# 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