Reference File Types

The linearity step uses a LINEARITY reference file.

LINEARITY Reference File

REFTYPE:

LINEARITY

Data model:

LinearityModel

The LINEARITY reference file contains pixel-by-pixel polynomial correction coefficients.

Reference Selection Keywords for LINEARITY

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

Instrument	Keywords
FGS	INSTRUME, DETECTOR, SUBARRAY, DATE-OBS, TIME-OBS
MIRI	INSTRUME, DETECTOR, SUBARRAY, BAND, FILTER, DATE-OBS, TIME-OBS
NIRCam	INSTRUME, DETECTOR, SUBARRAY, DATE-OBS, TIME-OBS
NIRISS	INSTRUME, DETECTOR, SUBARRAY, DATE-OBS, TIME-OBS
NIRSpec	INSTRUME, DETECTOR, SUBARRAY, 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

Type Specific Keywords for LINEARITY

In addition to the standard reference file keywords listed above, the following keywords are required in LINEARITY reference files, because they are used as CRDS selectors (see Reference Selection Keywords for LINEARITY):

Keyword	Data Model Name	Instruments
DETECTOR	model.meta.instrument.detector	All
SUBARRAY	model.meta.subarray.name	All
FILTER	model.meta.instrument.filter	MIRI only
BAND	model.meta.instrument.band	MIRI only

Reference File Format

LINEARITY reference files are FITS format, with 2 IMAGE extensions and 1 BINTABLE extension. The FITS primary HDU does not contain a data array. The format and content of the file is as follows:

EXTNAME	XTENSION	NAXIS	Dimensions	Data type
COEFFS	IMAGE	3	ncols x nrows x ncoeffs	float
DQ	IMAGE	2	ncols x nrows	integer
DQ_DEF	BINTABLE	2	TFIELDS = 4	N/A

Each plane of the COEFFS data cube contains the pixel-by-pixel coefficients for the associated order of the polynomial. There can be any number of planes to accommodate a polynomial of any order.

The DQ_DEF extension contains the bit assignments used in the DQ array. It contains the following 4 columns:

TTYPE	TFORM	Description
BIT	integer	The bit number, starting at zero
VALUE	integer	The equivalent base-10 value of BIT
NAME	string	The mnemonic name of the data quality condition
DESCRIPTION	string	A description of the data quality condition

NOTE: For more information on standard bit definitions see: Data Quality Flags.