software from the STSDAS hst_calib.wfc3
, to run MultiDrizzle
, obtained from STScI
data set, if needed for photometric calculations
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
) 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
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
uses the STSDAS
synthetic photometry package, synphot
, which requires accessing two reference files, GRAPHTAB
, 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:
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
. 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
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.
|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
). Alternately, the user may keep track of these numbers in any other preferred manner. Most users will only require knowledge of the PHOTFLAM
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).
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
and a single CR-combined crj
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
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
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.