Reference Files¶

The IPC deconvolution step uses an IPC reference file.

IPC Reference File¶

REFTYPE

IPC

Data model

IPCModel

The IPC reference file contains a deconvolution kernel.

Reference Selection Keywords for IPC¶

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

Instrument

Keywords

FGS

INSTRUME, DETECTOR, DATE-OBS, TIME-OBS

MIRI

INSTRUME, DETECTOR, DATE-OBS, TIME-OBS

NIRCam

INSTRUME, DETECTOR, DATE-OBS, TIME-OBS

NIRISS

INSTRUME, DETECTOR, DATE-OBS, TIME-OBS

NIRSpec

INSTRUME, DETECTOR, 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 IPC¶

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

Keyword

Data Model Name

DETECTOR

model.meta.instrument.detector

Reference File Format¶

IPC reference files are FITS format, with 1 IMAGE extension. The FITS primary HDU does not contain a data array. The format and content of the file can be one of two forms, as described below:

EXTNAME

XTENSION

NAXIS

Dimensions

Data type

SCI

IMAGE

2

nkern x nkern

float

or

SCI

IMAGE

4

ncols x nrows x nkern x nkern

float

Two formats are currently supported for the IPC kernel: a small 2-D array or a 4-D array. If the kernel is 2-D, its dimensions should be odd, for example 3 x 3 or 5 x 5 pixels. The value at the center pixel will be larger than 1 (e.g. 1.02533) and the sum of all pixel values will be equal to 1.

A 4-D kernel may be used to allow the IPC correction to vary from pixel to pixel across the image. In this case, the axes that are most rapidly varying (the last two in Python notation; the first two in IRAF/FITS notation) have dimensions equal to those of a full-frame image. At each point in that image, there will be a small, 2-D kernel as described in the previous paragraph.