Description

romancal.assign_wcs is the first step run on an image, after romancal.ramp_fitting. It associates a World Coordinate System (WCS) object with each science exposure. Note that no fitting is performed in this step; it only creates a WCS object that transforms positions in the detector frame to positions in the world coordinate frame (ICRS) based on the telescope pointing and reference files provided by CRDS. The constructed WCS object can be accessed as an attribute of the meta object when the 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.

romancal.assign_wcs uses GWCS - a package for managing the World Coordinate System of astronomical data. It expects to find the basic WCS keywords in the model.meta.wcsinfo structure. Distortions are stored in reference files in the ASDF format.

assign_wcs retrieves reference files from CRDS and creates a pipeline of transforms from input frame detector to the telescope frame v2v3 [1]. 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.

Note: in earlier builds of the pipeline the distortions are not available.

Basic WCS keywords and the transform from v2v3 to world

The following attributes in meta.wcsinfo are used 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]

RADESYS - standard coordinate system [ICRS]

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 science 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:

>>> from roman_datamodels import datamodels as rdm
>>> image = rdm.open('roman_assign_wcs.asdf')
>>> ra, dec = image.meta.wcs(x, y)

The WCS provides access to intermediate coordinate frames and transforms between any two frames in the WCS pipeline in forward or backward direction:

>>> image.meta.wcs.available_frames
    ['detector', 'v2v3', 'world']
>>> v2world = image.meta.wcs.get_transform('v2v3', 'world')
>>> ra, dec = v2world(v2, v3)
>>> x1, y1 = image.meta.wcs.invert(ra, dec)

There are methods which allow the result of evaluating the WCS object to be an astropy.SkyCoord objext (as opposed to numbers) which allows further transformation of coordinates to different coordinate frames.

Footnotes

[1]

V2V3 is a frame defined by the two perpendicular axes that lay along the primary’s mirror plane. For completeness, V1 is also part of the telescope frame system, being the axis perpendicular to the primary mirror (i.e. along the telecope’s optical axis).