The figure below shows simulated spectra from one of the dispersers used for Wide-Field Slitless Spectroscopy. This mode is supported by a matched pair of orthogonal grisms with R=150. These grisms, GR150C (grooves in the Horizontal direction disperse spectra vertically, along Columns) and GR150R (grooves in the Vertical direction disperse spectra horizontally, along Rows), are located in the filter wheel of NIRISS. The redshift of the object is proportional to the distance between the emission features in the horizontal and vertical spectra and the point at which the two spectra intersect.
Software is available to model wide-field slitless spectroscopic (WFSS) observations with NIRISS and to extract and calibrate the resulting spectra. The tools employed are the aXe package of programs, which was developed to support slitless spectroscopy with ACS and WFC2 on the Hubble Space Telescope, and Source Extractor, which builds a catalog of objects from an astronomical image.
In WFSS mode, the GR150C and GR150R grisms (highlighted in green) in the Filter Wheel can be used with the filters in the Pupil Wheel that are highlighted in green.