In February 2016, the pipeline began to process WFC3/UVIS data with calwf3
version 3.3, which incorporates two fundamental changes to the way exposures are calibrated and corrected. First, calwf3
applies pixel-based CTE (Charge Transfer Efficiency) corrections. (Calwf3
version 3.4, which added CTE corrections for subarray apertures, became operational in October 2016.) Second, photometric calibrations are determined and applied independently for each CCD chip. See WFC3 Space Telescope Analysis Newsletter issue 22
for a summary of the changes underlying and included in calwf3
version 3.3. WFC3 ISR 2016-01
and WFC3 ISR 2016-02
provide a reference guide and cookbook for calwf3
version 3.3, respectively. WFC3 ISR 2016-03
provides chip-dependent inverse sensitivities. WFC3 ISR 2016-04
describes the creation of chip-dependent flat fields, which included a correction for the effect of the crosshatch pattern in the UV filters on sensitivity calibrations, based on analysis presented in WFC3 ISR 2015-18
and described in Section 5.4.3
. WFC3 ISR 2016-07
documents the changes in WFC3/UVIS component files used by SYNPHOT and PYSYNPHOT to simulate HST photometric measurements.
The raw, uncalibrated FITS files are processed through calwf3
, the software task that calibrates the data for individual exposures, producing calibrated FITS files. Exposures that are obtained as part of an associated set, such as dithered images, have calwf3
calibration applied to the individual exposures before being processed as a set for the purpose of image combination. All calibrated images will be processed further with the STScI AstroDrizzle
software, for the purpose of removing geometric distortions from individual exposures and for combining associated exposures.
The FITS file name suffixes given to WFC3 raw and calibrated data products are described in Table E.1
and closely mimic the suffixes used by ACS and NICMOS. The initial input files to calwf3
are the raw (RAW) files from generic conversion and the association (ASN) table, if applicable, for the complete observation set. Most WFC3/UVIS RAW images first go through pixel-based CTE correction, producing a temporary, CTE-corrected RAW file with the suffix “RAC_TMP”. The RAC_TMP and original RAW files have the same calibration steps applied, producing two sets of final calibrated products (one uncorrected and one corrected for CTE). For WFC3/UVIS images, a temporary file, with the suffix “BLV_TMP” (BLC_TMP for CTE products), is created by calwf3
once bias levels have been subtracted and the overscan regions trimmed. This file is renamed using the “FLT” (“FLC” for CTE products) suffix after the remaining standard calibrations (dark subtraction, flat fielding, etc.) have been completed. For exposures taken as part of a UVIS CR-SPLIT or REPEAT-OBS set, a parallel set of processing is performed, using the BLV_TMP/BLC_TMP files as input to an image combination and cosmic ray rejection routine. The resulting CR-combined image, with a temporary file name suffix of “CRJ_TMP” (“CRC_TMP” for CTE products), then receives the remaining standard calibrations, after which it is renamed using the “CRJ” (“CRC” for CTE products) suffix.
The UVIS processing portion of calwf3
is based on calacs
calibrations of ACS/WFC CCD images, while the calwf3
IR processing is very similar to calnica
processing of NICMOS images. Calwf3
performs the following basic science data calibrations:
WFC3 grism observations are handled in a special way by the pipeline. Grism observations require a special flat-fielding procedure, where the flat-field value for each pixel is based on the wavelength of the detected signal. Calwf3
processing of grism images therefore uses an “identity” flat-field reference image (an image filled with values of 1.0 at each pixel), which allows for the gain calibration part of the flat-fielding step to still be applied without actually flat-fielding the science image. A separate software package, aXe, which was developed at ST-ECF and previously used for processing NICMOS and ACS spectral observations, is used to extract and calibrate one-dimensional spectra from WFC3 grism exposures (see Section 8.5
). The aXe software is available in STSDAS via PyRAF. The aXe software is used to locate and extract spectra of individual sources from calibrated images and performs wavelength calibration, background subtraction, flat fielding, and absolute flux calibration for the extracted spectra.
shows the values assigned to pixels in the DQ arrays of calibrated images, which indicate anomalous conditions and are frequently used in downstream processes to reject a pixel value. If more than one data quality condition applies to a pixel, the sum of the values is used. Note that some flag values have different meanings for UVIS and IR images.