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5.3 Relationship to Proposed Observations
Observers should recognize that their observations do not necessarily execute in the order listed in their Phase II proposals, but rather are scheduled so that they maximize the overall efficiency of HST. The first step in understanding how your data files relate to your original request is to examine the header keywords using iminfo or imheader. For example, the iminfo listing in Figure 5.1 says that exposure x2x10108t was a 722.4 second exposure of target BPM16274 using filters F253M+F4ND with the 256 x 256 format of the f/96 camera. It also gives the Exposure ID as 01-023, meaning the exposure listed under Visit 1, Exposure Logsheet line 23 of the Phase II proposal.
To see how the actual observation compares with the corresponding request, you can retrieve recent proposals via the HST Proposal Information Page at:
http://presto.stsci.edu/pub-lic/propinfo.html
Simply enter the Program ID (or proposal number; 6160 in the example above) into the box, click on the "Get Program Information" box and select either the full text or the formatted listing. Figure 5.2 shows an example of the formatted listing for the proposal at hand. Examination of this exposure logsheet shows that Line 23, Visit 1, requested one 423s exposure of BPM16274 using the F253M+F4ND filters and the 256 x 256 format of the f/96 camera. The EXPAND requirement increased the exposure time to fill the rest of the visibility period.
For Cycle 4 and earlier programs, the Exposure ID field reflected the use of RPSS instead of RPS2 for proposal submission. Entries in these Exposure ID fields look something like 23.000000, which means the exposure that corresponds to Exposure Logsheet Line 23. Where several exposures come from the same Exposure Logsheet line (e.g., if a spatial scan is used, or the Number_of_Iterations keyword is more than 1), the Exposure ID field contains a number like 23.0000000#001, to signify the first exposure corresponding to Exposure Logsheet line 23.
You may notice that the requested and actual exposure times differ for external FOC observations, even when no EXPAND requirement is specified. This disparity arises because the flight software that controls the FOC contains a bug that shortens the length of an exposure by approximately 3.5-4.5 seconds. Because typical FOC exposures last much longer than 4 seconds and the science header reports the correct exposure time, rewriting the software to correct the bug was deemed unnecessary.
Figure 5.2: Exposure Logsheet Via World Wide Web
Having matched the exposure logsheet lines to the data received, you then need to determine whether the exposure proceeded normally. The most important resource for assessing potential problems is the PDQ file (see Chapter 2), a text file created by OPUS that records information about the state of the observatory during the observation, along with any processing abnormalities. It reports ptoential problems in the free-form comment fields QUALITY, QUALCOM1, QUALCOM2, and QUALCOM 3, as well at the end of the file. Figure 5.3 gives an example of such a report.
Items to look for are:
- Was the FGS guiding mode the same as was requested?
The default guide mode for FOC observations is fine lock. If the guide star acquisition fails, it is possible to default to single-star guiding. In most cases, the effect on data quality is so small as to be unnoticeable. In the extracted OMS keywords section at the end of the PDQ file the keywords GUIDECMD and GUIDEACT should both be set to "FINE LOC".
- Were there any losses of lock or recenterings?
These glitches can degrade an observation slightly, although again the effect is small. Look at the OMS keywords NLOSSES and NRECENT.
- Were there any data dropouts?
The DCF fill and PODPS fill parameters in the .d0h data structure section should both be zero.
- Were there any instrument anomalies?
If the OPUS examination of the data detected any suspicious artifacts that might signify an instrumental problem, a comment will appear in one of the QUALCOM keywords, perhaps with some expansion in the "Additional Comments" section at the end.
- Were there any small-angle maneuvers executed by the telescope?
Such would be the case if an exposure were preceded by an Interactive Acquisition. If so, there will generally be an observer comments (.ocx) file giving the details of any such moves, and OPUS staff usually record the moves in the comments section at the end of every affected observation
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