The calwebb_coron3 module is stage 3 of the JWST Science Calibration Pipeline for NIRCam and MIRI coronagraphic data. 

The inputs to this stage are the calibrated slope images (calwebb_image2 output). In addition to the "crfints.fits" and "i2d.fits" products produced by other stage 3 processing modules, calwebb_coron3 produces 3 data product types that are particular to coronagraphic processing:

1. psfstack: A single 3-D product containing a stack of all the PSF images from the multiple input exposures. The size of the stack will be equal to the sum of the number of integration in each input PSF exposure. The output file name is source-based, using the product name specified in the ASN file, e.g., "jw86073-a3001_t001_nircam_f140m-maskbar_psfstack.fits".
   
   
2. psfalign: A 4-D data product, where the 3rd axis length is equal to the total number of reference PSF images in the input PSF stack ("_psfstack") product and the 4th axis length is equal to the number of integrations in the input science target product (n_cols × n_rows × n_psfs × n_ints). Image (n,m) in the 4-D data cube is the n^th PSF image aligned to the m^th science target integration. The file name is exposure-based, using the input science target exposure name as the root, with the addition of the association candidate ID and the “_psfalign” product type suffix, e.g., "jw8607342001_02102_00001_nrcb3_a3001_psfalign.fits".
   
   
3. psfsub: A 3-D stack of PSF-subtracted images of the science target, having the same dimensions as the input science target ("_calints") product. The PSF fitting and subtraction has been applied to each integration image independently. The file name syntax is exposure-based, using the root of the input “_calints” product, with the addition of the association candidate ID and the “_psfsub” product type suffix, e.g., "jw8607342001_02102_00001_nrcb3_a3001_psfsub.fits".

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.  Note that all steps in this module are common to both NIR and MIR data.

A brief description of each of the steps within calwebb_coron3 can be found below, along with links to further details (e.g., the relevant reference files) that can be found on the corresponding ReadTheDocs pages. Note, however, that the reference files themselves are all provided via CRDS. For instrument mode-specific notes on these pipeline steps see the corresponding known issues with JWST data articles.

---

Assemble reference PSFs

ReadTheDocs documentation: Stack Refs 
Package name: stack_refs

The available list of reference PSFs is generated. These reference PSFs can include reference stars specifically observed for the target as well as observations of the target taken at different orientations. Note that this step is in practice taken care of by the association generator, which lists all science and PSF reference inputs to be used by calwebb_coron3 processing.

---

Outlier detection

ReadTheDocs documentation: Outlier Detection 
Package name: outlier_detection

Outlier detection is done using the image stack. The majority of outliers will be due to cosmic rays undetected during the 1st pass at outlier detection done in calwebb_detector1. An iterative sigma clipping algorithm is used in pixel coordinates on the image stack. The presence of an outlier results in a pixel flag being set.

---

Align reference PSFs

ReadTheDocs documentation: Align Refs 
Package name: align_refs

This step uses a "PSFMASK" reference file to compute offsets between science target images and reference PSF images, and shift the PSF images into alignment. The "PSFMASK" reference file contains a 2-D mask that’s used as a weight function when computing the shifts between images using the Fourier LSQ algorithm to measure the shifts and the Fourier Shift algorithm to apply the shifts to each reference PSF integration. The parameters to override the automatic selection of a "PSFMASK" file from CRDS and specify your own file are of the form –override_psfmask. The pipeline documentation contains information on the required keywords for PSFMASK files.

---

Reference PSF subtraction

ReadTheDocs documentation: KLIP Processing 
Package name: klip

The reference PSF that is subtracted from each target integration is created using the list of reference PSFs and the KLIP algorithm (Soummer et al. 2012).

---

Image combination

ReadTheDocs documentation: Resample 
Package name: resample

The target images (including those at different rotations) are resampled and combined into a single image where the distortion has been removed, and where the pixels in the output image all have the same spatial scale, which is equal to the input detector pixel scale by default.

---

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, and 2nd pass outlier detection). These products are for the Archive and include the unrectified 2-D images and rectified 2-D images.

---

References

Soummer, R., Pueyo, R., Larkin, J., 2012, ApJ, 755, 28
Detection and Characterization of Exoplanets and Disks Using Projections on Karhunen-Loève Eigenimages

---

---