Reference File

The resample step uses the DRIZPARS reference file.

DRIZPARS Reference File

REFTYPE

DRIZPARS

Data model

DrizParsModel

The DRIZPARS reference file contains various drizzle parameter values that control the characteristics of a drizzled image and how it is built.

Reference Selection Keywords for DRIZPARS

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

Instrument	Keywords
MIRI	INSTRUME
NIRCam	INSTRUME
NIRISS	INSTRUME

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 DRIZPARS

No additional specific keywords are required in DRIZPARS reference files, because CRDS selection is based only on the instrument name (see Reference Selection Keywords for DRIZPARS).

Reference File Format

DRIZPARS reference files are FITS format, with 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
DRIZPARS	BINTABLE	2	TFIELDS = 7	N/A

The DRIZPARS extension contains various step parameter values to be used when processing certain types of image collections. The first two columns (numimages and filter) are used as row selectors within the table. Image collections that match those selectors then use the parameter values specified in the remainder of that table row. The table contains the following 7 columns:

TTYPE	TFORM	Description
numimages	integer	The number of images to be combined
filter	integer	The filter used to obtain the images
pixfrac	float	The pixel “shrinkage” fraction
kernel	string	The kernel function used to distribute flux
fillval	float	Value assigned to pixels with no input flux
wht_type	sting	The input image weighting type
stepsize	integer	Output WCS grid interpolation step size