DISTORTION Reference File

REFTYPE:

DISTORTION

Data model:

DistortionModel, DistortionMRSModel

Reference Selection Keywords for DISTORTION

CRDS selects appropriate DISTORTION references based on the following keywords. DISTORTION is not applicable for instruments not in the table. All keywords used for file selection are required.

Instrument	Keywords
FGS	INSTRUME, DETECTOR, EXP_TYPE, DATE-OBS, TIME-OBS
MIRI	INSTRUME, DETECTOR, EXP_TYPE, CHANNEL, BAND, DATE-OBS, TIME-OBS
NIRCam	INSTRUME, DETECTOR, EXP_TYPE, CHANNEL, FILTER, PUPIL, DATE-OBS, TIME-OBS
NIRISS	INSTRUME, EXP_TYPE, PUPIL, DATE-OBS, TIME-OBS

Standard Keywords

The following table lists the keywords that are required to be present in all reference files. The first column gives the FITS keyword names. The second column gives the jwst data model name for each keyword, which is useful when using data models in creating and populating a new reference file. The third column gives the equivalent meta tag in ASDF reference file headers, which is the same as the name within the data model meta tree (second column).

FITS Keyword	Data Model Name	ASDF meta tag
AUTHOR	model.meta.author	author
DATAMODL	model.meta.model_type	model_type
DATE	model.meta.date	date
DESCRIP	model.meta.description	description
FILENAME	model.meta.filename	N/A
INSTRUME	model.meta.instrument.name	instrument: {name}
PEDIGREE	model.meta.pedigree	pedigree
REFTYPE	model.meta.reftype	reftype
TELESCOP	model.meta.telescope	telescope
USEAFTER	model.meta.useafter	useafter

NOTE: More information on standard required keywords can be found here: Standard Required Keywords

Reference File Format

The distortion reference file contains a combination of astropy models, representing the transform from detector to the telescope V2, V3 system. The following conventions have been adopted:

1. The output in the V2, V3 system is in units of arcsec.

2. The input x and y are 0-based coordinates in the DMS system.

3. The center of the first pixel is (0, 0), so the first pixel goes from -0.5 to 0.5.

4. The origin of the transform is taken to be (0, 0). Note, that while a different origin can be used for some transforms the relevant offset should first be prepended to the distortion transform to account for the change in origin of the coordinate frame. For instance, MIRI takes input in (0,0)-indexed detector pixel coordinates, but shifts these around prior to calling transforms that are defined with respect to science-frame pixels that omit reference pixels.

Internally the WCS pipeline works with 0-based coordinates. When FITS header keywords are used, the 1 pixel offset in FITS coordinates is accounted for internally in the pipeline.

The model is a combination of polynomials.

model:

Transform from detector to an intermediate frame (instrument dependent).