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The Structure of Exposures within Orbits with RPS2

GHRS Instrument Handbook


RPS2 gives you the means to predict many details of how your observations will be carried out. But there are also missing pieces because the actual scheduling software for HST--upon which RPS2 is based - is very complex. Thus RPS2 is an approximation. Perhaps the single most important thing to bear in mind when you look at your RPS2 output for a GHRS program is that RPS2 assumes your observations fall into tidy orbits with a nominal observing period and without being interrupted by the South Atlantic Anomaly (SAA). But in practice those assumptions are unlikely to pertain. The target visibility available at the time your observation is actually scheduled may be longer than what RPS2 showed you (it is unlikely to be shorter), meaning that an observation you thought was starting at the beginning of an orbit is now starting near the end of the previous orbit. That can change significantly some aspects of your program. Even more important, the GHRS is the only interruptible instrument now on HST, making it likely that your program will actually run over more orbits than those RPS2 shows you because of the interruptions caused by passages through the SAA. You can ask that your program avoid the SAA, but that should be reserved for specific science needs. (Note that the time you are charged is that calculated by RPS2, even if more time is actually needed when the program executes.)

In some programs requiring very high wavelength precision you will not want to have your exposures interrupted by SAA passage or earth occultation because that time delay adds uncertainty. To ensure these interruptions do not occur, specify NO SPLIT as a Special Requirement on those exposures. Do not use END ORBIT with the GHRS; it is not appropriate to an interruptible instrument. Note that using NO SPLIT causes all sub-exposures associated with a single exposure to be put together as a single unit and therefore inefficient use of your orbits can result.