jwst.assign_wcs is one of the first steps in the level 2B JWST pipeline. It associates a WCS object with each science exposure. The WCS transforms positions on the detector to a world coordinate frame - ICRS and wavelength. In general there may be intermediate coordinate frames depending on the instrument. The WCS is saved in the FITS file. It can be accessed as an attribute of the meta object when the fits file is opened as a data model.

The forward direction of the transforms is from detector to world coordinates and the input positions are 0-based.

The basic WCS keywords are in the primary header and the distortion and spectral models are stored in reference files in the ASDF format.

For each observing mode, determined by the value of EXP_TYPE in the science header, assign_wcs retrieves reference files from CRDS and creates a pipeline of transforms from input frame detector to a frame v2v3. This part of the WCS pipeline may include intermediate coordinate frames. The basic WCS keywords are used to create the transform from frame v2v3 to frame world.

Basic WCS keywords and the transform from v2v3 to world

All JWST instruments use the following FITS header keywords to define the transform from v2v3 to world:

RA_REF, DEC_REF - a fiducial point on the sky, ICRS, [deg]

V2_REF, V3_REF - a point in the V2V3 system which maps to RA_REF, DEC_REF, [arcsec]

ROLL_REF - local roll angle associated with each aperture, [deg]

These quantities are used to create a 3D Euler angle rotation between the V2V3 spherical system, associated with the telescope, and a standard celestial system.

Using the WCS interactively

Once a FITS file is opened as a DataModel the WCS can be accessed as an attribute of the meta object. Calling it as a function with detector positions as inputs returns the corresponding world coordinates. Using MIRI LRS fixed slit as an example:

>>> from jwst.datamodels import ImageModel
>>> exp = ImagModel(miri_fixed_assign_wcs.fits')
>>> ra, dec, lam = exp.meta.wcs(x, y)
>>> print(ra, dec, lam)
    (329.97260532549336, 372.0242999250267, 5.4176100046836675)

The WCS provides access to intermediate coordinate frames and transforms between any two frames in the WCS pipeline in forward or backward direction. For example, for a NIRSPEC fixed slits exposure, which has been through the extract_2d step:

>>> exp = models.MultiSlitModel('nrs1_fixed_assign_wcs_extract_2d.fits')
>>> exp.slits[0].meta.wcs.available_frames
    ['detector', 'sca', 'bgwa', 'slit_frame', 'msa_frame', 'ote', 'v2v3', 'world']
>>> msa2detector = exp.slits[0].meta.wcs.get_transform('msa_frame', 'detector')
>>> msa2detector(0, 0, 2*10**-6)
    (5042.064255529629, 1119.8937888372516)

For each exposure, assign_wcs uses reference files and WCS header keywords to create the WCS object. What reference files are retrieved from CRDS is determined based on EXP_TYPE and other keywords in the science file header.

The assign_wcs step can accept any type of DataModel as input. In particular, for multiple-integration datasets the step will accept either of these data products: the slope results for each integration in the exposure, or the single slope image that is the result of averaging over all integrations.

jwst.assign_wcsis based on gwcs and uses the modeling, units and coordinates subpackages in astropy.

Software dependencies: