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Mirror Arrays (Theoretical DIY!)


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A nice sunny afternoon has got me daydreaming....

Since the big thing in very/extremely/massively/stupendously large telescopes these days seems to be mirror arrays, has anyone tried to build a small(!) DIY telescope using this principal?

If (big if) you could get hold of/make smallish hexagonal mirrors could you create a massive open-truss dob, or do they rely too heavily on complex additional optics to correct any distortion the mirror array causes?

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I think the prime reason why these large Telescopes use Mirror arrays, instead of a single large mirror.

Is because it's extremely hard to create such a huge single piece mirror. Not to mention the weight itself. Not to mention the sheer costs involved.

So to come back to your question.

I think creating an array mirror on a small scale would have the total opposite effect.

In that. That it's much harder to create, than a single piece mirror of the projected total "small" size.

Edited by JeroKane
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There have been a few amateur attempts. There was a thread about it recently somewhere; probably on cloudynights??

The results were 'interesting' -- and I mean that in a genuine way; the telescopes worked after a fashion -- but all the builders concluded it wasn't worth the hassle.

Most (all?) have been made by putting multiple circular on-axis mirrors (i.e. normal telescope mirrors) onto one mount, and aligning them carefully so that the images from the mirrors all overlap. That seemed to work reasonably well, after a fair bit of work. However, that's not optically the same as a large single primary mirror. If you want to make a true segmented mirror, all of your mirror segments need to off-axis paraboloids; which are an order of magnitude harder to make than normal on-axis paraboloids (where the optical axis of the mirror is in the physical centre of the mirror). You also want hexagons as you say, to pack them efficiently; again, that's harder than a circular mirror. You'd almost certainly have to get the mirrors made by a professional optical lab.

The control systems you'd need to implement to keep all the mirrors in alignment are significant. Especially if you want to work truely like a large single mirror, you'd need to "co-phase" all the segments, and that means aligning them (in 3 axes) to about 30 nanometers -- not for the feint-hearted! Several large segmented mirror telescopes have had severe problems with this part of the process.

As far as I can think, there are only 5 operational telescopes with segmented primary mirrors (Keck I and II, GranTeCan, HET and SALT). All of them are 10 meter telescopes. The majority of the big telescopes (>10) in the world have single mirrors up to 8.4m in diameter. Above this size, the logistics of making and transporting a single bit of glass become prohibitive, and segmented mirrors are the only option.

MMT was originally made with six 1.8m (?) circular mirrors acting as an array -- but it never worked that well, and they ripped it out and replaced it with a single 6.5m mirror about 10 years ago.

I think that what that all shows is that it really isn't worth it, unless you REALLY can't make a monolithic mirror... and that limit is somewhere around 8 meters.

I hope I don't sound too pessimistic! It would be a fun project, but probably a lot harder than you might have hoped... ;) If you want a big amateur telescope; put your efforts into making/buying a large single primary (you can get them up to 1m 'relatively' easily). If you're into fun complex engineering/control projects, it could be brilliant :)

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Wow, thanks for the detailed response TeaDwarf. Yes, it was mainly a theoretical question (not got the time nor money to attempt anything like this) but if I ever did, it would be entirely for a fun, interesting (and quite mad) challenge! :-)

It certainly seems that if this were to be undertaken as a DIY project, a good start would be building a computer-controlled mirror grinder! (an interesting project in itself).

Do you know of any published reading material about the optics of telescopes with segmented primary mirrors?

Edited by ixalon
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Do you know of any published reading material about the optics of telescopes with segmented primary mirrors?

Not any cheap ones... there are several good professional books on the subject; but they have professional book prices (i.e. ~100 quid). Do a search for "telescope optics" on books.google.co.uk -- there are previews of some good books on there. You might get lucky and get the relevant pages...

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