View Full Version : Foldable mirror?

2004-Jul-28, 03:41 AM
With Hubble possibly going out of commission soon, I was thinking about new space telescopes that might be put up soon. Obviously, when it comes to telescopes, the bigger the better. However, when talking about space telescopes, there's a definite limit to how large the mirror can be. A 10-meter space telescope would be great, but last time I checked there were no rockets capable of transporting a 10-meter mirror unless it were broken up into sections and reassembled in space. To reassemble such a delicate item to such exact tolerances on Earth would be nearly impossible, let alone in space, so I think that's right out.

But, as some of you might know, I recently started another thread concerning solar sails and their like, and I got to thinking that mirror could be constructed out of some flexible material that could be folded up for transport. My idea is to take a sheet of some maleable substance like mylar and coat one side of it with a reflective material - perhaps a metallic aerosol or something. The only real huge difficulty I can think of with this is somehow warping a flat disk of mylar to be parabolic like a telescopic reflector with the kind of strict tolerances necessary to deliver the kind of optics needed for such a device.

Is there any particular reason - well, obviously there must be SOME reason, so - what is the reason that this isn't being talked about by NASA? Or have I just not heard?

2004-Jul-28, 05:38 AM
I think that multiple mirrors could be assembled in space. Software could correct any anomolies such as the hairline cracks between mirrors and such.

Take hubble for example. They upgraded the software on the telescope to correct some problems, and now it works better than it ever has.

2004-Jul-28, 01:01 PM
The mirror for the James Webb Space Telescope (http://jwstsite.stsci.edu/telescope/nuts_.and._bolts/optics.shtml) is in 18 segments that will be folded up for launch and then deployed once the spacecraft gets on station.

2004-Jul-28, 02:06 PM
The mirror for the James Webb Space Telescope (http://jwstsite.stsci.edu/telescope/nuts_.and._bolts/optics.shtml) is in 18 segments that will be folded up for launch and then deployed once the spacecraft gets on station.

I coulda posted that first had I done so when first checking BABB from home... Anyway, at longer wavelengths the folding trick does work. The Japanese radio-astronomy satellite HALCA, http://www.vsop.isas.ac.jp/general/MUSES_B.html, used for interferometry with ground-based antennae from its high orbit, unfolded its parabolic dish (of roughly hexagonal shape). And in the less-clear world, it is widely believed that some geosynchronous electronic-intelligence satellites have unfurled parabolic dishes as much as 100 meters in diameter - see, for example, http://www.fas.org/spp/military/program/sigint/vortex2.htm. It still seems to be really tough to unfold and get the surface precision needed for optical or near-infrared use, but this could change as adaptive optics becomes more capable - since repairing a permanent wavefront error is no harder than a temporary one. Segments have been made to work on the ground already (Keck I and II, the GTC = Gran Telescopio de Canarias now under construction and a teeny bit larger, demonstrate that), and rumor has it that at least one bidder for the JWST primary segments has relevant experience at longer wavelengths for "unnamed but satisfied customers".

2004-Jul-28, 03:56 PM
Segmented telescopes are the way to go. The problem with mylar would be the precision required. ATM mirrors are measured in a fraction of a wavefront. A mirror that is off by 1/2 wave length is unusable, because wave interference darkens the image, the waves cancel eash other out. Even on large thin mirror telescopes a lot of thought is put into the cell, so the mirror is not compressed, or distorted. Imagine the problem with something like mylar.