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James Webb Space Telescope
NIRISS Wide-Field Slitless Spectroscopy

Slitless Spectroscopy with Orthogonal Grisms

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.

See the sensitivity of the NIRISS in its WFSS mode.

The image is a simulation of a field of view containing many faint galaxies whose light is dispersed by one of the grisms. The schematic inset explains how a pair of orthogonal spectra can be used to determine the redshift of a feature of known rest wavelength.

Simulating Slitless Spectroscopy with NIRISS

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.

Dual Wheel Configuration for Wide-Field Slitless Spectroscopy

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.

Dual wheel configuration for Wide-Field Slitless Spectroscopy