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, 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 convention was adopted:

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

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

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

  • 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 accounterd for internally in the pipeline.

The model is a combination of polynomials.

model

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