NIRCam Imaging

JWST NIRCam imaging is obtained at 0.6–2.3 μm (0.031 "/pix) and 2.4–5.0 μm (0.063 "/pix) simultaneously over a 9.7 arcmin² field of view.

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See also: NIRCam Imaging APT Template

Imaging is one of NIRCam's observing modes, which also include time-series imaging (rapid monitoring of bright variable sources) and coronagraphy (high-contrast imaging).

For standard imaging discussed here, NIRCam observes 9.7 arcmin² field of view simultaneously in a short wavelength channel (0.6–2.3 µm) and long wavelength channel (2.4–5.0 µm) via a beam splitting dichroic. Extra-wide, wide, medium, and narrow filters are available.

When creating a NIRCam imaging observation with the Astronomer's Proposal Tool (APT), the primary options are the filter, dither pattern, subarray, and exposure time (via the detector readout patterns). One filter is selected for each wavelength channel. All other parameter selections apply to both channels.

The Exposure Time Calculator (ETC) allows users to determine the observing parameters required for their science goals.


Table 1. Properties of NIRCam short and long wavelength channels


Short wavelength channelLong wavelength channel
Wavelength range0.6–2.3 μm2.4–5.0 μm
Nyquist wavelength12.0 μm4.0 μm
Fields of view2 × 2.2′ × 2.2′ (with 4"–5” gaps)2 × 2.2′ × 2.2′
Imaging pixels8 × 2040 × 2040 pixels2 × 2040 × 2040 pixels
Pixel scale0.031 ″/pixel0.063 ″/pixel

1 PSF FWHM ~ 2 pixels; undersampled at shorter wavelengths


Field of view

NIRCam's two modules operate in parallel to observe a 9.7 arcmin² field of view consisting of two 2.2' × 2.2' fields separated by a ~44" gap. Gaps of ~5" also separate the four detectors in each short wavelength module. If full field imaging is not required, individual modules or smaller subarrays may be used to increase readout speeds and reduce data volumes. To cover larger areas, mosaics are best obtained in combination with primary dithers.

Figure 1. NIRCam imaging fields of view overlaid to scale on an HST mosaic section from CANDELS (via 3DHST)

NIRCam images are, by default, obtained simultaneously with both modules using all 10 detectors at short and long wavelengths.  


Filters

Twenty-nine NIRCam filters are available for imaging. They are classified as extra-wide (R ~ 1), wide (R ~ 4), medium (R ~ 10), and narrow (R ~ 100). One filter is selected for each wavelength channel, and both wavelengths are observed simultaneously.


Dithering and mosaics

Dithering—required for all NIRCam imaging observations except time-series monitoringobtains multiple exposures slightly offset from one another on the sky. Primary dithers serve to fill image gaps, compensate for bad pixels, and mitigate flat field uncertainties. Smaller secondary dithers provide subpixel sampling to improve the resolution of reduced images.

Mosaics are used to image regions of sky larger than the NIRCam field of view. A mosaic consists of a grid of N × M telescope pointings. Large NIRCam mosaics are best combined with a FULL1 primary dither pattern.

1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.


Subarrays

If the full NIRCam field of view is not required for science, smaller subarrays may be read out from the detectors. Faster readout times for subarrays allow brighter objects to be observed without saturation in shorter exposures. Subarrays also deliver lower data volumes than full detector readouts. Users interested in rapid cadence monitoring of bright, time-variable sources should refer to the time-series imaging observing mode.



Exposure time

NIRCam's exposure time is governed by detector readout patterns.

Note that due to gaps between the detectors, dithered images will have uneven exposure times (with depth varying across the image).

The Exposure Time Calculator (ETC) allows users to determine the exposure time required to achieve their science goals.



Sensitivity

The following sensitivity and saturation estimates are provided as a rough guide. Please use the Exposure Time Calculator (ETC) for your proposed observations.

NIRCam imaging is capable of studying very faint sources. Typical 10 ks images in F200W and F322W2 may yield S/N = 10 detections of AB mag 29 (~9 nJy) point sources (and S/N = 5 detections of AB mag 29.75 [4.5 nJy] sources).

Figure 3. Approximate sensitivity of all NIRCam filters
Approximate sensitivity of all NIRCam filters
Sensitivity is shown as S/N = 10 detection limits for point sources in a 10 ks image (comprised of 10 exposures, 1 ks each). The sources are assumed to have flat spectra in nJy (and AB magnitudes). Zodiacal light is assumed to be 1.2 times the minimum. 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 sensitivity estimates for your specific proposed observations.


Saturation

Moderately bright sources (solar type, K-band Vega mag 14–17) will saturate the NIRCam detectors in full field broadband imaging. Narrower filters enable brighter saturation limits, as shown below. The smallest available subarray (64 × 64 pixels) brightens these limits by 6 magnitudes. The brightest objects are best studied using the NIRCam time-series observing modes, which allow the use of weak lenses (at short wavelengths) and grisms (at long wavelengths) to spread the light over more pixels.

Figure 4. Approximate saturation magnitudes in FULL frame imaging mode
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 80% 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.


References

University of Arizona NIRCam website




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Latest updates

  • Pixel scale values updated