NIRCam Overview

JWST’s Near Infrared Camera (NIRCam) offers imagingcoronagraphy, and grism slitless spectroscopy from 0.6 to 5.0 μm, as well as wavefront sensing measurements for JWST mirror alignment.

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The JWST Near Infrared Camera (NIRCam) observes from 0.6 to 5.0 μm and offers imagingcoronagraphy, and grismslitless spectroscopy. NIRCam has two modules pointing to adjacent fields of view. Each module uses a dichroic to observe simultaneously in a short wavelength channel (0.6–2.3 μm) and a long wavelength channel (2.4–5.0 μm).

 NIRCam has five observing modes for science:

  • Imaging of two 2.2' × 2.2' fields separated by 44" covering 9.7 arcmin² in total

NIRCam will also obtain wavefront sensing measurements used to align and phase JWST's primary mirror.

Figure 1. NIRCam field of view in the JWST focal plane

NIRCam in the JWST field of view



Observational capabilities

Each observing mode offers distinct capabilities and uses a subset of the optical elements available in the pupil and filter wheels.


Table 1. NIRCam observing mode parameters

Observing mode

Wavelength
coverage
(µm)

Field of view1

Pixel 
scale
(arcsec/pix)

Notes

Imaging

0.6–2.3

2 × 132" × 132"
(44" and 5" gaps)

0.031FWHM 2 pix
at 2.0 µm
2.4–5.0

2 × 129" × 129"
(48" gap) 

0.063FWHM 2 pix
at 4.0 µm

Coronagraphic
imaging

1.8–2.2 
2.8–5.0 

20" × 20"

0.031
0.063

Wide field
slitless spectroscopy
2.4–5.0 2 × 129" × 129"0.063R ~ 1,600
Time-series imaging0.6–2.3 
2.4–5.0

129" × 129";
132" × 132"

0.031
0.063

Grism time series2.4–5.0129" × 129"0.063R ~ 1,600

1 Smaller fields of view are available using subarrays in the imaging, time series imaging, and grism time series modes.

Field of view

Figure 2. NIRCam modules field of view

NIRCam field of view

The two NIRCam modules image a 5.1' × 2.2' field of view with a central gap of 44"; the two adjacent 2.2' × 2.2' fields cover 9.7 arcmin² in total. The coronagraphs obtain images in regions of the sky outside the imaging/grism field of view. These are projected onto the detectors by optical wedges located on the pupil plane Lyot stops.


Modules

NIRCam consists of two redundant modules—A and B—with identical optical elements and nearly identical throughput. The NIRCam observing modes include options to use either a single module or both simultaneously with identical integration times, readout patterns, and filters.



Channels

Each module includes a short (0.6–2.3 µm) and long (2.4–5.0 µm) wavelength channel. NIRCam uses a dichroic to observe at both wavelengths simultaneously in roughly the same field of view.


Table 2. Properties of short and long wavelength channels


Short wavelength channelsLong wavelength channels

Wavelength range

0.6–2.3 μm

2.4–5.0 μm

Fields of view

2 × 2.2' × 2.2' (with 4"–5” gaps)

2 × 2.2' × 2.2'

Pixels1

8 × 2040 × 2040 pixels

2 × 2040 × 2040 pixels

Pixel scale

0.031"/pixel

0.063″/pixel

PSF FWHM2

0.064" @ 2μm (Nyquist)

0.13" @ 4μm (Nyquist)

Grism slitless spectroscopy

N/A

R ~ 1600

Coronagraphic occulting masks

round: 2.1 μm

bar: 1.8–2.2 μm

round: 3.35, 4.3 μm

bar: 2.5–5.0 μm

1 Not including reference pixels insensitive to light that are four pixels along each outer edge.

2 PSF undersampled at lower wavelengths; slightly broader for coronagraphy.



Optical elements

Each channel includes a pupil wheel and a filter wheel, each with 12 optical elements, which may be used in various combinations. These elements include:

  • Filters with extra-wide, wide, medium and narrow passbands;

  • Coronagraphic Lyot stops to suppress light that passes the occulting masks;

  • Grism elements for slitless spectroscopy in the long wavelength channel;

  • Weak lenses to defocus light from bright sources to avoid detector saturation, also used for the telescope alignment.

Within each module, light may (optionally) be passed through a coronagraphic occulting mask (in the focal plane) before being divided by the dichroic beam splitter to the short and long wavelength channels.  

Figure 3. NIRCam pupil and filter wheels

The NIRCam pupil and filter wheels contain a total of 48 optical elements. Filters are color-coded by wavelength. Wider filters in the figure appear more transparent than narrower filters.

Filters

The 29 NIRCam filters have names "Fnnnx" where "nnn" refers to the central wavelength (e.g., 220 for 2.20 µm) and "x" refers to the filter width: "W2" for extra-wide (R ~ 1), "W" for wide (R ~ 4), "M" for medium (R ~ 10) and "N" for narrowband (R ~ 100).

Figure 4. NIRCam and JWST optical telescope element (OTE) filter throughputs

NIRCam and JWST Optical Telescope Element (OTE) filter throughputs (version 4.0: April 22, 2016). The vertical gray bar marks the approximate dichroic cutoff between the short and long wavelength channels. Filters marked "P" are located in the pupil wheel, requiring transmission through a second filter in the filter wheel, either F150W2, F322W2, or F444W. In those cases, the combined transmissions are plotted.


Detectors

NIRCam has 10 Teledyne HgCdTe H2RG detectors, or sensor chip assemblies (SCAs), each with 2040 × 2040 pixels sensitive to light. The eight short wavelength detectors cover roughly the same area on the sky as the two long-wavelength detectors. In full frame imaging mode, all 10 detectors are read out non-destructively every 10.74 s. The smallest supported science subarray (64 × 64 pixels) can be read out in 49 ms (the shortest exposure time).



Sensitivity

In a 10 ks image, NIRCam will obtain S/N = 10 detections of point sources as faint as ~10 nJy (AB mag 28.9) in some filters, and S/N = 5 detections of 5 nJy (AB mag 29.65) point sources.

Please use the Exposure Time Calculator (ETC) for the most up-to-date sensitivity estimates for specific proposed observations.

Figure 5. NIRCam sensitivity for wide filter imaging

NIRCam sensitivity for wide filter imaging

Sensitivity (detection limits for signal to noise = 10) for point sources in a 10 ks image (comprised of 10 exposures, 1 ks each). A flat source spectra in nJy and a zodiacal background at 1.2 times the minimum value are assumed. Wavelength is color-coded. Filter widths are shown as horizontal bars. Please use the ETC to calculate sensitivity estimates for your specific proposed observations.


Saturation

NIRCam's brightest saturation limits are achieved in the time-series observing modesWeak lenses (at short wavelengths) and grisms (at long wavelengths) spread the bright light over more detector pixels. Subarrays allow for shorter readout times. Stars observable without saturation (80% full well) include those visible to the naked eye: ~5th magnitude or brighter, depending on the observing mode, wavelength, and configuration.

Approximate saturation limits are given in time-series imaging and grism time series. Please use the ETC to calculate saturation estimates for your proposed observations.



Data calibration and analysis

Information about calibration is available at Data Processing and Calibration Files. This article and its links point to content about absolute astrometricflux, and wavelength calibration, as well as information on calibration reference files.

Details about the data can be found in the JWST File Names, Format, and Data Structures article. The JWST pipeline is described in JWST Data Reduction Pipeline and some information about post-pipeline processing can be found at JWST Post-Pipeline Data Analysis.



Acknowledgements

NIRCam was built by a team at the University of Arizona (UofA) and Lockheed Martin's Advanced Technology Center, led by Prof. Marcia Rieke at UoA.



References

Rieke, M. et al. 2005, SPIE, 5904, 1
Overview of James Webb Space Telescope and NIRCam's Role

Rieke, M. et al. 2003, SPIE, 4850, 478
NGST NIRCam Scientific Program and Design Concept

Beichman, C. et al. 2012, SPIE, 8442, 2
Science opportunities with the near-IR camera (NIRCam) on the James Webb Space Telescope (JWST)

NIRCam Papers from the August 2005 SPIE meeting

NIRCam Technical Documents 




Published

 

Latest updates
  •  
    Updated grism resolution

  •  
    Updated pixel scale values.