Each WFC3 image is calibrated with reference files particular to the observing mode used. Currently, raw images and telemetry files are processed and calibrated, using the most up-to-date reference files, parameters, and software (see Section 1.1.1 of the Introduction to the HST Data Handbooks). Then the data products, as well as the raw data are stored by MAST (Mikulski Archive for Space Telescopes).
calwf3 corrects for instrumental effects and generates calibrated frames, and
AstroDrizzle corrects for geometric distortion, cosmic-ray rejection, and combines associated dithered images. For information on
Astrodrizzle refer to the DrizzlePac webpages, drizzlepac.stsci.edu and the
DrizzlePac Handbook.
calwf3 and
AstroDrizzle are available in the stsci_python package. Users can customize their calibrations by running the software locally. Please refer to Sections
3.2.4 and
3.4 for more information.
The calwf3 software consists of individual tasks that:
calwf3 products may still contain artifacts such as hot pixels and cosmic rays.
calwf3 cosmic ray rejection in UVIS exposures will only occur if
CR-SPLIT or
REPEAT-OBS exposures are specified in the observing program. IR images are cosmic ray rejected through the combining of multiple non-destructive reads in an individual exposure. Hot pixels can only be removed through the use of dithered exposures, which are subsequently processed with
AstroDrizzle after
calwf3 processing.
Every raw WFC3 exposure is processed by calwf3 following the steps outlined in
Section 3.2. The final
calwf3 calibrated output product is the *_flt.fits file. The data in the SCI and ERR extensions of this file are in units of electrons for UVIS exposures and electrons per second for IR. This image is then processed with
AstroDrizzle to remove geometric distortions.
The same processing steps performed on single images are performed for the multiple images that make up an association. calwf3 will recognize and correctly process
CR-SPLIT and
REPEAT-OBS exposures by interpreting the various entries in the association table and thereby determining which exposures should be combined during processing.
UVIS CR-SPLIT and
REPEAT-OBS exposures receive identical processing. Initially, the raw images that make up each
CR-SPLIT or
REPEAT-OBS set are bias subtracted. These are then combined and simultaneously cosmic rays are identified and rejected. Next, the combined image is dark subtracted and flat fielded to create a single cosmic-ray rejected and calibrated image. This output product is given the *_crj.fits file name suffix. If the
EXPSCORR header keyword switch is set to
PERFORM then all the exposures that are part of a UVIS
CR-SPLIT or
REPEAT-OBS set will also be individually processed through all the steps of
calwf3, resulting in a set of *_flt.fits files along with the combined *_crj.fits product.
IR REPEAT-OBS exposures are individually run through all the calibration steps to create a fully calibrated, flat-fielded product for each exposure and given the
*_flt.fits file name suffix. The flt images are then combined and cosmic-ray rejected, using the same process as for UVIS
CR-SPLIT and
REPEAT-OBS exposures. The output product is given the _crj file name suffix.
Observations that use the default dither patterns provided in the Phase II proposal instructions (APT), and, pointing patterns created with
POS-TARGs are automatically associated. Associated observations are first processed with
calwf3 to produce calibrated flt products for each individual exposure and, if
CR-SPLIT or
REPEAT-OBS exposures were specified as part of the dither pattern, crj products for each dither position.
AstroDrizzle is used to combine all the individual flt products from
calwf3, producing a final distortion-corrected image. The
AstroDrizzle output product is given the
_drz file name suffix.
Note: For multiple exposures, AstroDrizzle supersedes the
calwf3 cosmic-ray rejection processing. It uses the
flt files produced by
calwf3 as input, and performs cosmic-ray rejection in the process of producing the final distortion-corrected drizzled image.
