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45.7 Recalibrating WF/PC-1 Data
This section describes the mechanics of doing a recalibration. The steps in recalibrating a dataset include choosing and assembling any necessary reference files or tables and the raw data files, setting the desired switches and updating the calibration reference file names in the header of the .d0h (raw data) file, and finally, running the calibration software, calwfp.
In order to recalibrate the data, all of the reference files and tables that are used by the calibration steps to be performed should be retrieved (see previous section for advise on choosing reference files). We suggest copying the raw data files required for input to the calibration routines and the required reference files and tables to a subdirectory to be used for recalibration. This will preserve the original files. How to identify and retrieve the calibration reference files necessary to recalibrate the data was described in the previous section.
The next step is to set the calibration switches and reference keywords in the header of the raw data file (.d0h); these control how the data is recalibrated and which reference files are used at each step in the process. The switches and keywords are changed with the chcalpar task but can also be done with hedit.
The chcalpar task in the STSDAS hst_calib.ctools package takes a single input parameter-the name(s) of the image files to be edited. When the chcalpar task is started, it will automatically determine the instrument used to produce that image and will open one of several parameter sets (psets) that will load it with the current values of the header keywords. The WF/PC-1 pset is ckwwfpc.
For example, one method of editing the calibration keyword values would be:
- Start the chcalpar task, specifying the image(s) in which the keyword values require changing. If more than one image is specified, for example using wildcards, the task will take the initial keyword values from the first image. For example, to change keywords for all WF/PC-1 raw science images in the current directory (with initial values from the first image), use the following command:
wf> chcalpar w*.d0h
- When chcalpar begins, it will bring up epar-the IRAF parameter editor, which can be used to edit the pset of calibration keywords. Any of the values of the calibration switches, reference files or tables to the values can be changed as desired for recalibration.
- To exit the editor, type :q two times. The task will inquire whether the current settings are correct. Typing "y" will save the settings and return the IRAF CL prompt. Typing "n" will bring back the editor, to re-define the settings. Typing "a" will return the IRAF CL prompt without saving any changes. For additional examples of updating the calibration keywords, check the online help by typing help chcalpar.
The calibration reference file names in the header of the raw data (i.e., the d0h file) are typically preceded by five characters (e.g., wref$ for calibration images and wtab$ for calibration tables) which are pointers to the location on disk where the files are to be found by the calibration software. Before running the calibration routines, these variables must be set to the location of the reference files (and x0h/q1h raw data files). For WF/PC-1 data, the variables to set are:
to> set wref = "/nemesis/hstdata/"
to> set wtab = "/nemesis/hstdata/"
to> set wcal = "/nemesis/hstdata/"
or, on VMS
to> set wtab = "DISK$SHARE:[HSTDATA]"
where the directory name in double quotes is the local directory where the calibration reference files and tables reside.
Once the calibration keywords in the header of the raw data file are set properly, the data can be recalibrated. The WF/PC-1 calibration software, calwfp, is executed by typing the name of the task followed by the rootname of the observation dataset. For example, to recalibrate the dataset w0w10e02t and write the log of the results to the file calwfpc.log (rather than to the screen), type:
hr> calwfp w0w10e02t > calwfpc.log
To avoid overwriting existing calibration files, specify a different output file name,
wf> calwfp w0w10e02t wfpc_out > wfpc.log
For more information about how these routines work, use the on-line help:
cl> help calwfp
45.7.1 Calculating Absolute Sensitivity for WF/PC-1
If DOPHOTOM is set to NO before running calwfp, then the values of the inverse sensitivity (PHOTFLAM), pivot wavelength (PHOTPLAM), RMS bandwidth (PHOTBW), zero point (PHOTZPT), and observation mode (PHOTMODE) will not be written to the header of the recalibrated data file. The DOPHOTOM calibration step does not alter the values of the data (which are always counts or data numbers in the calibrated file), but only writes the information necessary to convert counts to flux into the header file. Therefore, unless the absolute sensitivity for the observation must be recomputed (e.g., because a more recent throughput table exists for the observing mode), the keyword values from the original calibrated file can be used.
To recalculate the absolute sensitivity, set DOPHOTOM=YES in the .d0h file before running calwfp, or alternately, use the tasks in the synphot package of STSDAS.
See "Improving the Photometric Calibration" on page 46-17 for suggestions on
improving the WF/PC-1 photometric calibration.
To calculate the absolute sensitivity, calwfp and the synphot tasks use a series of component lookup and throughput tables. These tables are not part of STSDAS itself, but are part of the synphot dataset, which can be easily installed at your home site (see "Getting the synphot Database" on page A-15 for information about how to do this).
The synphot dataset must be installed in order to recalculate absolute sensitivity
for WF/PC-1 data using calwfp or synphot.
Table 46.4 provides a summary of the expected accuracy of calibrated WF/PC-1 data produced by the pipeline. For a more in depth discussion of the origin of these accuracies, of problems which can prevent these levels of accuracies from being obtained, and of ways to achieve higher accuracy than those routinely obtained, see Chapter 46.
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Last updated: 01/14/98 16:04:33