# 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.