NIRISS Sensitivity

Sensitivity estimates are available for the JWST NIRISS observing modes (wide field sliltess spectroscopy, imaging, single object slitless spectroscopy, and aperture masking interferometry).

 Users should ultimately use the Exposure Time Calculator (ETC) for all saturation/sensitivity calculations.

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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 three pixels in the cross-dispersion direction and two pixels in the wavelength direction. 

Figure 1. NIRISS GR150 continuum and line flux sensitivity


Colors correspond due to the different blocking filters: blue - F090W, orange - F115W, red - F150W, green - F140M , purple - F158M, brown - F200W.



NIRISS imaging sensitivity

See also: NIRISS Imaging

NIRISS imaging is not available as a primary observing mode but is offered as a coordinated parallel mode when another JWST instrument is the primary science instrument. Currently, NIRISS imaging in parallel is only offered when NIRCam imaging is the primary science mode. Other prime-parallel combinations involving NIRISS imaging in parallel will be considered for implementation in cycle 2.

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 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

Filter

Flux density

(nJy)

Magnitude

(Vega)

Pupil Wheel Filters
F090W14.628.0
F115W12.427.9
F140M17.027.2
F150W11.227.6
F158M15.627.1
F200W9.727.2
Filter Wheel Filters
F227W13.126.3
F356W13.725.7
F380M35.824.5
F430M50.123.9
F444W21.424.8
F480M60.923.5

† Magnitude in NIRISS filters assuming an average A0V star has the same magnitude in all filters and using the Bohlin Sirius model (2020) as a template for this average star at magnitude -1.401.


Figure 2. NIRISS imaging continuum sensitivity for various filters



NIRISS single object slitless spectroscopy

        See also: NIRISS Single Object Slitless Spectroscopy

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NIRISS single object slitless spectroscopy (SOSS) enables medium-resolution (R ≈ 700) spectroscopy at 0.6–2.8 μm, in 3 cross-dispersed orders for a single bright target. The SOSS observing mode makes use of the GR700XD grism and has 2 detector subarray readout options: SUBSTRIP256 and SUBSTRIP96. Full frame readout is also supported.

For SOSS mode the sensitivity is calculated by requiring signal to noise 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.

Figure 3. Estimated SOSS sensitivity for an exoplanet with contrast 0.1% per wavelength resolution element and for a 4-group NISRAPID full frame observation




NIRISS aperture masking interferometry

See also: HCI NIRISS Limiting Contrast, 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 for further information about the companion-to-host flux ratio of the minimum detectable companion.



References

Bohlin, R. C., Hubeny, I, & Rauch, T. 2020, AJ, 160, 21
New Grids of Pure-hydrogen White Dwarf NLTE Model Atmospheres and the HST/STIS Flux Calibration




Latest updates
  •  
    Updated WFSS line sensitivity and continuum sensitivity plots to be consistent with ETC v.1.3

  •  
    Updated WFSS line sensitivity plot and added WFSS continuum sensitivity plot.
Originally published