Description¶
The Source Type (srctype) step in the calibration pipeline checks or sets
whether a spectroscopic source should be treated as a point or extended object.
This information is then used in some subsequent spectroscopic processing
steps.
Depending on the JWST observing mode, the observer may have the option of
designating a source type in the APT template for the observations. They have
the choice of declaring whether or not the source should be considered
extended. If they don’t know the character of the source, they can also
choose a value of unknown. The observer’s choice in the APT is passed along
to DMS processing, which sets the value of the SRCTYPE keyword in the
primary header of the level-1b (_uncal.fits) product that’s used as input
to the calibration pipeline. The SRCTYPE keyword may have values of
POINT, EXTENDED, or UNKNOWN.
The srctype calibration step checks to see if the SRCTYPE keyword
has been populated and its value. If the observer did not provide a source
type value or the SRCTYPE keyword is set to UNKNOWN, the srctype
step will choose a suitable default value based on the observing mode of
the exposure.
The default values set by the step, as a function of exposure type (the value
of the EXP_TYPE keyword) is shown in the table below.
| EXP_TYPE | Exposure Type | Default SRCTYPE |
|---|---|---|
| MIR_LRS-FIXEDSLIT | MIRI LRS fixed-slit | Point |
| MIR_LRS-SLITLESS | MIRI LRS slitless | Point |
| MIR_MRS | MIRI MRS (IFU) | Extended |
| NIS_SOSS | NIRISS SOSS | Point |
| NRS_FIXEDSLIT | NIRSpec fixed-slit | Point |
| NRS_BRIGHTOBJ | NIRSpec bright object | Point |
| NRS_IFU | NIRSpec IFU | Point |
For NIRSpec MOS exposures (EXP_TYPE=”NRS_MSASPEC”), there are multiple
sources per exposure and hence a single parameter can’t be used in the
APT, nor a single keyword in the science product, to record the type of
each source. For these exposures, a stellarity value can be supplied by
the observer for each source used in the MSA Planning Tool (MPT). The
stellarity values are
in turn passed from the MPT to the MSA metadata (_msa.fits) file
created by DMS and used in the calibration pipeline. The stellarity
values from the MSA metadata file are loaded for each source/slitlet
by the assign_wcs step of the calwebb_spec2 pipeline and then
evaluated by the srctype step to determine whether each source
should be treated as point or extended.
If the stellarity value is less than zero, the source type is set to
UNKNOWN. If the stellarity value is between zero and 0.75, it is
set to EXTENDED, and if the stellarity value is greater than 0.75,
it is set to POINT. The resulting choice as stored in a SRCTYPE
keyword located in the header of the SCI extension associated with
each source/slitlet.
In the future, reference files will be used to set more detailed threshold values for stellarity, based on the particular filters, gratings, etc. of each exposure.