Based on the more recent of the Webb construction photos as well as the drawings in the deployment explorer, the JWST secondary mirror support booms look rather white, and even shiny. Won't they reflect stray light into the telescope from all directions? Why wouldn't they be matte black like other elements? I wondered if they might be black in infrared, but then remembered that JWST is supposed to be able to see light up into the red/orange range, so that doesn't explain it.
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1$\begingroup$ A part looking white or shiny in the visible range of light may be black in the invisible infrared range if it is very cold. There are filters used to block unwanted light into the red/orange range. $\endgroup$– UweJan 5, 2022 at 7:39
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1$\begingroup$ @CuteKItty_pleaseStopBArking, won't there also be reflections of all sorts of things off the primary mirror, off the booms, and then back off the primary mirror? I'd expect that to add some funky background. Maybe it doesn't shift around enough with orientation to matter? $\endgroup$– dfeuerJan 5, 2022 at 17:17
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2 Answers
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A smart-ass answer to the question “Why aren’t the booms painted black?” would be, “Because they are already made of black material.” This doesn’t answer the puzzle of, “Why are supports coated in reflective foil only on their sunward sides?”
Answer: The supports are covered in foil on their sunward side to prevent radiant heating (from the 40*K sunshield) from warping them.
The supports, as installed, were black (as seen in this photo). The primary material of the JWST bus is graphite composite https://en.wikipedia.org/wiki/Spacecraft_bus_(James_Webb_Space_Telescope) . The appearance of the supports is consistent with this material. IR emissivity for graphite composite is typically 0.65-0.85 .
https://phys.org/news/2016-12-nasa-webb-telescope-vibration-anomaly.html
Before launch, the supports were partially coated with reflective wrap. This photo was during final deployment testing so (presumably), this is how JWST was configured at launch.
And here is a photo during fueling with the wrap still in place
The reflective wrap was only placed on the sunward (sunshade) side of the supports. In this photo, the anti-sun support is closest to the camera.
My guess: if the supports were not shielded, the sunward side of the supports would be warmed by the “cold” side of the sunshade at 40K. The anti-sun sides of the supports would equilibrate with interplanetary temperature of 10K. This could give a 30*K gradient across the width of the supports. Graphite composites have a very low (even negative) coefficient of thermal expansion. But if all 3 supports bent in the same direction at the same time, this could have a measurable effect on the positioning of the secondary mirror.
Note: It is not wise to make assumptions about IR emissivity based upon the visible light appearance of a surface. Total emissivity of asphalt and snow are almost the same. https://en.wikipedia.org/wiki/Emissivity#Emissivities_of_common_surfaces
Emissivity varies with wavelength, emission angle, and surface treatment.
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$\begingroup$ So ... why were they covered in reflective foil? My question already discusses the IR vs. visible—the telescope observes red and orange light as well as IR. $\endgroup$– dfeuerJan 5, 2022 at 20:52
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$\begingroup$ @dfeuer The foil coverings are not symmetrical about the telescope's optic axis, so they are unlikely positioned for an observing purpose. $\endgroup$– WoodyJan 5, 2022 at 20:56
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$\begingroup$ Okay, so do you know if they're still there? Or would they have been removed before the fairing was lowered? Maybe we could tell from photos of that process? $\endgroup$– dfeuerJan 5, 2022 at 20:58
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2$\begingroup$ @dfeuer Good thought. I'll search for fairing installation photos. $\endgroup$– WoodyJan 5, 2022 at 20:59
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$\begingroup$ OK, so you think they're intended to shield the support booms from IR radiation coming from the sun shield. Only the "upper" boom has reflective shielding on the "inner" side, which will reflect light onto the primary mirror. Some of that light will be far infrared, either from the heat shield or from elsewhere, and is largely irrelevant. Some of it will be yellow or shorter wavelengths, and therefore mostly irrelevant. I guess the relevant reflection from the one boom is considered acceptable? $\endgroup$– dfeuerJan 6, 2022 at 3:33
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According to this NASA video,
... they're white there because they have some plastic protective coatings on it, kind of like you'd get something shipped in the mail that's got plastic bubble wrap.
So I guess the photos are of a temporary condition, and the online graphics are colored deceptively.
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3$\begingroup$ cdn.sci.esa.int/documents/34594/35357/… shows the JWST support booms without plastic wrapping. Looks black to me. Plus they are at 35K once JWST cools down fully. $\endgroup$– SheldonJan 5, 2022 at 19:43
