NIRCam Time-Series Observations

JWST NIRCam offers two observing modes for precision monitoring of time-variable sources: time-series imaging and grism time series.

See also: NIRCam Time-Series APT TemplateNIRCam Grism Time-Series APT Template

The NIRCam time-series modes provide stable, repeatable observations of time-variable sources, including stars that host transiting exoplanets and brown dwarfs. These observations enable high-precision, rapid-cadence, long-duration measurements of photometric and/or spectroscopic variations over time. The modes are also designed to accommodate bright sources.

The two NIRCam time-series observing modes are: 

These modes differ from other NIRCam observing modes in the following ways:

  • Dithering and mosaics are not allowed (no telescope movements)
  • Very long exposures (>50 hours in full frame mode, with some interrupts and limitations) consisting of many integrations (up to 65,535) are possible
  • Weak lenses to defocus light (in the short wavelength images) are available

In each mode, observations are obtained simultaneously at short and long wavelengths (via the dichroic). Depending on the mode, a variety of filters (broad-, medium-, and narrowband), weak lenses (at short wavelengths), and a grism (at long wavelengths) are available.


Table 1. Filters, weak lenses, and a grism available for time-series observations

Observing mode

Short wavelengths
(0.6–2.3 µm)

Long wavelengths
(2.4–5.0 µm)

NIRCam time-series imaging

Broad-, medium- or narrowband
filter with or without the weak lens:

  • WLP8 (+8 waves of defocus1)
Broad-, medium-, or narrowband filter
NIRCam grism time series

Medium- or narrowband
filteralways with a weak lens:

  • WLP8 (+8 waves of defocus1)
    -or- 
  • WLP4 (+4 waves of defocus1
Grism R (slitless spectroscopy)
with a wide filter

1 Waves of defocus are defined at 2.12 μm. For 8 waves, the PSF width is ~80 times larger.


The simultaneous observations at both wavelengths have identical integration times, exposure times, and readout patterns. Therefore, elements yielding similar sensitivity 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). Weak lens observations at short wavelengths may be paired with narrowband imaging or grism observations at long wavelengths. Some of these combinations are enforced by the Astronomer's Proposal Tool (APT).

Spatial scanning is not available for JWST observations, as it was for HST, to improve saturation limits and photometric precision. The reason is that JWST slews too slowly (0.03 "/s) to compete with the short readout times of NIRCam's subarrays, which enable bright objects to be observed without saturation. However, weak lenses and a grism are available to spread light over many pixels to improve photometric precision.



References

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

Greene, T., Chu, L., Hodapp, K. W., et al. 2016, SPIE 99040E
Slitless Spectroscopy with the James Webb Space Telescope Near-Infrared Camera (JWST NIRCam)




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