Description

The klip step is one of the coronagraphic-specific steps in the coron sub-package and is used in Stage 3 calwebb_coron3 processing. It applies the Karhunen-Loeve Image Plane (KLIP) algorithm to coronagraphic images, using an accompanying set of reference PSF images, in order to fit and subtract an optimal PSF from a source image. The KLIP algorithm uses a KL decomposition of the set of reference PSF’s, and generates a model PSF from the projection of the target on the KL vectors. The model PSF is then subtracted from the target image (Soummer, Pueyo, and Larkin 2012). KLIP is a Principle Component Analysis (PCA) method and is very similar to the Locally Optimized Combination of Images (LOCI) method. The main advantages of KLIP over LOCI are the possibility of direct forward modeling and a significant speed increase.

The KLIP algorithm consists of the following high-level steps:

  1. Partition the target and reference PSF images in a set of search areas, and subtract their average values so that they have zero mean

  2. Compute the KL transform of the set of reference PSF’s

  3. Choose the number of modes to keep in the estimated target PSF

  4. Compute the best estimate of the target PSF from the projection of the target image on the KL eigenvectors

  5. Calculate the PSF-subtracted target image

Arguments

The klip step has one optional argument:

--truncate

This is an integer parameter with a default value of 50 and is used to specify the number of KL transform rows to keep when computing the PSF fit to the target.

Inputs

The klip step takes two inputs: a science target exposure in the form of a 3D data cube and a 4D aligned PSF image (“_psfalign”) product.

3D calibrated images

Data model

CubeModel

File suffix

_calints

A 3D calibrated science target product containing a stack of per-integration images. This should be a “_calints” product created by the calwebb_image2 pipeline. Normally one of the science target exposures specified in the ASN file used as input to the calwebb_coron3 pipeline.

4D aligned PSF images

Data model

QuadModel

File suffix

_psfalign

A 4D collection of PSF images that have been aligned to each of the per-integration images contained in the science target “_calints” product, created by the align_refs step.

Outputs

3D PSF-subtracted images

Data model

CubeModel

File suffix

_psfsub

The output is a 3D 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.”

Reference Files

The klip step does not use any reference files.