Reference Files¶
The background image subtraction step uses reference files only when processing Wide-Field Slitless Spectroscopy (WFSS) exposures. Two reference files are used for WFSS mode.
WFSS Background reference file¶
| REFTYPE: | WFSSBKG |
|---|---|
| Data model: | WfssBkgModel |
The WFSS background reference file contains a “master” image of the dispersed background produced by a particular filter+grism combination.
CRDS Selection Criteria¶
WFSSBKG reference files are selected by:
INSTRUME, DETECTOR, EXP_TYPE, FILTER, and PUPIL
Required Keywords¶
The following table lists the keywords that are required to be present in a WFSSBKG reference file. An asterisk following a keyword name indicates a standard keyword that is required in all reference files, regardless of type.
| Keyword | Model Name |
|---|---|
| AUTHOR* | meta.author |
| DATAMODL* | meta.model_type |
| DATE* | meta.date |
| DESCRIP* | meta.description |
| DETECTOR | meta.instrument.detector |
| EXP_TYPE | meta.exposure.type |
| FILENAME* | meta.filename |
| FILTER | meta.instrument.filter |
| INSTRUME* | meta.instrument.name |
| PEDIGREE* | meta.pedigree |
| PUPIL | meta.instrument.pupil |
| REFTYPE* | meta.reftype |
| TELESCOP* | meta.telescope |
| USEAFTER* | meta.useafter |
Reference File Format¶
WFSSBKG reference files are FITS files with 3 IMAGE extensions and 1 BINTABLE 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 |
| ERR | IMAGE | 2 | ncols x nrows | float |
| DQ | IMAGE | 2 | ncols x nrows | integer |
| DQ_DEF | BINTABLE | 2 | TFIELDS = 4 | N/A |
The DQ_DEF extension contains the bit assignments used in the DQ array. It contains the following 4 columns:
| TTYPE | TFORM | Description |
|---|---|---|
| BIT | integer | The bit number, starting at zero |
| VALUE | integer | The equivalent base-10 value of BIT |
| NAME | string | The mnemonic name of the data quality condition |
| DESCRIPTION | string | A description of the data quality condition |
Wavelength Range reference file¶
| REFTYPE: | WAVELENGTHRANGE |
|---|---|
| Data model: | WavelengthrangeModel |
The wavelength range reference file contains information about the range of wavelengths in the exposure. It is used, together with a source catalog, to create a mask giving the locations of source spectra in the target image and hence where the background regions are.
CRDS Selection Criteria¶
Wavelengthrange reference files are selected by:
INSTRUME, EXP_TYPE, PUPIL (NIRCam only), and MODULE (NIRCam only)