Description

Overview

The extract_2d step extracts 2D arrays from spectral images. The extractions are performed within all of the SCI, ERR, and DQ arrays of the input image model. It also computes an array of wavelengths. The SCI, ERR, DQ and WAVELENGTH arrays are stored as one or more slit objects in an output MultiSlitModel and saved as separate extensions in the output FITS file.

Assumptions

This step uses the bounding_box attribute of the WCS stored in the data model, which is populated by the assign_wcs step. Hence the assign_wcs step must be applied to the science exposure before running this step.

For GRISM/WFSS modes in NIRCAM and NIRSS, no bounding_box has been attached to the datamodel. This is to keep the WCS flexible enough to be used with any source catalog that may be associated with the dispersed image. Instead, there is a helper method that is used to calculate the bounding boxes that contain the dispersed spectra for each object. One box is made for each order. extract2d uses the source catalog referenced in the input models meta information to create the list of objects and their corresponding bounding box, this list is used to make the 2D cutouts from the dispersed image.

Algorithm

The step is currently applied only to NIRSpec Fixed Slit, NIRSPEC MSA, NIRCAM GRISM and NIRISS WFSS observations.

For NIRSPEC:

If the step parameter which_subarray is left unspecified, the default behavior is to extract all slits which fall within a detector. Only one slit may be extracted by specifying the slit name with the which_subarray argument, using one of the following accepted names: S1600A1, S200A1, S200A2, S200B1, S400A1 or S200B1 in the case of NIRSPEC FS exposure or any of the slitlet names in the case of the MSA.

To find out what slits are available for extraction:

>>> from jwst.assign_wcs import nirspec
>>> nirspec.get_open_slits(input_model)

The corner locations of the regions to be extracted are determined from the bounding_box contained in the exposure’s WCS, which defines the range of valid inputs along each axis. The input coordinates are in the image frame, i.e. subarray shifts are accounted for.

The output MultiSlit data model will have the meta data associated with each slit region populated with the name and region characteristic for the slits, corresponding to the FITS keywords SLTNAME, SLTSTRT1, SLTSIZE1, SLTSTRT2, and SLTSIZE2.

For NIRCAM GRISM and NIRISS WFSS :

If the step parameter grism_objects is left unspecified, the default behavior is to use the source catalog that is specified in the input model’s meta information, input_model.meta.source_catalog.filename. Otherwise, a user can submit of list of GrismObjects that contains information about the objects that should be extracted. The GrismObject list can be created automatically by using the method in jwst.assign_wcs.utils.create_grism_bbox. This method also uses the name of the source catalog saved in the input model’s meta information. If it’s better to construct a list of GrismObjects outside of these, the GrismObject itself can be imported from jwst.transforms.models.

Step Arguments

The extract_2d step has two optional arguments for NIRSPEC observations:

  • --which_subarray: name (string value) of a specific slit region to extract. The default value of None will cause all known slits for the instrument mode to be extracted. Currently only used for NIRspec fixed slit exposures.
  • --apply_wavecorr: bool (default is True). Flag indicating whether to apply the
    Nirspec wavelength zero-point correction.

For NIRCAM and NIRISS, the extract_2d step has only one optional argument:

  • --grism_objects: list (default is empty). A list of jwst.transforms.models.GrismObject.

Reference Files

To apply the Nirspec wavelength zero-point correction, this step uses the WAVECORR reference file. The zero-point correction is applied to observations with EXP_TYPE of “NRS_FIXEDSLT”, “NRS_BRIGHTOBJ” or “NRS_MSASPEC”. This is an optional correction (on by default). It can be turned off by specifying apply_wavecorr=False when running the step.

NIRCAM GRISM and NIRISS WFSS observations use the wavelengthrange reference file in order to construct the bounding boxes around each objects orders. If a list of GrismObject is supplied, then no reference file is neccessary.