Reference Files¶

The refpix step uses a REFPIX reference file, but only when processing NIRSpec exposures that have been acquired using an IRS2 readout pattern. No other instruments or exposure modes require a reference file for this step.

REFPIX Reference File¶

REFTYPE

REFPIX

Data model

IRS2Model

The REFPIX reference file contains the complex coefficients for the correction.

Reference Selection Keywords for REFPIX¶

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

Instrument

Keywords

NIRSpec

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 REFPIX¶

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

Keyword

Data Model Name

DETECTOR

model.meta.instrument.detector

Reference File Format¶

REFPIX reference files are FITS format, with two BINTABLE extensions. The FITS primary HDU does not contain a data array. The first BINTABLE extension is labeled with EXTNAME = “IRS2” and has the following column characteristics:

Column

Data type

alpha_0

float32

alpha_1

float32

alpha_2

float32

alpha_3

float32

beta_0

float32

beta_1

float32

beta_2

float32

beta_3

float32

The “alpha” arrays contain correction multipliers to the reference output, and the “beta” arrays contain correction multipliers to the interleaved reference pixels. Both arrays have 4 components, one for each sector (amplifier output). These are applied to (i.e. multiplied by) the Fourier transform of the interleaved reference pixel data. The coefficients are intrinsically complex values, but have their real and imaginary parts stored in alternating table rows, i.e. row 1 contains the real components of all coefficients and row 2 contains the corresponding imaginary components for each. This storage scheme results in a total of 2916352 (2048 * 712 * 2) rows in the table.

The second BINTABLE extension is labeled with EXTNAME = “DQ” and has the following column characteristics:

Column

Data type

output

int16

odd_even

int16