The JWST NIRISS WFSS enables slitless spectroscopy in the wavelength range 0.8–2.2 μm. Estimates of the line flux sensitivity are provided in this article.
The NIRISS wide field slitless spectroscopy (WFSS) mode provides low resolution (R ~ 150) spectroscopy over the wavelength range 0.8–2.2 μm for every object within the 2.2' × 2.2' field of view. Blocking filters in the pupil wheel limit the wavelength coverage and spatial extent of the spectra on the detector. WFSS offers two identical grisms (GR150C and GR150R) that are mounted orthogonally to each other in the filter wheel. Use of both filters helps to mitigate contamination from overlapping spectra.
Figure 1 illustrates the S/N = 10 line flux sensitivity for a 10 ks observation through each of the blocking filters for an unresolved source. The extraction aperture is assumed to be 3 pixels in the cross-dispersion direction and 2 pixels in the wavelength direction.
The continuum sensitivity values are generally below 2 μJy for a 10 ks exposure over most of the wavelength range, rising to larger values at the shortest wavelengths. In terms of Vega magnitudes, where on average the A0V stars have the same magnitude in all filters, the continuum sensitivity is between 22.0 and 24.5 over the whole wavelength range.