Space Telescope Science Institute
WFC3 Data Handbook 2.1 May 2011
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WFC3 Data Handbook > Chapter 3: WFC3 Data Calibration > 3.1 Calibration Overview

Each WFC3 image is calibrated with reference files particular to the observing mode used. Once requested via the HDA, WFC3 data are processed with “On-The-Fly-Reprocessing” (OTFR), which processes and calibrates raw telemetry files using the most up-to-date reference files, parameters, and software (see Section 1.1.1 of the Introduction to the HST Data Handbooks).
OTFR calls calwf3 to correct for instrumental effects and generate calibrated frames, and MultiDrizzle to correct for geometric distortion, cosmic-ray rejection, and to combine associated dithered images. See Chapter 4 for information on MultiDrizzle.
calwf3 and MultiDrizzle are available in the STSDAS package. Users can customize their calibrations by running the software locally. Please refer to Sections 3.2.4 and 5.2.1 for more information.
The calwf3 software consists of individual tasks that:
Combine images and reject cosmic rays from CR-SPLIT and REPEAT-OBS exposures
calwf3 products may still contain artifacts such as hot pixels and cosmic rays. calwf3 cosmic rays rejection in UVIS exposures will only occur if CR-SPLIT or REPEAT-OBS exposures are specified in the observing program. IR images through the combining of multiple non-destructive reads in an individual exposure, naturally receive cosmic ray rejection. Hot pixels can only be removed through the use of dithered exposures, which are subsequently processed with MultiDrizzle after calwf3 processing.
Processing of single exposures
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 file and has the suffix “_flt.” 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 MultiDrizzle to remove geometric distortions.
Processing of CR-SPLIT/REPEAT-OBS exposures
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 have the bias subtracted. These are then combined and simultaneously have cosmic rays 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” 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 files along with the combined crj 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” 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.
Processing of dithered exposures
Observations that use the default dither patterns provided in the 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. MultiDrizzle is used to combine all the individual flt products from calwf3, producing a final distortion-corrected image. The MultiDrizzle output product is given the “_drz” file name suffix.
All WFC3 observations, not just dithered images, need to be processed with MultiDrizzle to correct for geometric distortion and pixel area effects. Please refer to Chapter 4 for more information.
For multiple exposures, MultiDrizzle 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.

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