Reference Files

The photom step uses PHOTOM reference files.

PHOTOM Reference File

REFTYPE:

PHOTOM

Data models:

WfiImgPhotomRefModel

The PHOTOM reference file contains conversion factors for putting pixel values into physical units.

Reference Selection Keywords for PHOTOM

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

Instrument	Keyword Attributes
WFI	instrument, date, time

Standard ASDF metadata

The following table lists the attributes that are required to be present in all reference files. The first column shows the attribute in the ASDF reference file headers, which is the same as the name within the data model meta tree (second column). The second column gives the roman data model name for each attribute, which is useful when using data models in creating and populating a new reference file.

Attribute	Fully Qualified Path
author	model.meta.author
model_type	model.meta.model_type
date	model.meta.date
description	model.meta.description
instrument	model.meta.instrument.name
reftype	model.meta.reftype
telescope	model.meta.telescope
useafter	model.meta.useafter

NOTE: More information on standard required attributes can be found here: Standard ASDF metadata

Type Specific Keywords for PHOTOM

In addition to the standard reference file keywords listed above, the following keywords are required in PHOTOM reference files. The first (detector) is needed because it is used as a CRDS selector. The second (optical_element) is used to select the appropriate set of photometric parameters. (see Reference Selection Keywords for PHOTOM):

Attribute	Fully qualified path	Instruments
detector	model.meta.instrument.detector	WFI
optical_element	model.meta.instrument.optical_element	WFI

Tabular PHOTOM Reference File Format

PHOTOM reference files are ASDF format, with data in the phot_table attribute. The format and content of the file is as follows (see WfiImgPhotomRefModel):

Data is stored in a 2D table, with optical elements for the row names:

Instrument	Row names
WFI	F062, F087, F106, F129, F146, F158, F184, F213, GRISM, PRISM, DARK

And the variable attributes for the columns (with data type):

Instrument	Column name	Data type	Dimensions	Units
WFI	photmjsr	quantity	scalar	MJy/steradian
	uncertainty	quantity	scalar	MJy/steradian
	pixelareasr	quantity	scalar	steradian

The pixelareasr variable attribute gives the average pixel area in units of steradians.