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I came across this diagram in the JWST User documentation which explains how light is directed into the ISIM (Integrated Scientific Instrument Module):

Diagram of JWST OTE optics

But the ISIM contains four detectors (MIRI, NIRCam, NIRSpec and NIRISS), How does the ISIM split this beam for each detector?

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Found this report which includes this drawing:

enter image description here

The report contains a bunch of tables describing the wavelength bands, filter paths, etc. Note particularly the "Dichroic Beamsplitter"

PostScript: several nice references on the Wikipedia page for ISIM

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    $\begingroup$ That diagram is showing the inside of NIRCam, 1 of the JWST instruments, which comes after the ISIM. Those beamsplitters split the NIRCam imaging beam into short (0.6-2.3um) and long (2.4-5um) wavelength channels [ref]( jwst-docs.stsci.edu/near-infrared-camera/nircam-observing-modes/…). The ISIM seems to use pickoff mirrors to direct different bits of the OTE beam into the different instruments as shown in this FOV layout - not found an ISIM optical diagram yet.. $\endgroup$ – astrosnapper Jul 30 '20 at 23:19
  • $\begingroup$ @astrosnapper I don't see anything in that Image-Plane layout drawing which suggest pickoff mirrors, or for that matter upstream dichroics or other splitter/separator elements. It's purely showing the ultimate placement of sensors in the main telescope's FOV $\endgroup$ – Carl Witthoft Jul 31 '20 at 10:48
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    $\begingroup$ I agree, the only clue is that none of the instruments are on axis and brief references in the caption of Fig 2 on this page, the Gardener et al 2008 ref and other instruments such as NIRSS to pick-off mirrors (and as the first element in your posted NIRcam diagram). All beamsplitters would be in the instruments, ISIM (optically at least) is just pickoff mirrors around the telescope pupil $\endgroup$ – astrosnapper Jul 31 '20 at 16:31

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