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

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