calwebb_image3
The algorithms for calwebb_image3, which is stage 3 of the JWST Science Calibration Pipeline for imaging data, are described. These algorithms process the data from calibrated slope images to mosaics and source catalogs.
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Unless otherwise stated, the algorithms described are the baseline version.
Figure 1. calwebb_image3
Graphical representation of all the steps in the calwebb_image3 module.
Steps for all data
Note that all steps in this module are common to both NIR and MIR data.
Refine relative WCS
Software documentation outside JDox: Tweakreg
The relative WCS information between the different images is refined using the locations of common sources between images, and the WCS information associated with each image is updated by this step.
Moving target WCS
Software documentation outside JDox: Moving Target WCS
The step runs only for moving target data, where it takes the original WCS and modifies it such that the output frame of the final WCS is centered at the average location of the moving target.
Background matching ("Skymatch")
Software documentation outside JDox: Skymatch
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.
Outlier detection
Software documentation outside JDox: Outlier Detection
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 presence of an outlier results in a pixel flag being set.
It should be noted that if a moving object is present in observations that are specified as a fixed target, this step can can erase the signal from such an object (and vice versa; for observations that are specified as moving target, this step can erase the signal from fixed objects such as stars or galaxies which will trail across the detector during the integrations). This is discussed further in this article: .Moving Target Calibration and Processing.
Imaging combination
Software documentation outside JDox: Resampling
The combination of multiple images into a mosaic is carried out by removing the distortion and combining the resampled images onto a grid where all the pixels have the same spatial scale, which is equal to the input detector pixel scale by default. This is done for all the images obtained in a specific filter in an observation, grouped into an association file (ASN), which is required for this step. 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.
Source catalog
Software documentation outside JDox: Source Catalog
Source catalogs are created using the Astropy Photutils package. The goal of this step is to produce a good quality catalog that can be used as a basic 1st-pass source list. This step also produces the segmentation map for the identified sources. The catalog is subsequently used for the reduction of NIRISS/NIRCam WFSS observations and to set up NIRSpec MSA observations.
Update exposure 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 2-D images and rectified 2D images.