# Reference Files¶

The superbias subtraction step uses a SUPERBIAS reference file.

## SUPERBIAS Reference File¶

REFTYPE

SUPERBIAS

Data model

SuperBiasModel

The SUPERBIAS reference file contains a 2-D image of the detector bias (“zeroth” read) structure.

### Reference Selection Keywords for SUPERBIAS¶

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

Instrument

Keywords

FGS

INSTRUME, DETECTOR, READPATT, SUBARRAY, DATE-OBS, TIME-OBS

NIRCam

INSTRUME, DETECTOR, READPATT, 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 SUPERBIAS¶

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

Keyword

Data Model Name

Instruments

DETECTOR

model.meta.instrument.detector

FGS, NIRCam, NIRISS, NIRSpec

FGS, NIRCam, NIRISS, NIRSpec

SUBARRAY

model.meta.subarray.name

FGS, NIRCam, NIRISS, NIRSpec

SUBSTRT1

model.meta.subarray.xstart

NIRSpec only

SUBSTRT2

model.meta.subarray.ystart

NIRSpec only

SUBSIZE1

model.meta.subarray.xsize

NIRSpec only

SUBSIZE2

model.meta.subarray.ysize

NIRSpec only

### Reference File Format¶

SUPERBIAS reference files are FITS format, with 3 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

SCI

IMAGE

2

ncols x nrows

float

ERR

IMAGE

2

ncols x nrows

float

DQ

IMAGE

2

ncols x nrows

integer

DQ_DEF

BINTABLE

2

TFIELDS = 4

N/A

The SCI array contains the super-bias image of the detector. The ERR array contains uncertainties in the super-bias values and the DQ array contains data quality flags associated with the super-bias image.

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.