The algorithms for CALWEBB_IMAGE3, the 3rd stage of the JWST calibration pipeline for imaging data, are described. These algorithms process the data from calibrated slope images to mosaics and source catalogs.
The CALWEBB_IMAGE3 module is the 3rd stage in the calibration pipeline for all imaging data. The inputs to this stage are the calibrated slope images (CALWEBB_IMAGE2 output) and the outputs are mosaics and source catalogs. 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.
Refine relative WCS
The relative WCS information between the different images is refined using the locations of common sources between images. The AstroDrizzle code is used for this step. The WCS information associated with each image is updated by this step.
The background levels in images can vary as a function of time due to the thermal telescope emission, zodiacal emission, etc. This step corrects the overall background level of each image so that the overlapping regions of the images have the same background. The AstroDrizzle code is used for this step.
A 2nd pass at outlier detection is done using the overlapping regions observed in different exposures. The majority of the outliers will be due to cosmic rays undetected during the 1st pass at outlier detection done in CALWEBB_DETECTOR1. The AstroDrizzle code is used for this step. The presence of an outlier results in a pixel flag being set.
The combination of multiple images into a mosaic is done using the AstroDrizzle code with the output pixel size set to the input pixel size. This is done for all the images taken in a specific band in an observation. Pixels flagged as "bad" (outliers, hot pixels, etc.) are not used in the creation of the mosaic.
Note that moving targets are supported and the mosaics are created in the moving target reference frame.
Create expourse level products
The exposure level products are re-created at this stage to provide the highest quality products that include the results of the ensemble processing (updated WCS, matching backgrounds, a 2nd pass outlier detection). These products are for the archive and include the unrectified 2D images and rectified 2D images.
Source catalogs are created using the Astropy Photutils package. The goal of this step is to produce a good quality catalog that will provide a 1st pass at such a catalog. This catalog is needed for the reduction of NIRISS/NIRCam WFSS observations and to setup NIRSpec MSA observations.