SPECWCS Reference File

REFTYPE:

SPECWCS

Data model:

SpecwcsModel

Reference Selection Keywords for SPECWCS

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

Instrument	Keywords
MIRI	INSTRUME, EXP_TYPE, DETECTOR, CHANNEL, BAND, SUBARRAY, DATE-OBS, TIME-OBS
NIRCam	INSTRUME, EXP_TYPE, MODULE, PUPIL, DATE-OBS, TIME-OBS
NIRISS	INSTRUME, EXP_TYPE, FILTER, 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 exact format and contents of the SPECWCS reference file varies depending on the instrument observing mode, as explained in the following sections.

MIRI LRS mode

For the MIRI LRS mode the SPECWCS file is in FITS format. The reference file contains the zero point offset for the slit relative to the full field of view. For the Fixed Slit exposure type the zero points in X and Y are stored in the header of the second HDU in the ‘IMX’ and ‘IMY’ keywords. For the Slitless exposure type they are stored in the header of the second HDU in FITS keywords ‘IMXSLTl’ and ‘IMYSLTl’. For both of the exposure types, the zero point offset is 1-indexed and the X (e.g., IMX) refers to the column and Y refers to the row.

MIRI MRS mode

For the MIRI MRS the SPECWCS file is in ASDF format with the following structure.

channel:

The MIRI channels in the observation, e.g. “12”.

band:

The band for the observation (one of “LONG”, “MEDIUM”, “SHORT”).

model:

slice_number:

The wavelength solution for each slice. <slice_number> is the actual slice number (s), computed by s = channel * 100 + slice

NIRISS SOSS mode

For NIRISS SOSS mode the SPECWCS file is in ASDF format with the following structure.

model:

A tabular model with the wavelength solution.

NIRCam Grism modes

For NIRCam WFSS and TSGRISM modes the SPECWCS file is in ASDF format with the following structure:

displ:

The wavelength transform models

dispx:

The x-dispersion models

dispy:

The y-dispersion models

invdispx:

The inverse x-dispersion models

invdispy:

The inverse y-dispersion models

invdispl:

The inverse wavelength transform models

orders:

a list of order numbers that the models relate to, in the same order as the models

NIRISS WFSS mode

For NIRISS WFSS mode the SPECWCS file is in ASDF format with the following structure:

displ:

The wavelength transform models

dispx:

The x-dispersion models

dispy:

The y-dispersion models

invdispx:

The inverse x-dispersion models

invdispl:

The inverse wavelength transform models

fwcpos_ref:

The reference filter wheel position in degrees

orders:

a list of order numbers that the models relate to, in the same order as the models