Reference File

The dark step uses a DARK reference file.

DARK Reference File

REFTYPE:

DARK

Data models:

DarkModel, DarkMIRIModel

The DARK reference file contains pixel-by-pixel and frame-by-frame dark current values for a given detector readout mode.

Reference Selection Keywords for DARK

CRDS selects appropriate DARK references based on the following keywords. DARK is not applicable for instruments not in the table.

Instrument	Keywords
FGS	INSTRUME, DETECTOR, READPATT, SUBARRAY, DATE-OBS, TIME-OBS
MIRI	INSTRUME, DETECTOR, READPATT, SUBARRAY, DATE-OBS, TIME-OBS
NIRCam	INSTRUME, DETECTOR, EXP_TYPE, NOUTPUTS, SUBARRAY, DATE-OBS, TIME-OBS
NIRISS	INSTRUME, DETECTOR, READPATT, SUBARRAY, DATE-OBS, TIME-OBS
NIRSpec	INSTRUME, DETECTOR, READPATT, SUBARRAY, SUBSTRT1, SUBSTRT2, SUBSIZE1, SUBSIZE2, 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 DARK

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

Keyword	Data Model Name	Instruments
DETECTOR	model.meta.instrument.detector	All
EXP_TYPE	model.meta.exposure.type	NIRCam
NOUTPUTS	model.meta.exposure.noutputs	NIRCam
READPATT	model.meta.exposure.readpatt	FGS, MIRI, NIRISS, NIRSpec
SUBARRAY	model.meta.subarray.name	All
SUBSTRT1	model.meta.subarray.xstart	NIRSpec
SUBSTRT2	model.meta.subarray.ystart	NIRSpec
SUBSIZE1	model.meta.subarray.xsize	NIRSpec
SUBSIZE2	model.meta.subarray.ysize	NIRSpec

Reference File Format

DARK reference files are FITS format, with 3 IMAGE extensions and 1 BINTABLE extension. The FITS primary data array is assumed to be empty. The format and content of the files is different for MIRI than the near-IR instruments, as shown below.

Near-IR Detectors

Characteristics of the three IMAGE extensions for DARK files used with the Near-IR instruments are as follows (see DarkModel):

EXTNAME	NAXIS	Dimensions	Data type
SCI	3	ncols x nrows x ngroups	float
ERR	3	ncols x nrows x ngroups	float
DQ	2	ncols x nrows	integer
DQ_DEF	2	TFIELDS = 4	N/A

MIRI Detectors

The DARK reference files for the MIRI detectors depend on the integration number, because the first integration of MIRI exposures contains effects from the detector reset and are slightly different from subsequent integrations. Currently the MIRI DARK reference files contain a correction for only two integrations: the first integration of the DARK is subtracted from the first integration of the science data, while the second DARK integration is subtracted from all subsequent science integrations. The format of the MIRI DARK reference files is as follows (see DarkMIRIModel):

EXTNAME	NAXIS	Dimensions	Data type
SCI	4	ncols x nrows x ngroups x nints	float
ERR	4	ncols x nrows x ngroups x nints	float
DQ	4	ncols x nrows x 1 x nints	integer
DQ_DEF	2	TFIELDS = 4	N/A

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.