NIRCam Time-Series Imaging

JWST NIRCam's time-series imaging observing mode performs rapid photometric monitoring of bright, time-variable sources. Weak lenses and subarrays may be used to improve saturation limits.

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See also: NIRCam Time-Series APT Template

The NIRCam time-series imaging mode was designed to enable precise measurements of photometric variations in relatively bright sources. It is one of 2 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. 

Dithers and mosaics are not allowed in this mode. (In standard imaging mode, dithers are required.)

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 2 wide filters (e.g., F150W and F356W) or 2 narrow filters (e.g., F212N and F323N).

 

Weak lens

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.)

 

Saturation limits

Use of the weak lens (at short wavelengths) in conjunction with the 160 × 160 pixel subarray increases saturation limits by ~10 magnitudes compared to standard full field imaging.

Figure 1. Saturation magnitudes for short wavelength NIRCam filters using the WLP8 weak lens with a 160 × 160 pixel subarray

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 ~0.84 s exposure (2 readouts of the subarray) from the Exposure Time Calculator (ETC) v4.0. 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 ETC.

References

University of Arizona NIRCam website

Beichman, C. et al. 2014, PASP, 126, 1134
Observations of Transiting Exoplanets with the James Webb Space Telescope (JWST)

Rigby, J. et al 2023, PASP, 135, 048001 
The Science Performance of JWST as Characterized in Commissioning



Notable updates
  •  
    ETC v4 updates to saturation plots

  •  
    ETC v2 updates to saturation plots

  •  
    Updated saturation limits to 70% well capacity (ETC v1.5)
Originally published