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 grisms (GR150C and GR150R) that are mounted orthogonally to each other in the filter wheel. The grisms are very similar, though GR150C has increased throughput in the 1st order compared with GR150R. 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. 

Figure 1. NIRISS GR150 continuum and line flux sensitivity
Colors corresponding to the different blocking filters: blue for F090W, orange for F115W, red for F150W, green for F140M, purple for F158M, and brown for F200W. The sensitivities shown are for the GR150C grism which has higher throughput in the 1st order compared with the GR150R grism.


NIRISS imaging sensitivity

See also: NIRISS Imaging

NIRISS imaging is available as a prime observing mode and as a coordinated parallel mode when NIRCam imaging is the primary science mode. 

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

Filter

Flux density

(nJy)

Magnitude†

(Sirius)

Pupil Wheel Filters
F090W15.927.9
F115W14.127.8
F140M19.227.1
F150W12.827.4
F158M17.827.0
F200W11.327.1
Filter Wheel Filters
F227W11.726.4
F356W12.625.8
F380M33.524.6
F430M46.124.0
F444W20.524.9
F480M59.523.5

† 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). 


Figure 2. NIRISS imaging continuum sensitivity for various filters


NIRISS single object slitless spectroscopy

        See also: NIRISS Single Object Slitless Spectroscopy

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

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 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.

Figure 3. 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 article for further information about the companion-to-host flux ratio of the minimum detectable companion.


References

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



Notable updates
  •  
    Updated to match ETC 4.0 sensitivities where new PSFs were created to include effects of IPC and an updated background model was implemented. Imaging sensitivities changed by several percent in the long wavelength filters due to changes in the background.

  •  
    Updated to match ETC 3.0 sensitivities. Biggest change is to assume a default extraction aperture of 2.5 pixels for sensitivity calculations. 

  •  
    Updated to use in-flight sensitivities measured during JWST commissioning.

  •  
    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