Each NIRCam module has short wavelength (0.6–2.3 μm) and long wavelength (2.4–5.0 μm) channels, each with a pupil wheel and filter wheel of 12 elements each. Many combinations of pupil and filter wheel elements are allowed as described below. These elements include:
- Filters: extra-wide, wide, medium, and narrow-band with R = λ/Δλ ~ 1, 4, 10, and 100, respectively.
- Coronagraph Lyot stops to suppress diffracted light that passes the occulting masks on the focal plane.
- Grism elements in the long wavelength channel. Two elements are available, dispersing light along detector rows and columns, respectively.
- Weak lenses to defocus light from bright sources to avoid detector saturation. These are also used for telescope alignment.
- Clear positions (empty holes) in the pupil wheel to be used in combination with filter wheel elements.
- Dark elements used for calibration.
- Other elements used only for wavefront sensing and telescope alignment, including the PAPPA (pupil alignment pinhole projector assembly), DHS (dispersed Hartman sensing), and IPR Wedge (internal phase retrieval).
Pupil and filter wheels characteristics
Table 1. NIRCam pupil and filter wheel elements
Short wavelength channel
Long wavelength channel
Black: bandpass filters named W2, W, M, N have R = λ/Δλ ~ 1, 4, 10, 100, respectively
Green: grism elements disperse spectra along detector rows (R) or columns (C), used in conjunction with filters
Magenta: WL, weak lens, used to defocus for mirror alignment or bright star imaging without saturation
Gray: CLEAR positions, empty holes in the pupil wheel
Wavefront sensing/calibration (not available for science)
- DHS, dispersed Hartman sensing (subaperture grisms): coarse phasing alignment of JWST's mirrors
- PAPPA, pupil alignment pinhole projector assembly: align NIRCam with the JWST Optical Telescope Element (OTE)
- IPR, internal phase retrieval, wedge: measure NIRCam wavefront errors using LEDs mounted on coronagraphs
- Dark: blocks incoming light for dark current measurements
Allowed combinations between pupil and filter wheel elements