Imaging saturation limits

Table 1. NIRCam imager bright source limits (ETC v4.0)

Figure 1. Approximate saturation magnitudes in full frame imaging mode

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 N_groups = 2 (reset-read-read). 

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

Grism saturation limits

Long wavelength

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. In addition to FULL, the grism time series offers several subarray options. When the long wavelength channel grism time series is paired with imaging on the short wavelength channel, 3 subarrays are offered,  each along the bottom of module A: SUBGRISM256 (2048 × 256), SUBGRISM128 (2048 × 128), and SUBGRISM64 (2048 × 64), see the NIRCam Grism Time Series article. When the long wavelength channel grism time series is paired with short wavelength spectroscopy, 4 MULTISTRIPE subarrays are offered SUB40STRIPE1_DHS, SUB80STRIPE2_DHS, SUB160STRIPE4_DHS and SUB256STRIPE4_DHS (40 ×, 80 ×, 160 ×, 256 × 2048 pixels respectively, see the NIRCam Short Wavelength Grism Time Series and NIRCam Multistripe Subarrays articles for more details). Table 3 lists the bright limits for the 2048 × 64 subarray calculated using the ETC v4.0, assuming that the source reaches 70% full well using 2 groups of the RAPID readout pattern (reset-read-read). The SUB40STRIPE1_DHS subarray has a faster frametime than SUBGRISM64 (0.21485 vs 0.34061 s), resulting in ~0.5 magnitudes (a factor of ~1.5 in flux) brighter limits.

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

For larger subarrays and/or a single detector output, the minimum integration times increase, and a K ~ 4.5 Vega mag star would saturate the detector. Table 3 shows approximate saturation limits for the various subarrays, again assuming 2 detector reads between resets (RAPID readout pattern). These limits are given for 2.7 µm, the wavelength most prone to saturation. F277W and F322W2 observations will experience such saturation. Longer wavelength observations may observe somewhat brighter stars without saturating. Please consult the Exposure Time Calculator (ETC).

Table 4. Subarray saturation limits

Short wavelength

The NIRCam short wavelength grism time-series mode uses the Dispersed Hartmann Sensor (DHS) to perform R ~ 300 (1st order) or R ~600 (2nd order) monitoring of bright, time-variable sources at 0.6–2.3 µm 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 saturation limits are relatively high compared to the long-wavelength grism time series values. The saturation limit varies by sub-aperture, with DHS sub-aperture #7 having the highest throughput. Depending on how many sub-aperture spectra the user has chosen to collect (2, 4 or 8), saturation of the subaperture 7 spectrum may or may not be a particular concern. Figure 5 shows the saturation limit for sub-aperture 7, thus providing the most conservative estimate.

Table 5. NIRCam SW grism brightness limits in module A 2048 × 40 pixel subarray SUB40STRIPE1_DHS (ETC v4.0)

References