NIRCam Sensitivity

The sensitivity estimates for JWST NIRCam presented here have been derived using the Exposure Time Calculator (ETC). They are intended to provide reference values for a few representative cases. 

For initial exploration and quick feasibility check, users can use the JWST Interactive Sensitivity Tool (JIST). However, for detailed calculations tailored to their specific science cases users should ultimately use the Exposure Time Calculator (ETC) . 

On this page

The Exposure Time Calculator (ETC) determines the efficiency with which photons striking the JWST primary mirror will be converted into measured signal at the NIRCam detectors. It uses a model that accounts for the measured transmission/reflection values for all optical elements and quantum efficiency of the detectors. Noise is estimated based on characterization data for the detectors, including read noise, dark current, and 1/f components, and includes the usual photon statistics for light from sources and predicted background levels. The expected point spread function is computed using WebbPSF.



Calculation

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



Imaging

Table 1 and Figure 1 show the Signal-to-Noise ratio achieved by NIRCam in imaging mode for a 10 ks integration using a circular photometric aperture 2.5 pixels in radius. NIRCam imaging is capable of studying very faint sources. Typical 10 ks images in F200W and F322W2 yield S/N = 10 detections of AB mag 29 (~9 nJy) point sources (S/N = 5 at AB mag 29.75 [4.5 nJy]).


Table 1. S/N = 10 in 104 s for the NIRCam imaging filters

Filter

Flux density1 (nJy)

F070W22.5
F090W15.3
F115W13.2
F150W10.6
F200W9.1
F277W14.3
F322W29.1
F356W12.1
F444W23.6
F140M19.4
F162M21.4
F182M16.1
F210M14.9
F250M32.1
F300M25.8
F335M21.8
F360M20.7
F410M24.7
F430M50.9
F460M46.5
F480M67.9
F164N145
F187N133
F212N129
F323N194
F405N158
F466N274
F470N302

1  Sensitivites given assume a zodiacal light background at the position RA = 17:26:44, Dec = -73:19:56  on June 19, 2019. This value is equal to 1.2 × the minimum zodiacal light background. Additional information on the NIRCam imaging sensitivity can be found in this article: NIRCam Imaging Sensitivity.

Figure 1. Expected NIRCam imaging point source sensitivity

Approximate sensitivity of all NIRCam filters

This assumes circular photometric apertures 2.5 pixels in radius and 1.2 × minimum zodiacal light background.


Coronagraphy

The NIRCam coronagraphic occulting masks will occult light from point sources, enabling deep searches for nearby companions and extended sources. The sensitivity to nearby companions is given by a combination of reduced throughput, due to the insertion of the Lyot stop to mitigate diffraction effects, and  limiting contrast. The Lyot stop transmit ~20% of the light, corresponding to a loss of about 2 magnitudes. In the vicinity of the occulted star the sensitivity loss increases due to a combination of photon and residual speckle noise, so the sensitivity also depends on the brightness of the primary as described by the limiting contrast curve. Users should use the Exposure Time Calculator (ETC) to evaluate NIRCam coronagraphic performance.



Grism

The NIRCam grisms disperse light for the NIRCam wide field slitless spectroscopy and NIRCam grism time-series modes. Approximate continuum and line sensitivities are shown in Figure 2 for a 10 ks integration using a 2 × 5 pixel extraction aperture (2 pixels in the spectral direction by 5 pixels in the spatial direction).  Tables 2 and 3 show the sensitivity values for modules A and B, respectively. Users should consult the Exposure Time Calculator (ETC) to assess NIRCam slitless spectroscopy performance.


Table 2. S/N = 10 in 104 s for the NIRCam grism module A

λ (micron)Fcont(microJy)Fline(erg s-1 cm-2)Filter
2.59.19.7E-18F322W2
2.76.86.3E-18F322W2
2.96.15.0E-18F322W2
3.16.64.8E-18F322W2
3.35.33.5E-18F322W2
3.55.23.2E-18F322W2
3.75.12.9E-18F322W2
3.95.63.0E-18F322W2
4.17.84.0E-18F444W
4.38.74.3E-18F444W
4.5104.8E-18F444W
4.7135.9E-18F444W
4.9177.5E-18F444W


Table 3. S/N = 10 in 104 s for the NIRCam grism module B
λ (micron)Fcont(microJy)Fline(erg s-1 cm-2)Filter
2.5111.2E-17F322W2
2.78.67.9E-18F322W2
2.97.76.2E-18F322W2
3.18.25.9E-18F322W2
3.36.54.3E-18F322W2
3.56.23.8E-18F322W2
3.763.4E-18F322W2
3.96.33.3E-18F322W2
4.19.54.8E-18F444W
4.3105.0E-18F444W
4.5125.4E-18F444W
4.7146.3E-18F444W
4.9177.4E-18F444W

Figure 2. Expected NIRCam grism line and continuum sensitivities in modules A and B

Grism module A and module B sensitivities from Greene et al. (2017)—tables 2 and 3. This assumes a 2 × 5 pixel extraction aperture (2 pixels in the spectral direction by 5 pixels in the spatial direction). Module B's sensitivity is ~75% that of module A because module B's grism has a less effective AR coating.


References

Greene, T. et al. 2017, JATIS, 035001

λ = 2.4 to 5 μm spectroscopy with the James Webb Space Telescope Near-Infrared Camera

NIRCam Design Features and Performance website (U. Arizona)




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