NIRCam Modules

JWST NIRCam consists of two modules with nearly identical optics and detectors. When both are used in tandem, they double the NIRCam field of view.

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NIRCam's two modules—A and B—provide redundancy and double the available field of view in the imaging and wide field slitless spectroscopy (WFSS) observing modes. For observing modes covering smaller fields of view, a single module, A or B, may be used, based on optimal performance. The two modules have nearly identical optical elements and detectors with a few notable differences.



Notable differences between modules

Throughput

Module B has better throughput at the longest wavelengths (see comparisons at NIRCam filters page). This is most pronounced for the F480M filter, which has ~33% and ~42% throughput in modules A and B, respectively (this includes all telescope throughputs, filter transmissions, and detector efficiencies).

Grism performance

Module A grisms have anti-reflection coating on both sides. Module B grisms are only coated on their flat (non-grooved) sides, reducing their efficiency by ~26% and introducing some ghosts of bright spectra (Greene et al., 2016).

Coronagraph performance

The module A coronagraph optical substrate has a significantly better anti-reflection coating than the module B substrate. The module A coronagraph mask neutral density squares are also superior. Coronagraph mask occulters are arranged in different orders in each module (both sets are equally good).

Field of view

In both modules, the fields of view of the short wavelength (SW) and long wavelengh (LW) channels are almost, but not entirely, identical. However, module A provides slightly better overlap. The slight difference in overlap will not affect most observations, but may impact observations that use the smallest subarrays



Views of the NIRCam modules

Figure 1. NIRCam modules

NIRCam module B image
NIRCam module A image

The two NIRCam modules, A (top) and B (bottom), are nearly identical mirror images of one another.
Figure 2. NIRCam module optical path

NIRCam module image for reference with optical path diagram

NIRCam module optical path

 Module A (top) and a diagram of the optical path through its components (bottom), shown with the same orientation. The optical path illustration reproduced from Greenhouse et al. 2011, Figure 2.
Figure 3. NIRCam modules mounted together

NIRCam modules mounted together

The two NIRCam modules are mounted back-to-back on the Integrated Science Instrument Module (ISIM). Here, module B is shown on top, and module A is barely visible below it.


References

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)

Greenhouse, M. et al. 2011, SPIE 814606G
Status of the James Webb Space Telescope integrated science instrument module system




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