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
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 based on the Exposure Time Calculator (ETC) v2.0 using code available on GitHub. 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 (S/N = 5 at AB mag 29.75 [4 nJy]).
Table 1. S/N = 10 in 104 s for the NIRCam imaging filters
| Filter | Flux density* (nJy) | Magnitude (AB) |
|---|---|---|
| F070W | 14.5 | 28.5 |
| F090W | 11.9 | 28.7 |
| F115W | 11.1 | 28.8 |
| F150W | 9.0 | 29.0 |
| F200W | 8.0 | 29.1 |
| F277W | 12.9 | 28.6 |
| F322W2 | 8.3 | 29.1 |
| F356W | 12.0 | 28.7 |
| F444W | 16.1 | 28.4 |
| F140M | 15.0 | 28.5 |
| F162M | 13.7 | 28.6 |
| F182M | 11.7 | 28.7 |
| F210M | 13.5 | 28.6 |
| F250M | 29.4 | 27.7 |
| F300M | 19.9 | 28.2 |
| F335M | 19.0 | 28.2 |
| F360M | 18.9 | 28.2 |
| F410M | 21.6 | 28.1 |
| F430M | 34.4 | 27.6 |
| F460M | 45.9 | 27.3 |
| F480M | 43.9 | 27.3 |
| F164N | 75 | 26.7 |
| F187N | 72 | 26.8 |
| F212N | 73 | 26.8 |
| F323N | 138 | 26.1 |
| F405N | 118 | 26.2 |
| F466N | 161 | 25.9 |
| F470N | 180 | 25.8 |
* 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.
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, from Greene et al. (2017). 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 |
|---|---|---|---|
| 2.5 | 9.1 | 9.7E-18 | F322W2 |
| 2.7 | 6.8 | 6.3E-18 | F322W2 |
| 2.9 | 6.1 | 5.0E-18 | F322W2 |
| 3.1 | 6.6 | 4.8E-18 | F322W2 |
| 3.3 | 5.3 | 3.5E-18 | F322W2 |
| 3.5 | 5.2 | 3.2E-18 | F322W2 |
| 3.7 | 5.1 | 2.9E-18 | F322W2 |
| 3.9 | 5.6 | 3.0E-18 | F322W2 |
| 4.1 | 7.8 | 4.0E-18 | F444W |
| 4.3 | 8.7 | 4.3E-18 | F444W |
| 4.5 | 10 | 4.8E-18 | F444W |
| 4.7 | 13 | 5.9E-18 | F444W |
| 4.9 | 17 | 7.5E-18 | F444W |
| λ (micron) | Fcont(microJy) | Fline(erg s-1 cm-2) | Filter |
|---|---|---|---|
| 2.5 | 11 | 1.2E-17 | F322W2 |
| 2.7 | 8.6 | 7.9E-18 | F322W2 |
| 2.9 | 7.7 | 6.2E-18 | F322W2 |
| 3.1 | 8.2 | 5.9E-18 | F322W2 |
| 3.3 | 6.5 | 4.3E-18 | F322W2 |
| 3.5 | 6.2 | 3.8E-18 | F322W2 |
| 3.7 | 6 | 3.4E-18 | F322W2 |
| 3.9 | 6.3 | 3.3E-18 | F322W2 |
| 4.1 | 9.5 | 4.8E-18 | F444W |
| 4.3 | 10 | 5.0E-18 | F444W |
| 4.5 | 12 | 5.4E-18 | F444W |
| 4.7 | 14 | 6.3E-18 | F444W |
| 4.9 | 17 | 7.4E-18 | F444W |
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. © Greene et al. (2017).
This figure refers to pre-mission expectations and will be updated when further analysis of calibration data is completed.
References
Greene, T. et al. 2017, JATIS, 035001
NIRCam Design Features and Performance website (U. Arizona)