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 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 (MEDIUM8 with 10 groups and 10 exposures) using a circular photometric aperture 2.5 pixels in radius based on the Exposure Time Calculator (ETC) v4.0. 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 (~8 nJy) point sources.

 

Table 1. S/N = 10 in 104 s for the NIRCam imaging filters, from ETC v4

Filter

Flux density* (nJy)

Magnitude (AB)
F070W14.228.5
F090W11.928.7
F115W11.128.8
F150W9.129.0
F150W24.529.8
F200W8.029.1
F277W12.128.7
F322W28.229.1
F356W11.828.7
F444W17.428.3
F140M15.128.5
F162M14.728.5
F182M11.828.7
F210M13.628.6
F250M27.327.8
F300M18.928.2
F335M18.228.3
F360M19.128.2
F410M22.128.0
F430M36.627.5
F460M49.227.2
F480M47.927.2
F164N90.426.5
F187N80.926.6
F212N80.226.6
F323N12826.1
F405N12926.1
F466N16625.9
F470N19125.7

* Sensitivites assume point sources with photometric apertures 2.5 pixels in radius and a benchmark background (1.2 × minimum zodiacal light) described in JWST Background Model. Additional information on the NIRCam imaging sensitivity can be found in this article: NIRCam Imaging Sensitivity.


Figure 1. Expected NIRCam imaging point source sensitivity
This assumes circular photometric apertures 2.5 pixels in radius and a benchmark background (1.2 × minimum zodiacal light) described in JWST Background Model.

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 wide field slitless spectroscopy, long wavelength grism time-series, and short wavelength grism time-series modes. 

Long wavelength

Approximate continuum and line sensitivities are shown in Figure 2  and Table 2 for a 10 ks integration on module A. Module B grisms have 30% lower transmission. 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, from ETC v4

λ (micron)Fcont(microJy)Fline(erg s-1 cm-2)Filter
2.513.41.6E-17F322W2
2.710.01.1E-17F322W2
2.98.47.9E-18F322W2
3.17.66.5E-18F322W2
3.37.05.5E-18F322W2
3.56.64.7E-18F322W2
3.76.64.3E-18F322W2
3.97.14.4E-18F322W2
4.18.95.2E-18F444W
4.39.95.4E-18F444W
4.511.55.9E-18F444W
4.714.47.0E-18F444W
4.918.98.8E-18F444W


Figure 2. Grism sensitivities in module A 



Sensitivity of the NIRCam LW grism on module A, expressed as the flux required to reach SNR ~ 10 in a ~10 ks observation. Top panel: results for a continuum source. Bottom panel: results for individual, unresolved emission lines. For clarity, the medium filters are shown only in the top panel. In this background-dominated SNR regime, the effect of the wavelength-integrated, slitless background as a function of filter width is clearly seen as the sensitivity gets better (smaller flux) for narrower passbands. In both cases the 10 ks observation was configured as 10 exposures of one integration each, using the MEDIUM8 readout pattern with 10 groups. This choice keeps each exposure around the maximum recommended NIRCam exposure time of 1,000s, which avoids excessive cosmic rays contamination. The sky background level was set to Medium. The cross-dispersion extraction area was set to 3 pixels and the background extraction annulus to 10–25 pixels from the source. For the continuum case, the target SNR is expressed per spectral pixel (as the ETC provides results per detector pixel). For the emission line case, the SNR is obtained after integrating the line flux over 3 spectral pixels. These plots were obtained using results from the ETC v4.0.

Short wavelength

The NIRCam short wavelength grism time-series mode uses the Dispersed Hartmann Sensor (DHS) to perform grism time-series observations in the short wavelength channel. The DHS pupil wheel element is composed of 10 separate grisms that occupy rectangular sub-apertures which sample very small fractions of the primary mirror, so sensitivity is relatively low compared to the long-wavelength grism time-series values. 

Figure 3. Sensitivity for the DHS vs subarray size and blocking filter.



Sensitivity of the DHS vs wavelength for the 2, 4 and 8 spectra cases (STRIPE1, 2 and 4 subarrays, respectively) in the F070W, F090W, F150W2 and F200W filters. Note that this has been computed per spectral pixel, rather than per resolution element. The spectral resolution is determined by the size of the LSF, which changes linearly with wavelength (i.e., the resolving power is constant), and goes from 20 pixels at 2 µm to 10 pixels at 1 µm. The measurement time of 100 s and the SNR of 100 have been chosen as representative of the typical usage of this mode for transiting exoplanet studies. Sensitivities for F070W and F090W are for 2nd-order spectra, F150W2 and F200W values are for the 1st order spectrum, and all are for a single integration. The gray shaded area below 0.8 μm indicates that the DHS performance in this spectral region has not been fully characterized yet. Note that the F150W2 cutoff at 2.0 µm is related to the fact that 1st and 2nd order in this filter overlap beyond this wavelength, therefore results in this regime are not reported, although, nominally the F150W2 bandpass extends beyond it. Sensitivity for the SUB256STRIPE4 subarray is very close to that of SUB160STRIPE4. Observers should use the ETC to evaluate SNR for their particular targets.

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)



Notable updates
    • ETC v4 updates to imaging and grism sensitivities
    • Added section with sensitivity for new SW grism time-series mode

  •  
    ETC v2 updates to imaging sensitivities (not grism)
Originally published