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James Webb Space Telescope
NIRCam Instrument Design

NIRCam is comprised of a single optical imaging assembly mounted into the Integrated Science Instrument Module (ISIM) of JWST.

Modules

Figure 1. CAD layout of the NIRCam optical imaging assembly
  • The optical assembly consists of two modules, each imaging a 2.16' x 2.16' field of view.
  • The modules are built on two optical benches mounted back-to-back. As a result, the two fields of views imaged by the modules are adjacent.
  • The modules are functionally identical, with identical optical and focal plane components. They are mirror images of each other except for coronagraphic pupil masks.
Figure 2. Layout of a NIRCam module

Channels

  • As shown in Figure 2, in each module the incoming light initially reflects off the pick-off mirror, then passes through a folding mirror, a collimator and dichroic which splits the beam at about 2.35 µm reflecting the short wavelengths (0.6-2.3 µm) and transmitting the long wavelengths (2.4-5.0 µm) beams.
  • The components associated with each of these beams are collectively referred to as a channel.
  • The short wavelength channel is based on a 4096 x 4096 pixels Focal Plane Array with 0.0317"/pixel scale. The long wavelength channel is based on a 2048 x 2048 pixel chip with 0.0648"/pixel scale.
  • The scales of the short and long wavelength arms are tuned to provide Nyquist sampling at 2 µm and 4 µm, respectively.
  • Each channel has a pupil and filter wheel. The contents of the pupil and filter wheels are different for SW and LW channels.

NIRCam Field of View

Figure 3 shows an example of single NIRCam exposure. The upper 5 images are from Module A, the lower 5 images are from Module B. For each module, the image produced by the short-wavelength channel is on the left, the one produce by the long wavelength channel is on the right. The images of the two channels have the same total Field of Views. For the purpose of illustration, the galaxies are twice as large and one quarter as dense as they are expected to appear.

Figure 3. An example of single NIRCam exposure

Filters/Pupil Wheel

  • Each channel is equipped with a 12 slot pupil wheel containing coronagraphic pupil masks, wavefront sensing optics, filters, and calibration sources.
  • The pupil wheel is followed in the optical train by a 12 slot filter wheel containing wide-band and narrow-band filters. The pupil wheel also contains some of the filters.
  • The short-wavelength channel contains 5 wide-band (R~4), 4 medium (R~10), and 3 narrow- band (R~100) filters, while the long-wavelength channel contains 3 wide, 8 medium, 4 narrow-band filters as shown in the table below.
Short Wavelength FiltersLong Wavelength Filters
F070WF277W
F090WF356W
F115WF444W
F150WF250M
F200WF300M
F140MF335M
F162MF360M
F182MF410M
F210MF430M
F164NF460M
F187NF480M
F212NF323N
F405N
F466N
F470N

Detectors

  • NIRCam is based on 2048x2048 format chips (sensor chip assemblies or SCAs), with 18 µm x 18 µm HgCdTe photodiode pixels. They are produced by Teledyne.
  • The chips used for the short and long wavelength channels are based on different types of HgCdTe. However, in what concerns their functionality and control they are basically identical.
  • The sensitive area of a chip is 2040x2040 pixels, due to a 4 pixel wide border of reference pixels along all edges. Reference pixels are used for tracking bias drifts during an exposure.
  • The finer pixel scale of the short-wavelength channels requires a Focal Plane Array, i.e. a 2x2 mosaic of chips butted with a small gap (5”) between them. The resulting gaps in the images have to be filled by dithering moves of the telescope. The long wavelength channels use a single chip.
  • The fields observed by the two modules are adjacent with about 50" separation. Also this larger gap in the images has to be filled by dithering moves.

Figure 4 shows the layout of the FPAs.

Figure 4. Schematic showing the layout of the short-wave (upper) and long-wave (lower) Focal Plane Assemblies (FPAs), with the designations for each of the chips/Sensor Chip Assemblies (SCAs) that make up the FPAs. The black squares in one corner of each chip show the location of the first pixel addressed during chip readout, which is also the origin (0,0) of the pixel coordinate system on each chip. The 4 gray regions on each chip show the portions served by the 4 parallel output channels. The approximate projection of the coronagraph onto the FPAs is shown. The V1, V2, V3 coordinate system is shown for reference along with the FPA X, Y coordinate system.
The optical bench is perpendicular to the page and bisects modules A and B.