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Why glass ?


ZOG

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We all know that a telescope mirror is made from a glass blank with a reflective (aluminium?) coating.

I read that in the latest reactive optics systems, the mirror is wafer thin, so that got me thinking, why glass ? Ok it's the traditional way of making a mirror, and the latest optical machines in China are banging mirrors out cheaper than ever, but glass does have it's problems, the main two being the weight and how fragile the stuff is.That got me thinking, in this day and age, there must be a material that mirrors could be made from that is lighter, stronger, and cheaper than glass. Precision grinding techniques to tight tolerances, using a variety of materials are standard in engineering, so why not in mirror production.

just a thought, but one that I couldn't think of a reasonable answer to.

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Must have something to do with the properties of glass and the way in which light is reflected. Agree though that in today's times, when we can put men on the moon etc, you would think a more robust material other than glass would be available. The biggest problem would be cleaning the surface as it would have to be very resistent to scratching.

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I saw an article a few months back that suggested the next generation of mirrors for scopes may be made from plastic.

Theres a cheap way to make shaped plastic for mirror by having a plate of molten plastic and spinning it at a precise speed. As the speed builds up the outer edges rise up and create a curve on the surface - the same shape as a mirror. Keep the spin up till the plastic sets, add some final polishing ( much faster cos its plastic and not glass ) add an aluminum coating - voila - one mirror.

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Sorry to dissappoint you!

The plastic mirrors are NBG! Not thermally stable and you can't get the surface accuracy ( 1/8 wave) required!

I've tried solid aluminium, Mylar ( with vacuum) slumped glass ( only 6mm thick) but NEVER achieved the results you get from a good strain free piece of glass (or pyrex..)

Glass is a super cooled liquid with a very, very fine crystal structure which can take an excellent polish and is very stable....

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Would have thought that they would have a plastic that can be machined to shape then coated for a mirror. I used a cutter for IR mirrors and lens made from Germanium many years ago, but the cutting unit did cost £1m then.

Tried aluminium on it, not a good idea:evil6: and the surface had lots of small groves, like an LP:eek: so it acted like a diffraction grating and the everything was a spectrum. So although you could make the basic mirror from aluminium it would still need finishing.

As I said I would have though that a mirror could be cut from a suitable plastic. Assumes that there is a suitable plastic. So that would imply that the remainder of the process is impractical.

As glass is still used to the exclusion of everthing else I would say that a plastic mirror simply would not last for a usable length of time so is not used. Perhaps the silvering will not take or last.

They cast aspheric lens wonder if they will eventually cast a mirror from an exopy then silver it? :)

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I'm with Merlin on this one...

Glass has the right "compromise" of properties to make it the preffered solution at the moment. It is possible to electroplate palstics by first using a series of intermediate compounds alright for a security mirror or a vandal proof one in the "powder room" but not much cop for a ultra precise scope mirror

Billy...

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I was astonished when I watched a prog. the other night with that little guy Who is on Top Gear with Clarkson.

He was in an engineering programme, part of which covered large telescopes. The astonishing bit, was the laser polishing of mirror segments for the Keck scopes. How technology has moved on.

I thought Cerium Oxide and water were here to stay.

Apparently the new process gets down to the molecular level. Well, it would need to wouldn't it.

Pardon me for the slight deviation Zog.:)

Ron.

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In the 1970's a University somewhere (Scotland ?) produced a prototype of a flexible mirror that was created from thin film something like Mylar that was kept in a parabolic figure by pumping the air out from behind it. In principle it worked but how thermally stable it was I don't really know. I've not heard more of it so I guess it's drawbacks outweighed it's advantages, lightness being one of them (an advantage) of course.

John

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The Mylar mirrors were made in Strathclyde Uni, Glasgow by my tutor, Peter Waddell; I have copies of the design papers etc in my collection.

They were successfully used as "radiation collectors", unfortunately the surface accuracy wasn't as good as planned ( the Mylar was uneven thickness and didn't maintain a parabolic figure under vacuum) also wrinkles at the edge retainer meant that the usable surface was only about 80% of the aperture.

Maurice Gavin made one in the 80's ( I think) and published in the BAA.

I also tried vacuum forming thin ( about 3mm) glass sheet; didn't get better than about f12 before it broke, but as a long focus mirror would have done well.

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The Mylar mirrors were made in Strathclyde Uni, Glasgow by my tutor, Peter Waddell; I have copies of the design papers etc in my collection.

They were successfully used as "radiation collectors", unfortunately the surface accuracy wasn't as good as planned ( the Mylar was uneven thickness and didn't maintain a parabolic figure under vacuum) also wrinkles at the edge retainer meant that the usable surface was only about 80% of the aperture.

Maurice Gavin made one in the 80's ( I think) and published in the BAA.

I also tried vacuum forming thin ( about 3mm) glass sheet; didn't get better than about f12 before it broke, but as a long focus mirror would have done well.

Thanks for that - I'm glad my memory was not playing tricks on me :)

Just got the decade wrong !.

John

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Tangentially, one or more large 19th century mirrors, 40" or so, were made from organ pipe metal, cast, ground and polished. They worked reasonably well. I've heard talk about making a mirror by spinning a dish of mercury at the right speed, and using a system of flat mirrors to point it at various objects. Collimating that would be hell!

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All reflecting telescopes had all metal mirror's prior to 1840.

It was Speculum a mixture of copper and tin; the Melbourne 48" was the last large metal mirror telescope.

The ability to deposit silver on glass changed things for ever.

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I liked a comment about polishing the palomar mirror. At one point in the process they rubbed inperfections off by hand but the heat of the finger meant they had to leave the mirror to cool overnight before checking if the curve was now right!

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Lulu, I saw a programme about a guy who made a magnifying glass out of ice, but the weird thing was when he tried to use it to light a fire by magnifying the suns rays. You can guess what happened, thing melted before he could get any flames going..

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Hi from N Wales

New to the forum so hi to everybody.

I work in the high end optics industry and a material that is being seriously looked at for mirrors is silicon carbide, same stuff as is in masonry drills. The Herschel Space Tele has a mirror and primary load bearing structure made from it, I saw it at Boostec in France, mega impressive. The mirrors are hard, scratch resistant, extremely lIght, and astro expensive. It has fabrication problems at much over 1M at the moment which is a bit limiting.

Plastic is not good as it absorbs moisture and expands/contracts so changes shape which is a touch inconvenIANT.

MRF polishing is another superfinishing surface correction process used today as well as ion beam figuring.

Regards

XLRACER

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  • 1 year later...

if you love unusual optics ,

then try to borrow or purchase the book 'unusual telescopes'

by peter manly. it contains loads of weird and wonderfull optics

including the mylar sheet stretched over a vacuum, and a mirror

made by rotating a pool of mercury.

lance

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