing Changes > 1 Degree Require a New Visit
Target separations smaller than 2 arc minutes introduce SAMs (see the HST Primer: Orbital Visibility, Acquisition Times, and Overheads) that consume orbital visibility. For separations larger than that, major spacecraft slews are generated which can be scheduled between orbital visibility periods. These slews do not interfere with the amount of time available for science exposures. However, target pointing and orbital viewing (exposure) requirements determine the orbital visibility period and the number of days of scheduling opportunities for a visit. Since each target on the sky has its own unique target availability (days per year), the intersection of the individual target scheduling windows determines the total suitability for the visit during the observing cycle. Therefore, combining targets with large separations into single visits can have the effect of restricting the total number of scheduling opportunities for the visit. The visit may even be completely unschedulable. The suitability problem increases with target separation. Placing widely separated targets in separate visits maximizes the scheduling opportunities for all targets. Given this, target separations up to 1 degree are allowed in a single visit subject to the guidelines mentioned above. Separations greater than 1 degree are disallowed in a single visit to help ensure that visit structures are not created which are unnecessarily restrictive or even completely unschedulable.
Unused Orbital Visibility Periods Prohibited in a Visit
Visit structures that create full visibility gaps are disallowed since they would result in very inefficient use of the telescope. A new visit should be created in this case.
Telescope Roll Orientation Restrictions Within a Visit
HST operational limitations require that a new visit must be created when the telescope roll orientation needs to be changed.