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 during ground testing.

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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 obtained during ground testing. 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

 Users should ultimately use the Exposure Time Calculator (ETC) for all saturation/sensitivity calculations.


Imaging saturation limits

The NIRCam imager is the prime near-infrared imager for JWST. It offers seven subarrays: (1) FULL 1, SUB640, SUB320, and SUB160 that are located in the center of module B and (2) SUB400P, SUB160P, and SUB64P that are located in the top-right corner of module B. The subarrays are designed to accommodate observations of bright sources. Table 1 lists the bright limits (95% full well) for the largest (FULL) and smallest (SUB64P) NIRCam Imaging subarrays, assuming the RAPID readout pattern with Ngroups = 2 (reset-read-read). Figures 1 and 2 illustrate these limits.


Table 1.NIRCam imager bright source limits

Filter

Bright source limit (Vega mags)
Ksat (G2V)
FULL (21.4 s)

Bright source limit (Vega mags)
Ksat (G2V)
SUB64P (~0.1 s)

F070W14.438.63
F090W15.249.44
F115W15.449.64
F140M14.628.82
F150W15.379.57
F150W216.6010.80
F162M14.408.60
F164N11.996.19
F182M14.368.56
F187N11.685.88
F200W14.809.00
F210M13.667.86
F212N11.385.58
F250M14.148.34
F277W15.239.43
F300M13.938.13
F322W215.669.86
F323N11.025.22
F335M13.657.85
F356W14.428.62
F360M13.457.65
F405N10.444.64
F410M13.067.26
F430M12.096.29
F444W13.687.88
F460M11.525.72
F466N9.643.84
F470N9.453.65
F480M11.555.75

Figure 1. Approximate saturation magnitudes in full frame imaging mode

The figure shows saturation, in magnitudes (Vega K-band), for a solar type G2V star in 21.4 s (based on two readouts of the full detector), filling pixel wells to 95% capacity. Brighter saturation limits may be achieved by using subarrays to reduce the exposure time, and/or using time-series observations with the weak lenses or grism. Filter widths are shown as horizontal bars. Extra-wide, wide, medium, and narrow filters are labeled in normal, bold, and italic text, respectively, each with progressively thicker bars. Please use the Exposure Time Calculator (ETC) to calculate saturation estimates for your specific proposed observations.
Figure 2. Saturation magnitudes for NIRCam filters in a 64 × 64 pixel subarray

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The figure shows approximate saturation magnitudes (Vega K-band for a solar type G2V star) in the 64 × 64 pixel subarray for a ~0.1 s exposure (two readouts of the subarray). Saturation is defined here as 95% of the pixel well capacity. Filters are color-coded, with widths shown as horizontal bars. More precise saturation estimates may be obtained from the Exposure Time Calculator (ETC). Limits ~5 magnitudes brighter than those shown here may be achieved at 1.3–2.2 µm by using the +8-wave weak lens (WLP8) with the 160 × 160 pixel subarray.
Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.


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 three 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). 

Table 2. NIRCam Time-series imaging WLP8 bright source limits
Filter

Bright source limit (Vega mags)
Ksat (G2V)
SUB160P (~0.55 s)

F150W4.63
F200W4.37
F140M3.69
F182M3.93
F210M3.43
F187N1.34
F212N1.22

Figure 3. Saturation magnitudes for short wavelength NIRCam filters using the WLP8 weak lens with a 160 × 160 pixel subarray

The figure shows approximate saturation magnitudes (Vega K-band for a solar type G2V star), using the WLP8 weak lens, with a 160 × 160 pixel subarray for a ~0.55 s exposure (two readouts of the subarray). Saturation is defined here as 70% of the pixel well capacity. Filters are color-coded, with widths shown as horizontal bars. More precise saturation estimates may be obtained from the Exposure Time Calculator (ETC).


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 three 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 v1.5, assuming that the source reaches 70% full well using 2 groups of the RAPID readout pattern (reset-read-read).

 

Table 3. NIRCam grism brightness limits in Module A 2048 × 64 pixel subarray SUBGRISM64
lambda (μm)

Bright source limit
(K-band Vega mags)
A0V star

Bright source limit
(K-band Vega mags)
G2V star

Bright source limit
(K-band Vega mags)
M2V star

Filter
2.54.44.44.0F322W2
2.74.54.54.4F322W2
2.94.54.54.4F322W2
3.14.24.34.2F322W2
3.34.24.34.4F322W2
3.54.24.24.3F322W2
3.74.04.14.2F322W2
3.93.83.94.1F322W2
4.13.63.73.8F444W
4.33.43.43.5F444W
4.53.13.13.2F444W
4.72.72.82.8F444W
4.92.42.42.6F444W

Figure 4. Grism saturation limits in the 2048 × 64 pixel subarray

Approximate grism saturation limits in K-band Vega magnitudes for 3 stellar types (A0V, G2V, M2V) in the Module A 2048 × 64 pixel subarray with stripe mode readout (4 outputs), assuming a detector reset and two reads (0.68 s integration). Results are from the Exposure Time Calculator (ETC) v1.5. Please use the ETC to obtain saturation limits for your proposed observations.


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)




Published

 

Latest updates
  •  
    Updated bright source saturation limits (ETC v1.5) to 95% (imaging) and 70% (time-series) well capacity