NIRCam Bright Source Limits
The bright source limits of JWST's Near Infrared Camera (NIRCam) are predicted by a saturation model that uses measurements obtained from in-flight data acquired during JWST instrument commissioning.
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The bright source limits for various modes and subarrays of JWST's NIRCam are predicted by a saturation model that uses measurements from in-flight data. 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 the 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.
Saturation limits quoted below are based on the Exposure Time Calculator (ETC) v2.0 using code available on GitHub.
Users should ultimately use the Exposure Time Calculator (ETC) for all saturation calculations.
Imaging saturation limits
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Table 1. NIRCam imager bright source limits (ETC v2.0)
Filter | Bright source limit (Vega mags) | Bright source limit (Vega mags) |
---|---|---|
F070W | 15.98 | 10.15 |
F090W | 16.27 | 10.44 |
F115W | 16.19 | 10.36 |
F140M | 15.29 | 9.47 |
F150W | 16.06 | 10.23 |
F150W2 | 17.41 | 11.59 |
F162M | 15.12 | 9.29 |
F164N | 12.69 | 6.87 |
F182M | 14.96 | 9.13 |
F187N | 12.30 | 6.48 |
F200W | 15.38 | 9.55 |
F210M | 14.22 | 8.39 |
F212N | 11.87 | 6.04 |
F250M | 14.46 | 8.63 |
F277W | 15.60 | 9.77 |
F300M | 14.43 | 8.61 |
F322W2 | 16.08 | 10.25 |
F323N | 11.42 | 5.59 |
F335M | 14.11 | 8.28 |
F356W | 14.84 | 9.01 |
F360M | 13.88 | 8.05 |
F405N | 10.85 | 5.03 |
F410M | 13.39 | 7.56 |
F430M | 12.50 | 6.67 |
F444W | 14.07 | 8.25 |
F460M | 11.94 | 6.11 |
F466N | 9.96 | 4.14 |
F470N | 9.83 | 4.00 |
F480M | 12.06 | 6.24 |
Time-series imaging saturation limits
The NIRCam time-series mode was designed to enable precise measurements of photometric variations in relatively bright sources. For the short wavelength channel, the WLP8 weak lens may be used to defocus the light, raising the saturation limit by ~5 magnitudes. The NIRCam time-series mode offers 3 subarrays: SUB400P, SUB160P, and SUB64P (all located in the top-right corner of module B). Table 2 lists the bright limits (70% full well) for the SUB160P subarray, assuming the RAPID readout pattern with Ngroups = 2 (reset-read-read).
Filter | Bright source limit (Vega mags) |
---|---|
F150W | 4.66 |
F200W | 4.57 |
F140M | 3.77 |
F182M | 4.12 |
F210M | 3.53 |
F187N | 1.64 |
F212N | 1.28 |
Grism saturation limits
The NIRCam wide field slitless spectroscopy and NIRCam grism time-series modes provide R ~ 1,600 spectra of all objects within (or just outside) the field of view in the long wavelength channel. The wide field mode offers one subarray: FULL. The grism time series offers 3 subarrays, each along the bottom of module A: SUBGRISM256 (2048 × 256), SUBGRISM128 (2048 × 128), and SUBGRISM64 (2048 × 64). Table 3 lists the bright limits for the 2048 × 64 subarray calculated using the ETC v2.0, assuming that the source reaches 70% full well using 2 groups of the RAPID readout pattern (reset-read-read).
lambda (μm) | Bright source limit | Bright source limit (K-band Vega mags) G2V star | Bright source limit | Filter |
---|---|---|---|---|
2.5 | 4.4 | 4.4 | 4.2 | F322W2 |
2.7 | 4.5 | 4.5 | 4.4 | F322W2 |
2.9 | 4.5 | 4.5 | 4.5 | F322W2 |
3.1 | 4.4 | 4.4 | 4.5 | F322W2 |
3.3 | 4.3 | 4.4 | 4.5 | F322W2 |
3.5 | 4.3 | 4.3 | 4.5 | F322W2 |
3.7 | 4.1 | 4.1 | 4.4 | F322W2 |
3.9 | 3.9 | 3.9 | 4.2 | F322W2 |
4.1 | 3.7 | 3.7 | 4.0 | F444W |
4.3 | 3.4 | 3.4 | 3.7 | F444W |
4.5 | 3.1 | 3.1 | 3.3 | F444W |
4.7 | 2.8 | 2.8 | 3.0 | F444W |
4.9 | 2.4 | 2.4 | 2.7 | F444W |
References
Greene, T. et al. 2017, JATIS, 035001
University of Arizona NIRCam website