WFC3 Data Handbook V. 4.0

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3.2	Overview of calwf3

3.2.1 Structure and Processing Flow

3.2.2 UVIS Processing

3.2.3 IR Processing

3.2.4 Running calwf3

3.2.1 Structure and Processing Flow

calwf3 uses separate paths for processing UVIS and IR images, as listed in Table 3.1. calwf3 automatically calls the appropriate tasks, but each may be run separately if the user desires some variation from the normal processing. wf3ccd and wf32d are used for processing UVIS images, while IR image processing is done with wf3ir. The wf3rej program is used for both UVIS and IR images to combine multiple exposures contained in a CR-SPLIT or REPEAT-OBS set. Figure 3.2 shows the calwf3 flow of data in the calwf3 pipeline

Table 3.1: WFC3 Calibration Pipeline Tasks.

Task	Function	UVIS	IR
wf3ccd	UVIS CCD-specific calibrations	Y	N/A
wf32d	UVIS generic 2D image calibrations	Y	N/A
wf3ir	IR calibrations	N/A	Y
wf3rej	Image combination/Cosmic-ray rejection task	Y	Y

UVIS Images

wf3ccd first subtracts the bias and trims the overscan regions from the image. If an associated set of UVIS CR-SPLIT or REPEAT-OBS images is being processed, all of the overscan-trimmed images are sent through wf3rej to be combined and receive cosmic-ray rejection. The resulting combined image then receives final calibration with wf32d, which includes dark subtraction and flat fielding. If there are multiple sets of CR-SPLIT or REPEAT-OBS images in an association, each set goes through the cycle of wf3ccd-wf3rej-wf32d processing.

IR Images

All individual IR images are processed with wf3ir. If an association of IR REPEAT-OBS images is being processed, the calibrated images are combined into a single product using wf3rej. If there are multiple sets of REPEAT-OBS images in an association, each set goes through the cycle of wf3ir-wf3rej processing.

All Images

During automatic pipeline processing, AstroDrizzle follows calwf3. All calibrated images are corrected for geometric distortion correction and associated sets of dithered images are combined into a single product. See Chapter 4 for more information on geometric distortion.

Figure 3.1: Flow diagram for WFC3 data shown with calwf3 task names.

3.2.2 UVIS Processing

The individual UVIS processing steps performed by wf3ccd and wf32d are shown in Figure 3.2 and Figure 3.3, respectively. The reference files needed for each step and the calibration switches controlling them are also listed. The calibration steps are as follows:

•	Calculate a noise model for each pixel and record in the error (ERR) array

•	Flag known bad pixels and saturated pixels in the data quality (DQ) array

•	Correct for A-to-D conversion errors, if necessary

•	Subtract bias level determined from overscan regions

•	Subtract bias image

•	Subtract post-flash image, if necessary

•	Perform cosmic-ray (CR) rejection and combining of CR-SPLIT or REPEAT-OBS data, if necessary

•	Scale and subtract dark image and calculate mean dark value

•	Perform flat fielding and gain conversion

•	Perform shutter-shading correction, if necessary (if not done during cosmic-ray rejection)

•	Calculate photometric header keyword values for flux conversion

•	Calculate image statistics

Each step is described in detail in Section 3.4.

Figure 3.2: Flow diagram for CCD data using wf3ccd in calwf3.

Figure 3.3: Flow diagram for overscan-trimmed CCD data using wf32d in calwf3.

3.2.3 IR Processing

Individual IR processing steps performed by wf3ir are shown in Figure 3.4, along with the names of reference files and calibration switches. The steps are as follows:

•	Flag known bad pixels in the data quality (DQ) array

•	Identify pixels in the initial (‘zeroth’) read that contain detectable source signal

•	Subtract bias drifts determined from the reference pixels

•	Subtract the zeroth read

•	Calculate a noise model for each pixel and record in the error (ERR) array

•	Correct for photometric non-linearity and flag saturated pixels

•	Subtract dark image

•	Calculate photometric header keyword values for flux conversion

•	Convert the data from counts to count rates

•	Perform flat fielding and gain conversion

•	Calculate image statistics

Detailed descriptions of each calibration step are given in Section 3.4. In general, the first nine steps - through conversion to count rates - are applied independently to each readout of the IR MultiAccum exposure stack. The up-the-ramp fitting processes then produces a single image representing the best-fit count rate for each pixel. The final two calibration steps are then applied to that single image, as well as to each read of the MultiAccum stack.

3.2.4 Running calwf3

The calwf3 processing software and its associated routines, such as wf3ccd and wf3ir, are now available independent of the IRAF environment as part of the HSTCAL package.

Users may access the routines while running PyRAF by importing the new wfc3tools package (released as part of the latest stsci_python).The calibration routines may also be called directly from the command line prompt. The wfc3tools package and the calwf3 executable are included with the HSTCAL package released with the latest STSDAS public release and can be downloaded from:

http://www.stsci.edu/institute/software_hardware/stsdas/download-stsdas.

 

Warning: Users wishing to run the most recent version of the calibration pipeline should NOT use the version of the software found in the IRAF stsdas.hst_calib.wfc3 package. The previous IRAF/PyRAF version, while still available within the current STSDAS/IRAF release, will no longer be maintained or updated. The STScI archive pipeline began using the HSTCAL-version of calwf3 as of Sept 26, 2012.

Examples of how to run the code:

In Python, without Teal:

>>> from wfc3tools import calwf3

>>> calwf3.calwf3(filename)

 

In Python, with Teal:

>>> from stsci.tools import teal

>>> from wfc3tools import calwf3

>>> teal.teal('calwf3')

 

In PyRAF:

>>> import wfc3tools

>>> epar calwf3

 

From the command line:

> calwf3.e filename

 

All versions of calwf3 past 3.0 represent the new HSTCAL software.

Table 3.2: Types of files used as input to calwf3.

INPUT	FILE DESCRIPTION
_asn	filename of association table
_raw	filename of individual, uncalibrated exposure
_crj	filename of any sub-product from an association table

While both CR-SPLIT and REPEAT-OBS exposures from an association get combined using calwf3, dithered observations from an association do not. AstroDrizzle must be used to create a dither-combined product.

Warning: All WFC3 observations, not just dithered images, need to be processed with AstroDrizzle to correct for geometric distortion and pixel area effects.

Figure 3.4: Flow diagram for IR data using wf3ir in calwf3.