Reference Files

The align_refs step uses a PSFMASK reference file.

PSFMASK Reference File

REFTYPE:

PSFMASK

Data model:

PsfMaskModel

The PSFMASK reference file contains a 2-D mask that’s used as a weight function when computing shifts between images.

Reference Selection Keywords for PSFMASK

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

Instrument	Keywords
MIRI	INSTRUME, FILTER, CORONMSK, SUBARRAY, DATE-OBS, TIME-OBS
NIRCam	INSTRUME, FILTER, CORONMSK, SUBARRAY, 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 at Standard Required Keywords.

Type Specific Keywords for PSFMASK

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

Keyword	Data Model Name
FILTER	model.meta.instrument.filter
CORONMSK	model.meta.instrument.coronagraph
SUBARRAY	model.meta.subarray.name

Reference File Format

PSFMASK reference files are in FITS format, with 1 IMAGE 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
SCI	IMAGE	2	ncols x nrows	float

The values in the SCI array give the mask values to be applied to the images when computing relative shifts. The mask acts as a weighting function when performing Fourier fits. The values range from zero (full weighting) to one (pixel completely masked out).