Sensitivity estimates are available for the JWST NIRISS observing modes (wide field sliltess spectroscopy, imaging, single object slitless spectroscopy, and aperture masking interferometry).
NIRISS WFSS sensitivity
See also: NIRISS Wide Field Slitless Spectroscopy
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 2 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 and continuum 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.
NIRISS imaging sensitivity
See also: NIRISS Imaging
Images are obtained through filters in the pupil wheel, spanning wavelengths 0.8–2.2 μm, and in the filter wheel, covering wavelengths 2.5–5.0 μm. Five medium-band filters (F140M, F158M, F380M, F430M, F480M) and 7 wideband filters (F090W, F115W, F150W, F200W, F277W, F356W, F444W) are available.
NIRISS imaging is also used to take direct images before and after grism exposures in the wide field slitless spectroscopy mode. Direct images allow identification of objects in the grism exposures and absolute wavelength calibration. Direct images are only available through the short wavelength filters in the pupil wheel: F090W, F115W, F140M, F150W, F158M, F200W.
Table 1 lists the filter sensitivities corresponding to a S/N = 10 for a 10 ks observation of a point source. These sensitivities are also plotted in Figure 2.
Table 1. Filter point source imaging sensitivity for S/N = 10 in 10ks
|Pupil Wheel Filters|
|Filter Wheel Filters|
† The magnitude in NIRISS filters are equivalent to the previous "Vegamag" system where the CALSPEC Sirius model from Bohlin 2022 is used as a template for an A0V star with a magnitude of -1.395 in all filters (Rieke et al. 2022).
NIRISS single object slitless spectroscopy
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For the SOSS mode, the sensitivity is calculated by requiring a signal-to-noise ratio of 1,000 per wavelength resolution element from 4 NISRAPID groups in full frame readout. At this signal-to-noise ratio, a transiting exoplanet with contrast 0.001 with respect to the star should be detectable. Figure 2 shows the Vega magnitude values for this level of contrast in the first and second orders, for a star of spectral type A0V. Given a longer time baseline or with smoothing of the spectrum, the sensitivity will be better than shown in the plot.
NIRISS aperture masking interferometry
NIRISS aperture masking interferometry (AMI) offers high spatial resolution imaging at 2.77, 3.80, 4.30, and 4.80 μm for bright objects at separations of 70–400 mas through the use of a non-redundant mask (NRM). AMI enables the detection of faint objects around bright objects through high-contrast imaging. See the HCI NIRISS Limiting Contrast article for further information about the companion-to-host flux ratio of the minimum detectable companion.
Rieke, G., H., Su, K., Sloan, G., C., Schlawin, E., 2022, AJ, 163, 45
Infrared Absolute Calibration. I. Comparison of Sirius with Fainter Calibration Stars