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