James Webb Space Telescope
NIRISS Wide-Field Slitless Spectroscopy

• Wavelength range: 1 – 2.5 μm
• Spectral resolving power: 150
• Provides a spectrum for every source in the field of view
• Spectrophotometric accuracy: 15%
• Optimized for the detection of Lyman alpha emitters ( 10 < z < 17 )

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, G150H (horizontal) and G150V (vertical), 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.

Dual Wheel Configuration for Wide-Field Slitless Spectroscopy

In WFSS mode, the G150H and G150V grisms (highlighted in green) in the Filter Wheel can be used with the filters in the Pupil Wheel that are highlighted in green.