Tasks in the package

The coronagraphic package currently consists of the following tasks:

  • stack_refs
  • align_refs
  • klip
  • hlsp

Briefly, the stack_refs step is used to load images of reference PSF targets, as listed in an Association file, and stack the images into a data cube in a single file to be used in subsequent processing steps. The align_refs step is then used to align the stacked reference PSF images with the images contained in a science target exposure. The klip step applies the Karhunen-Loeve Image Plane (KLIP) algorithm to the aligned reference PSF and science target images and produces PSF-subtracted science target images. The hlsp task produces high-level science products (HLSP’s) from a KLIP-subtracted image.

CALWEBB_CORON3

Currently the individual steps can only be run in a convenient way by running the calwebb_coron3 pipeline, which calls the individual steps and takes care of all the necessary loading and passing of data models for the input and output products of each step. The input to the calwebb_coron3 pipeline is expected to be an ASN file. The ASN file should define a single output product, which will be the combined image formed from the PSF-subtracted results of all the input science target data. That output product should then define, as its members, the various input reference PSF and science target files to be used in the processing. An example ASN file is shown below.

{"asn_rule": "CORON", "target": "NGC-3603", "asn_pool": "jw00017_001_01_pool", "program": "00017",
"products": [
    {"prodtype": "coroncmb", "name": "jw89001-c1001_t001_nircam_f160w",
     "members": [
         {"exptype": "science", "expname": "test_targ1_calints.fits"},
         {"exptype": "science", "expname": "test_targ2_calints.fits"},
         {"exptype": "psf", "expname": "test_psf1_calints.fits"},
         {"exptype": "psf", "expname": "test_psf2_calints.fits"},
         {"exptype": "psf", "expname": "test_psf3_calints.fits"}]}],
"asn_type": "coron",
"asn_id": "c1001"}

In this example the output product “jw89001-c1001_t001_nircam_f160w” is defined to consist of 2 science target inputs and 3 reference psf inputs. Note that the values of the exptype attribute for each member are very important and used by the calwebb_coron3 pipeline to know which members are to be used as reference PSF data and which are data for the science target. The output product name listed in the ASN file is used as the root name for some of the products created by the calwebb_coron3 pipeline. This includes:

  • rootname_psfstack: the output of the stack_refs step
  • rootname_i2d: the final combined target image

Other products will be created for each individual science target member, in which case the root names of the original input science target products will be used as a basis for the output products. These products include:

  • targetname_psfalign: the output of the align_refs step
  • targetname_psfsub: the output of the klip step

Stack_refs

Overview

The stack_refs step takes a list of reference PSF products and stacks all of the images in the PSF products into a single 3D data cube. It is assumed that the reference PSF products are in the form of a data cube (jwst CubeModel type data model) to begin with, in which images from individual integrations are stacked along the 3rd axis of the data cube. Each data cube from an input reference PSF file will be appended to a new output 3D data cube (again a CubeModel), such that the dimension of the 3rd axis of the output data cube will be equal to the total number of integrations contained in all of the input files.

Inputs and Outputs

The stack_refs step is called from the calwebb_coron3 pipeline module. The calwebb_coron3 pipeline will find all of the psf members listed in the input ASN file, load each one into a CubeModel data model, and construct a ModelContainer that is the list of all psf CubeModels. The ModelContainer is passed as input to the stack_refs step. The output of stack_refs will be a single CubeModel containing all of the concatenated data cubes from the input psf files.

Align_refs

Overview

The align_refs step is used to compute offsets between science target images and the reference PSF images and shift the PSF images into alignment. Each integration contained in the stacked PSF data is aligned to each integration within a given science target product. The calwebb_coron3 pipeline applies the align_refs step to each input science target product individually, resulting in a set of PSF images that are aligned to the images in that science target product.

Inputs and Outputs

The align_refs step takes 2 inputs: a science target product, in the form of a CubeModel data model, and the stacked PSF product, also in the form of a CubeModel data model. The resulting output is a 4D data model (QuadModel), where the 3rd axis has length equal to the total number of reference PSF images in the input PSF stack and the 4th axis has length equal to the number of integrations in the input science target product.

Klip

Overview

The klip task applies the KLIP algorithm to coronagraphic images, using an accompanying set of reference PSF images, in order to fit and subtract an optimal PSF from the source. 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 LOCI. The main advantages of KLIP over LOCI is the possibility of direct forward modeling and a significant speed increase.

The KLIP algorithm consists of the following steps:

  1. Partition the target and reference 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

Inputs and Outputs

The klip task takes two inputs: a science target product, in the form of a 3D CubeModel data model, and a set of aligned PSF images, in the form of a 4D QuadModel data model. Each ‘layer’ in the 4th dimension of the PSF data contains all of the aligned PSF images corresponding to a given integration (3rd dimension) in the science target cube. The output from the klip step is a 3D CubeModel data model, having the same dimensions as the input science target product, and contains the PSF-subtracted images for every integration of the science target product.

Arguments

The task takes one optional argument, truncate, which is used to specify the number of KL transform rows to keep when computing the PSF fit to the target. The default value is 50.

HLSP

Overview

The hlsp task produces high-level science products for KLIP-processed images. The task currently produces two such products: a signal-to-noise ratio (SNR) image and a table of contrast data. The SNR image is computed by simply taking the ratio of the SCI and ERR arrays of the input target image. The contrast data are in the form of azimuthally-averaged noise versus radius. The noise is computed as the 1-sigma standard deviation within a set of concentric annuli centered in the input image. The annuli regions are computed to the nearest whole pixel; no sub-pixel calculations are performed.

Input Arguments

The hlsp task takes one input file name argument, which is the name of the KLIP-processed target product to be analyzed. One optional argument is available, annuli_width, which specifies the width (in pixels) of the annuli to use in calculating the contrast data. The default value is 2 pixels.

Outputs

The hslp task produces two output products. The first is the snr image (file name suffix “_snr”) and the second is the table of contrast data (file name suffix “_contrast”). The contrast data are stored as a 2-column table giving radius (in pixels) and noise (1-sigma).