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, 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

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.