I searched for use of an umbrella mechanism for a reflecting telescope but have not found anything. I was imagining that use of enough actuators on enough umbrella arms would allow fine tuning an inner mirrored umbrella surface to enough accuracy to serve as a primary telescope reflector - has anyone any refs or alternatively a 'showstopper' ?
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1$\begingroup$ Is this a design for a folding telescope mirror, with a flexible surface. The accuracy required is extreme: less than the wavelength of the light you are collecting. That would make this challenging from an engineering perspective. It probably possible in the microwave regime. $\endgroup$– James KSep 10, 2022 at 5:33
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$\begingroup$ Yes, folding mirror is what I'm after. A bit more searching turned up a few umbrella type mechanisms like 'raincube' , at Oxford they have 'hinged rib and 'wrapped rib' designs where the hinged rib is more or less umbrella-like. All these are for radar as you mention which relaxes the surface requirements - what I was wondering basically was the feasibility for optical . $\endgroup$– jeremy_rutmanSep 10, 2022 at 11:35
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$\begingroup$ I'd say it's impossible for optical. At least it is not practical. You'd need nanometer scale alignment, which would require so many precise actuators that it would be cheaper, simpler lighter, less bulky to just have a piece of mirrored glass. But I might be wrong, I hope someone with direct knowledge can anwer. $\endgroup$– James KSep 10, 2022 at 12:11
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$\begingroup$ this is something i've wondered: with an umbrella reflector, if the imperfections of the reflecting surfaces are known precisely, then it should be possible to deconvolute the image and result in a perfect image $\endgroup$– Aaron FSep 11, 2022 at 11:52
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I searched for use of an umbrella mechanism for a reflecting telescope but have not found anything. I was imagining that use of enough actuators on enough umbrella arms would allow fine tuning an inner mirrored umbrella surface to enough accuracy to serve as a primary telescope reflector - has anyone any refs or alternatively a 'showstopper'?
Fascinating question! This works for me, except for the nitty gritty of reality
I see that you have included the space-telescope tag and I am pretty sure that JWST is the first "unfolding" optical telescope in space. There are certainly several umbrella-like unfolding dish antennas for radio + microwave wavelengths.1 And X-ray telescopes often "telescope" or "scissor" out2 to their full lengths like the handle of a collapsible umbrella.
Packing the entire spacecraft:
- folded mirror
- folded secondary support (optical train)
- rolled-out sunshield
- spacecraft bus and propulsion system
- electronics
- and antenna
- solar panels
into the nosecone of a rocket in such a way that it survived the jarring launch, hot sun and journey to its halo orbit was quite an amazing feat!
There's no wind forces or the "pull of gravity" on the telescope relative to its mount in space, so one might think a flimsy support structure and good actuators is all that's necessary to hold a bunch of mirror segments in place to tens of nanometer accuracy. And for the most part, one would be correct in thinking that, if it weren't for (at least) three things:
- thermal expansion/contraction due to inhomogeneous and variable heating and cooling
- mechanical vibrations
- limits to the rate at which position errors of the mirrors can be measured and updated, and to the rate at which actuators can compensate this.
It's just the nuts and bolts, nitty gritty design engineering of the whole system that makes a fairly rigid, high thermal inertia (more mass means temperatures change more slowly) passive structure like that used in JWST is the best bet for lowest risk, highest probability of success based on the technology that was available when they froze the basic design.
An an unfolding umbrella tiled with actuator-controlled mirrors is 100% certainly possible, and you may even see one like that in the future!
But as discussed in https://space.stackexchange.com/q/57345/12102 the "next generation" JWST-like unfolding space telescope will look a lot like JWST, except a much bigger mirror that unfolds twice, and especially a much bigger heat shield), different wavelengths, many more segmented mirror tiles, and a new "swivel joint" that allows the telescope to point somewhat independently of its heat shield.
If there's a show-stopper, it's in the budget and the probability of success!
At 1 billion dollars, the JWST was already absurdly over budget at deliriously late.
The umbrella collectors I've seen in space to date are all for microwaves and radio where the umbrella is the primary surface itself. It's hard enough getting the actuators and optical feedback systems to handle the slow thermal drift and residual flexure and vibration of the spacecraft when they are attached to a rigid frame. The flimsy umbrella would just make their jobs much harder.
Lessons learned from JWST could be applied to refine future unfolding supports for mirror segments like the Launch Pad Astronomy video below "4 Future Space Telescopes NASA wants to build" shows.
1A few ("flimsy") umbrellas in space:
above: Venera 4's unfolding dish from This answer to Have solid dish antennas on deep space spacecraft (as opposed to meshes) ever provided any other helpful function? As meteor shields perhaps?. below: Click for larger view of flexible mesh antenna of TDRS-13 shown horizontally to because am I the only one reminded of "Audrey II, an evil and boisterous flytrap-like extraterrestrial plant" from Frank Oz's 1986 Little Shop of Horrors?) from https://space.stackexchange.com/q/51300/12102 and adapted from Source
above: Queqiao from tweet from Why is Queqiao's dish antenna so big? (Chang'e-4 relay satellite) Largest ever on/near the Moon? below: Spektr-R Source: @BowlOfRed's answer to What artificial satellite has the farthest orbit around the Earth?
2"scissor-like extension of X-ray telescopes
Note that all the optics is rigidly mounted in one end, the other end only holds the 2D sensor, so wiggling/flexure can be corrected by tracking multiple objects in the FOV (if you're lucky) or through other sensors, but those would not otherwise affect the relationship between critical optical surfaces.
above: from NuStar below: from How does IXPE measure polarization, and why does it have three identical X-ray telescopes?
