HST Data Handbook for WFPC2

2.3 Correlating Phase II Exposures with Data Files

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Because of the need to schedule observations as efficiently as possible, the order in which exposures are executed may be different from the order in which they appear in the observer's Phase II submission, unless an explicit special requirement dictates otherwise. As a result, the data may be received in an order that differs from that originally proposed by the observer. In this section, we discuss how to correlate the data received with the exposures specified in the Phase II submission, which can often facilitate the process of interpreting the observations and their interconnection. The Phase II proposal specifications submitted by the investigator can be found by entering the proposal ID on the HST Program Information page or in the Program Status box on the top HST page.

The first step is often identifying the program to which a data file belongs, and can be accomplished via the header keyword PROPOSID. Data files can then be associated with the corresponding exposure line in the Phase II proposal by comparing exposure information in the Phase II proposal with data file header keywords. For WFPC2 data, the most useful comparisons are shown in the table below.

Table 2.5: Comparing Phase II Proposal Keywords to Data Header Keywords

Phase II	Data Header
Proposal ID	PROPOSID
Target_Name	TARGNAME
Position	RA_TARG, DEC_TARG
Spectral Element	FILTNAM1, FILTNAM2
Time_Per_Exposure	EXPTIME

 

A convenient tool for viewing some of the more important data header keywords in an easy-to-read formatted output is the STSDAS task, iminfo. An example of the output of this task in shown in figure 2.1. Note that the data header keywords are expanded to standard English words in this output. The header file (.c0h) can also be examined with IRAF tasks hedit or imhead, with any standard text editor, or simply by listing the contents of the file. Again, please note that a standard text editor will show only the comments for the group keywords, not the values which are stored in the image file.

Figure 2.1: Displaying WFPC2 Header Keywords with iminfo
 

In the specific example shown in figure 2.1 the Proposal ID is given as 05837. Entering the proposal ID into the space provided on either the HST Program Information web page or Program Status box on the HST web page (without the leading 0) and clicking on [Get Program Information], will bring up the Program Information page for that proposal. Under Program Contents on that page you can choose either the file typed in during Phase II or a formatted output. The latter may be easier to read; we reproduce a portion of that file in table 2.6.

The Exposure ID listed by iminfo is 02-023. This corresponds to visit 02, exposure 23. A different format was used in Cycles 0 through 4; exposures in these proposals have a single, unique numeric identifier. To reach this exposure line, page down through the proposal until visit 02 is reached. Now search for exposure 23 in visit 02. As shown in table 2.6, this exposure requested a single 5 second exposure of target CAL-GANY-W through filter F410M. A quick comparison with the keywords listed by iminfo shows that, indeed, this data file contains the observation specified in this exposure line.

Table 2.6: Exposure Log Sheet for WFPC2

A comparison of these keywords should quickly reveal the data file corresponding to a given exposure line. There are, however, two cases in which such a comparison is somewhat more complicated.

It is recommended that WFPC2 exposures longer than 600 seconds be split into two shorter exposures to facilitate the removal of cosmic rays. If the optional parameters CR-SPLIT and CR-TOLERANCE are omitted in the Phase II submission, and the exposure is longer than 600 seconds, it will be split into two exposures for more efficient scheduling. The default CR-TOLERANCE of 0.2 can be used, meaning that the split exposure times could each range from 30-70% of the total exposure, with their sum equal to the original total exposure time.

Exposure times may occasionally be shortened or lengthened by up to 20% without the approval of the PI, provided that the resulting S/N is 90% of that with the original exposure time. Such changes may be required to fit observations into specific orbital time slots. If, after examining your exposure headers, you still have questions regarding the execution of your observing plan, we recommend you speak with your program coordinator or email help@stsci.edu.

The output generated by iminfo contains information on the target, exposure time, and filters used, but nothing on guide star status. Any of the STSDAS tasks (hselect, hedit, or imhead), see table 2.2, can provide access to that information by printing the FGSLOCK header keyword to the screen.

The FGSLOCK keyword can have the values FINE, COARSE, GYROS, or UNKNOWN. Coarse tracking is no longer allowed, so this FGSLOCK in recent data will most likely read either FINE or GYROS. Gyro tracking allows a drift rate of approximately 1 mas/sec. It would only be used if requested by the proposer. FINE tracking typically holds pointing with an rms error of less than 7 mas. Normally, two guide stars are used in HST observations, but on occasion only one appropriate guide star can be found. Such observations will suffer from small drift rates (a few mas in a 1000 second exposure). To determine the quality of tracking obtained during your observations, please review appendix C of the HST Introduction which describes how to determine the number and quality of guide stars actually used, as well as how to use the OMS jitter files.