Reference File¶
The gain_scale
step uses the GAIN reference file.
It requires this reference file only to
get the value of the “GAINFACT” keyword in the header of the file.
This is the value used to rescale the science data. The ramp_fit
step also uses the GAIN reference file and if it succeeded in finding
the “GAINFACT” keyword when it was executed, it will have already
stored the keyword value in the science data, for later use by the
gain_scale
step. In this case the gain_scale
step will not read
the GAIN reference file again when it runs.
GAIN reference file¶
- REFTYPE
GAIN
- Data model
The GAIN reference file contains a pixel-by-pixel gain map, which can be used to convert pixel values from units of DN to electrons. The gain values are assumed to be in units of e/DN.
Reference Selection Keywords for GAIN¶
CRDS selects appropriate GAIN references based on the following keywords. GAIN is not applicable for instruments not in the table. All keywords used for file selection are required.
Instrument |
Keywords |
---|---|
FGS |
INSTRUME, DETECTOR, SUBARRAY, DATE-OBS, TIME-OBS |
MIRI |
INSTRUME, DETECTOR, SUBARRAY, DATE-OBS, TIME-OBS |
NIRCam |
INSTRUME, DETECTOR, SUBARRAY, DATE-OBS, TIME-OBS |
NIRISS |
INSTRUME, DETECTOR, SUBARRAY, DATE-OBS, TIME-OBS |
NIRSpec |
INSTRUME, DETECTOR, 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: Standard Required Keywords
Type Specific Keywords for GAIN¶
In addition to the standard reference file keywords listed above, the following keywords are required in GAIN reference files, because they are used as CRDS selectors (see Reference Selection Keywords for GAIN):
Keyword |
Data Model Name |
---|---|
DETECTOR |
model.meta.instrument.detector |
SUBARRAY |
model.meta.subarray.name |
BUNIT1 |
model.meta.bunit_data |
1 BUNIT is not used as a CRDS selector, but is required in the “SCI” extension header of GAIN reference files to document the units of the data. The expected value is “ELECTRONS/DN”.
Reference File Format¶
GAIN reference files are FITS files with a single IMAGE extension. The FITS primary data array is assumed to be empty. The characteristics of the FITS extensions are as follows:
EXTNAME |
XTENSION |
NAXIS |
Dimensions |
Data type |
---|---|---|---|---|
SCI |
IMAGE |
2 |
ncols x nrows |
float |