The NIRCam time-series imaging mode was designed to enable precise measurements of photometric variations in relatively bright sources. It is one of two modes available for NIRCam time-series observations, the other being grism time series. These modes provide maximum stability in the observations and electronics. They are designed to accommodate bright sources. They also allow for very long uninterrupted observations, consisting of many integrations executed at high cadence and observing efficiency.
Simultaneous imaging is obtained via a dichroic at short (0.6–2.3 µm) and long (2.4–5.0 µm) wavelengths in various extra-wide, wide, medium, and narrow NIRCam filters. Exposure times and readout patterns will be identical at both wavelengths. Therefore, filters with similar sensitivities and saturation limits should be used for both wavelengths. For example, imaging may be obtained simultaneously in two wide filters (e.g., F150W and F356W) or two narrow filters (e.g., F212N and F323N).
The weak lens WLP8 is available in the short wavelength channel to defocus the image of a bright source, improving the saturation limit by several magnitudes. To improve this limit further, the weak lens may be used in conjunction with subarrays, which should be 160 × 160 pixels or larger to encompass most of the defocused image and attain a proper background subtraction.
The weak lens may be paired with select filters between 1.3–2.2 µm. While the weak lens is being used in the short wavelength channel, long wavelength imaging is restricted to narrowband filters to avoid saturation. (The long wavelength grism may also be used in the NIRCam grism time-series observing mode.)
Use of the weak lens (at short wavelengths) in conjunction with the 160 × 160 pixel subarray increases saturation limits by ~11 magnitudes compared to standard full field imaging. The smallest NIRCam subarray (64 × 64 pixels; without the weak lens) enables saturation limits ~6 magnitudes brighter than full field imaging at both short and long wavelengths.
NIRCam Design Features and Performance (U. Arizona)
Beichman, C. et al. 2014, PASP, 126, 1134