Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

Mirror Arrays (Theoretical DIY!)


ixalon
 Share

Recommended Posts

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?

Link to comment
Share on other sites

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
Link to comment
Share on other sites

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 :)

Link to comment
Share on other sites

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
Link to comment
Share on other sites

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...

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.