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calwf3 software from the STSDAS hst_calib.wfc3 package
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PyRAF, to run MultiDrizzle and PyDrizzle, obtained from STScI
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synphot data set, if needed for photometric calculations
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Before any recalibration can be done, the user’s local directory containing the calibration reference files must be defined for the software tasks. For WFC3, this directory is referred to as “
iref”. The raw image headers already contain the appropriate keywords that list the reference file names that were assigned during STScI pipeline processing. The user must simply define the location of the “
iref” directory in the Unix environment.
If done from the command line, this setup must be done in the same window in which
IRAF (or
PyRAF) will be started. Setting “
iref” from within
IRAF will not work, even though subsequently typing “
show iref” would suggest it might. For convenience, this setup command can be added to your
.setenv file, so that the
iref environment variable will always be defined.
When retrieving data from the HDA, OTFR uses the latest available calibration reference files by default. In order to use non-default reference files, manual recalibration is required. The calibration reference file keywords will need to be updated manually in the raw data files with the desired file names before running
calwf3. In addition, the user can choose to change which calibration steps are performed by
calwf3 by resetting the values of the calibration switch keywords. These keywords are listed in
Table 3.8 along with their default values as used in the STScI pipeline. To change the values of any of the keyword switches, use a FITS keyword editor, such as the IRAF
hedit task:
calwf3 does not alter the units of the pixels in the image when calculating photometric information. Instead it calculates and writes the inverse sensitivity conversion factors (PHOTFLAM and PHOTFNU) and the ST magnitude scale zero point (PHOTZPT) into header keywords in the calibrated data files. Refer to subsections on
PHOTCORR in
Section 3.4.2 (UVIS) and
Section 3.4.3 (IR) for more information.
To compute values for the photometric keywords during the PHOTCORR step,
calwf3 uses the
STSDAS synthetic photometry package,
synphot, which requires accessing two reference files,
GRAPHTAB and
COMPTAB, which are included in the
synphot data set. This data set must be installed on the user’s system if this step is to be performed during recalibration (see Section 4.5.1 of the
Introduction to the HST Data Handbooks). In order for
calwf3 to access the
synphot files, environment variables pointing to the local
synphot directories must be defined as follows:
The synphot data set contains numerous files that are updated on a regular basis, making it cumbersome for the user to maintain. A simple alternative is to set the
PHOTCORR calibration switch to “
OMIT” in the primary header of the
raw file. This avoids the need for downloading and maintaining your own copy of the
synphot data files. The user may then simply copy the photometric keyword values from the previously calibrated data files into the
raw file header and then run
calwf3, skipping the
PHOTCORR step. This is shown in the examples in
3.7.2.
Reprocessing WFC3 UVIS and IR datasets can stress some computing platforms because of the potentially large data volume and CPU-intensive calculations. Great care has been taken to minimize the memory requirements of the pipeline software. Line-by-line I/O used during UVIS processing is particularly useful when more than one image is operated on at a time, such as during flat-field application or combining images. Unfortunately, this places an extra burden on the I/O capabilities of the computer.
calwf3 requires up to 130MB of memory to process a full-frame UVIS image and up to 250MB for an IR exposure containing a full set of 16 non-destructive reads.
MultiDrizzle requires up to 400MB.
Timing tests for processing WFC3 datasets using calwf3 are given in
Table 3.9. Geometric correction or dither-combining using
MultiDrizzle will take extra time, because these are performed separately. The CPU usage column reports the amount of time the CPU was active and reflects the amount of time waiting for disk I/O. WFC3 observers should keep these requirements in mind when securing computing resources for data processing.
This section presents several examples of calwf3 reprocessing. The boxes show commands and output to the screen. The lines beginning with the “
iraf>” symbol, indicate commands typed into
IRAF or
PyRAF. Lines with no symbol indicate output from
IRAF.
The following example uses hypothetical UVIS observations of a stellar cluster, observed with the
F814W filter. The exposures are
CR-SPLIT into two exposures of 20 seconds each. The association table for this observation is
i8bt07020_asn.fits. Typing “
tprint i8bt07020_asn.fits” reveals the rootnames of the individual exposures:
i8bt07oyq and
i8bt07ozq.
For the purposes of this first example, assume that the observer desires to reprocess only one of these exposures. This example illustrates the steps required to reprocess a single exposure after changing the bias reference file from the default value to a file specified by the user.
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4.
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Now set the PHOTCORR processing step to “ OMIT” and copy the photometric keyword values from the previously calibrated image to the raw image. Notice that the PHOTCORR keyword resides in the primary header of the FITS file, while the remaining PHOT* keywords are located in the SCI image extension headers (see Tables 2.7 and 2.8). Alternately, the user may keep track of these numbers in any other preferred manner. Most users will only require knowledge of the PHOTFLAM or PHOTFNU keywords for photometric calibration. Setting PHOTCORR=OMIT allows users to skip this synphot-based calibration step (see See “Bypassing the PHOTCORR Step” for more information).
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This example uses the same data from Example 1 and illustrates the steps required to reprocess a WFC3 association after changing the bias reference file from the default value to a file specified by the user. The steps required are similar to the previous example, with a few modifications.
IRAF output comments that are similar to Example 1 have been omitted.
Note: If this command is executed in the same directory in which you have run the previous example, then one of the
flt files will already exist and
calwf3 will not overwrite existing images. Either delete the existing
flt file, move it to a separate directory, or rename it.
The products will be two separate calibrated flt images (
i8bt07oyq_flt.fits,
i8bt07oyq_flt.fits)
and a single CR-combined
crj image (
i8bt07021_crj.fits).
The following example uses IR images that are part of a 2-point line dither pattern. This example illustrates the steps required to reprocess images that are part of a dither pattern using a non-default dark reference file. The steps are similar to Example 2, but the format of the association and the data products are unique.
The output products will be two separate calibrated datasets, consisting of ima and
flt files for each of the input images. In subsequent processing (see
Chapter 4 for details),
MultiDrizzle can be used to combine the two
flt files into a single
drz image (
i8e654010_drz.fits).
The following example is for a hypothetical IR exposure that has some number of individual readouts affected by an anomaly, such as scattered Earth light. In this example we reprocess the raw data using
calwf3 after flagging all the pixels in the last 3 readouts of the exposure, so that the data from those readouts is not used in the ramp fitting process (CRCORR step). A convenient data quality flag value to use is 256, which causes the ramp fitting step to ignore any flagged reads as if the data were saturated.
Raw WFC3 FITS files contain null DQ arrays, so it is not possible to directly edit the pixel values in the DQ extensions of the raw files. Instead, the DQ extension keyword “pixvalue” is modified, which will cause all pixels within the DQ extensions to take on that value when the raw files are read into calwf3. The modified raw image file is then processed normally with calwf3.
