Reference File

The gain_scale step uses the GAIN reference file. It requires this reference file only to get the value of the “GAINFACT” keyword in the header of the file. This is the value used to rescale the science data. The ramp_fit step also uses the GAIN reference file and if it succeeded in finding the “GAINFACT” keyword when it was executed, it will have already stored the keyword value in the science data, for later use by the gain_scale step. In this case the gain_scale step will not read the GAIN reference file again when it runs.

GAIN reference file

REFTYPE

GAIN

Data model

GainModel

The GAIN reference file contains a pixel-by-pixel gain map, which can be used to convert pixel values from units of DN to electrons. The gain values are assumed to be in units of e/DN.

Reference Selection Keywords for GAIN

CRDS selects appropriate GAIN references based on the following keywords. GAIN 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, 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 GAIN

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

Keyword	Data Model Name
DETECTOR	model.meta.instrument.detector
SUBARRAY	model.meta.subarray.name
BUNIT1	model.meta.bunit_data

¹ BUNIT is not used as a CRDS selector, but is required in the “SCI” extension header of GAIN reference files to document the units of the data. The expected value is “ELECTRONS/DN”.

Reference File Format

GAIN reference files are FITS files with a single IMAGE extension. The FITS primary data array is assumed to be empty. The characteristics of the FITS extensions are as follows:

EXTNAME	XTENSION	NAXIS	Dimensions	Data type
SCI	IMAGE	2	ncols x nrows	float