1. An Overview of RPS2

1.5.1 Orbit Structure

RPS2 can help you use your orbit allocation as efficiently as possible. One way you can do this is through the graphical timeline located in the structure section of the Visit Analysis Report (see The Visit Analysis Report). Although this is a good model of what will happen during your observations, it is not an exact timeline of the observing events. If you need help optimizing your proposal, talk with your PC.

Some of the graphical timeline limitations are due to conceptual simplifications built into RPS2. One important example is "alignment" overhead. Alignments are generally groups of exposures that use the same aperture and pointing, but the detailed rules are much more complex. As a simplification, RPS2 does not display the alignment structure, so it cannot show you where the alignment overheads actually occur. In RPS2 all alignment overhead is distributed evenly among the exposures within the alignment.

Since HST is in low earth orbit, the target viewing time varies over the course of the year. Unless you specify timing special requirements, you will not be guaranteed the precise structure given in the Visit Analysis Report. Generally, this is not a problem, but it can be if timing is critical.

• · For instance, after viewing the RPS2 output, you may assume that your exposures will be taken every 96 minutes for ~50 minutes. But your visit might actually be scheduled when the target is in or near the Continuous Viewing Zone (CVZ), in which case the exposures would be taken back-to-back over a shorter period of time.
• · Another example is a group of exposures that appear in the same orbit in RPS2 may not end up back-to-back because the exposures may be scheduled in orbits affected by the South Atlantic Anomaly (SAA).

If exposures must be done back-to-back, you should use the SEQuence NON-INTerruptible Special Requirement. Using timing special requirements can increase overhead and decrease the scheduling flexibility of your visit, so you should use them only if scientifically justified. (See the Phase II Proposal Instructions for information regarding timing special requirements.)

1.5.2 Scheduling

While RPS2 has, at its heart, the same software that is used at STScI to evaluate and prepare a proposal, not all of the STScI processing capabilities are available. Therefore, there are some situations in which RPS2 will display a visit as schedulable at a particular time when it is actually not schedulable, and other situations where the converse is true.

For example, the current version of RPS2 does not have the capability to test for guide star availability. Therefore, a visit which appears to be schedulable in RPS2 may in fact not be schedulable due to lack of appropriate guide stars for the target. This can be determined only after your Phase II proposal has been submitted and processed at STScI.