The Source Type (srctype) step in the calibration pipeline attempts to determine whether a spectroscopic source should be considered to be a point or extended object, populating the “SRCTYPE” keyword with a value of either “POINT” or “EXTENDED.” This information is then used in some subsequent spectroscopic processing steps to apply source-dependent corrections.

Single Source Observations

For JWST observing modes that use a single primary target (e.g. MIRI MRS and LRS spectroscopy and NIRSpec IFU and Fixed-Slit spectroscopy), the observer has the option of designating a source type in the APT template for the observation. 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 is passed along to DMS processing, which sets the value of the “SRCTYAPT” keyword in the primary header of the products used as input to the calibration pipeline. If the user has selected a value in the APT, the “SRCTYAPT” keyword will be set to “POINT”, “EXTENDED”, or “UNKNOWN.” If the selection is not available for a given observing mode or a choice wasn’t made, the “SRCTYAPT” keyword will not appear in the uncalibrated product header.

The srctype step sets a value for the “SRCTYPE” keyword that is stored in the “SCI” extension header(s) of data products. The step sets the value of “SRCTYPE” based on input from the user given in the “SRCTYAPT” keyword, as well as other rules that can override the “SRCTYAPT” values.

The srctype step first checks to see if the “SRCTYAPT” keyword is present and has already been populated. If “SRCTYAPT” is not present or is set to “UNKNOWN”, the step determines a suitable value based on the observing mode and other characteristics of the exposure. The following choices are used, in order of priority:

  • Background target exposures default to a source type of “EXTENDED.” Background exposures are identified by the keyword “BKGDTARG” set to True.

  • TSO exposures default to a source type of “POINT.” TSO exposures are identified by EXP_TYPE=”NRC_TSGRISM” or “NRS_BRIGHTOBJ”, or TSOVISIT=True.

  • Exposures that are part of a nodded dither pattern, which are assumed to only be used with point-like targets, default to a source type of “POINT.” Nodded exposures are usually identified by the “PATTTYPE” keyword either being set to a value of “POINT-SOURCE” or containing the sub-string “NOD” (NIRSpec IFU and Fixed Slit). For MIRI MRS exposures the keyword “DITHOPFR” (DITHer pattern OPtimized FoR) is used instead of “PATTTYPE”. If it has a value of “POINT-SOURCE”, the source type is set to “POINT”.

  • If none of the above conditions apply, and the user did not choose a value in the APT, the following table of defaults is used, based on the “EXP_TYPE” keyword value:


Exposure Type



MIRI LRS fixed-slit



MIRI LRS slitless









NIRSpec fixed-slit



NIRSpec bright object





If the EXP_TYPE value of the input image is not in the above list, SRCTYPE will be set to “UNKNOWN”.

NOTE: NIRSpec fixed-slit (EXP_TYPE=”NRS_FIXEDSLIT”) exposures are unique in that a single target is specified in the APT, yet data for multiple slits can be contained within an exposure, depending on the size of the readout used (e.g. SUBARRAY=”ALLSLITS”). For this observing mode, the source type selection resulting from the logic outlined above is used to populate the SRCTYPE keyword associated with the data for the primary slit instance in the pipeline data products. The primary slit is determined from the value of the “FXD_SLIT” keyword. Any additional slit instances contained within the data product will have their SRCTYPE value set to the default for NIRSpec fixed-slit, which, as indicated in the table above, is “POINT”.

Multi-Source Observations


For NIRSpec MOS exposures (EXP_TYPE=”NRS_MSASPEC”), there are multiple sources per exposure and hence a single user-selected 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 for a given source in the MSA metadata is less than zero, the source type defaults to “POINT.” 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 is stored in the “SRCTYPE” keyword located in the header of the SCI extension associated with each 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.


It is not possible to specify ahead of time the source types for spectra that may show up in a Wide-Field Slitless Spectroscopy exposure. So for these modes the srctype step simply sets the SRCTYPE keyword value to “UNKNOWN” and the actual source sizes are derived from the catalog information generated from direct images that are obtained as part of a WFSS observation.