The algorithms for CALWEBB_IMAGE2, the 2nd stage of the JWST calibration pipeline for imaging data, are described. These algorithms process the data from uncalibrated slope images to calibrated slope images.
The CALWEBB_IMAGE2 module is the 2nd stage in the calibration pipeline for all imaging observations. The input to this stage is the uncalibrated slope images (CALWEBB_DETECTOR1 output) and the output is calibrated slope images. The steps are listed in Figure 1 with the flow from the top to the bottom.
Unless otherwise stated, the algorithms described are the baseline version.
There are only common NIR/MIR steps for this module.
The information to transfer the pixel coordinates to astronomical coordinates (e.g., ra & dec) is added to the data with this step. The needed information is described in the standard World Coordinate System (WCS) format—basically standard FITS WCS keywords and a distortion model. The WCS information and distortion model are provided by instrument and detector specific calibration reference files. The image data itself is not modified by this step.
If an APT specified background target was observed, a combined background image constructed from all exposures of that target is subtracted from the science target exposure. The background observations can be specified by the user when the calibration pipeline is being manually rerun.
Flat field correction
The flat field corrects for the pixel-to-pixel variations and large scale variations in the instrument+telescope responsivity. The flat field image is taken from instrument and detector specific calibration reference files.
The multiplicative conversion factor between counts/sec and MJy/sr is attached to the data. A second conversion factor between counts/sec and micro-Jy/sq arcsec is also be attached. Basically, these are FITS keywords. The pixel area reference file is also attached to the data allowing conversion between surface brightness and flux density for each pixel. Details of the calculation of these calibration factors can be found at Absolute Flux Calibration.
Rectified 2D product
As a product for the archive, rectified 2D images are created using the AstroDrizzle algorithm and the attached WCS information. This rectified 2D product is not used in the pipeline itself, it is useful for visual browsing of the data.