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From simple curiosity, does anybody know why there are around 5 layers of solar radiation reflecting foil on the James Webb Telescope, rather than one or two?

Is it to save weight, or (probably) another, more basic physical reason, that it needs to be more complicated than a single heavier thickness foil.

enter image description here

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A solid piece of metal with the thickness of 5 layers of foil is just like 5 layers of foil except that, since it is a single solid piece, the "layers" are able to conduct heat (quite quickly!) directly from one to the next.

If the layers are instead separated by vacuum, the conduction of heat is eliminated, and heat transfer is greatly reduced (radiative transfer is by comparison much slower), and therefore the telescope is protected from the thermal expansion that would ruin its precision.

It's the same reason why your vacuum thermos has two layers (an inner and an outer) instead of just one solid layer - the separation eliminates conduction. Your thermos would be even more effective at insulating if it had five layers, each separated by vacuum.

Thanks to @Ernie for finding a link to the official answer, which confirms this is the reason.

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  • $\begingroup$ Thanks very much, I thought it was because each layer was filtering out a different range of wavelengths. I know NASA would stick to the KISS principle, so I knew there had to be some reason. Fingers crossed it works. $\endgroup$
    – user81619
    Aug 31, 2015 at 23:14
  • $\begingroup$ No problem. I may not have the whole story, but I'm confident that's the main reason. $\endgroup$
    – Brionius
    Aug 31, 2015 at 23:15
  • $\begingroup$ @Brionius you are correct. Scroll down to point #12 in this link from NASA: jwst.nasa.gov/faq.html#sunshield $\endgroup$
    – Ernie
    Sep 1, 2015 at 0:09
  • $\begingroup$ For a somewhat more detailed discussion, see en.wikipedia.org/wiki/Multi-layer_insulation $\endgroup$ Sep 1, 2015 at 1:49

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