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.