NIRCam Sensitivity

The sensitivity model for JWST NIRCam incorporates ground testing measurements into a full sensitivity model that includes parameters such as background, measured transmission/reflectivity for optical elements, encircled energy, and detector noise. 

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The efficiency with which photons striking the JWST primary mirror will be converted into measured signal at the NIRCam detectors has been predicted using measured transmission/reflection values for all NIRCam 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

Approximate sensitivities are shown in Figure 1 for a 10 ks integration using a circular photometric aperture 2.5 pixels in radius. Please use the Exposure Time Calculator (ETC) for your proposed observations.

NIRCam imaging is capable of studying very faint sources. Typical 10 ks images in F200W and F322W2 may yield S/N = 10 detections of AB mag 29 (~9 nJy) point sources (and S/N = 5 detections of AB mag 29.75 [4.5 nJy] sources).


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 the limiting contrast.



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. Please consult the Exposure Time Calculator (ETC) for your proposed observations.


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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