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-SLISTLESS 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.