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30.4 Evaluating Planned Observations

30.4.1 Planned Observations (Exposure Logsheet)

An examination of the RPS2 (RPSS for pre-COSTAR) input file or, more commonly, the concise proposal summary logsheet can be helpful in understanding the structure and content of an observation sequence. As will be emphasized in Chapter 32, the observational strategies employed, such as target acquisition or limitation of wavelength readout, can have a very profound effect on the quality of FOS data. An understanding of what was planned can be beneficial in comprehending what was actually obtained.

The exposure logsheets and RPS2 input files for post-COSTAR programs are available from the PRESTO web page:


http://presto.stsci.edu/public/propinfo.html

FOS program logsheets and RPS2 (RPSS) input files are available via the FOS WWW page propfind query:


http://www.stsci.edu/ftp/instrument_news/FOS/propfind.html

In Figure 30.11 we show one exposure logsheet as an example. This will be used occasionally in the following to outline a few checks you can run on your data in order to assess whether what you see is what you expected. In our example the FOS observations started with a four-stage ACQ/PEAK target acquisition at the beginning of the visibility period, followed by science observations using the FOS/RD detector with the G270H grating and the circular 1.0" aperture. The investigators used the occultation time after the second visibility period to execute a side switch from FOS/RD to FOS/BL, which takes about 50 minutes. Then, in exposure 5, they re-acquired the target with a single-stage peak-up in order to insure good pointing accuracy and then continued their science observations, now using the G190H grating for a total of 100 minutes. By looking at the data files, but not the exposure logsheet or the RPS2, one can find out that this long integration was split into three separate exposures: the first filling the remainder of the re-acquisition orbit and then one each in the subsequent two orbits. Naturally, each such exposure will receive its own rootname and complete set of data files, and each will produce its own set of diagnostic paper product pages.

In summary, since FOS exposures are not interruptible, a single exposure logsheet line or RPS2 exposure line may produce several distinct FOS exposures and hence several sets of data files. Each set of data files will be identified by unique, usually consecutively ordered, rootnames.

Figure 30.10 shows in graphical form the time sequence of events and exposures during the execution of lines 1 through 6 of the sample exposure logsheet. For the example orbits shown the target visibility is 55 minutes and Earth occultation lasts 41 minutes. Note that the exposure for logsheet line 4 was shortened from 33 to 26 minutes in order to fit it into the available target visibility. The HST scheduling system routinely shortened or lengthened exposures by up to 20% in such cases

Figure 30.10: Timeline for FOS Activities in Sample Program

Figure 30.11: Sample FOS Exposure Logsheet

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