What ever happened to alloy mirrors?

[Lumen Miner (Mitchell)]

Been doing a bit of research of late, just for interest sake.

I would like to get your opinion on whether alloy mirrors, became an outdated technology, by circumstance rather than necessity. eg, could further analysis provide adequate affirmation to its usage or would it conclude its un-doing?


From reading many, far too many, analysis reports on the comparisons between substrates such as stainless steel, aluminium, etc it has become apparent that not only could these materials be used by the ATM's out there, for sub glass/pyrex prices, they could well out preform them in many catagories.



So I ask you why did we stop using alloys?

Was it a death too soon or a death well deserved?

[mental4astro (Alexander)]

Hi Lumen Miner,

Just a few things on alloy mirrors no longer being used:

*They are not as dimensionally stable as low expansion glass

*Much harder to shape, by ATM's, especially stainless steel

*Would attract dew faster

*Their density is greater than glass (except maybe al), therefore heavier for the same diameter

*When needing to repolish the reflective surface it means regrinding the alloy face, not just a chemical strip & reapplication of al or ag & not touching the shape of the polished glass.

[Lumen Miner (Mitchell)]

Originally Posted by mental4astro

Hi Lumen Miner,

Just a few things on alloy mirrors no longer being used:

*They are not as dimensionally stable as low expansion glass

*Much harder to shape, by ATM's, especially stainless steel

*Would attract dew faster

*Their density is greater than glass (except maybe al), therefore heavier for the same diameter

*When needing to repolish the reflective surface it means regrinding the alloy face, not just a chemical strip & reapplication of al or ag & not touching the shape of the polished glass.

You have some interesting points there. I do not wish to debate this topic, rather gather input. Could I ask your point of view though on a few points?

When you say not as "dimensionally stable", does that refer to expansion and contraction under extreme conditions, if so are these conditions within our realms or, high and low heat conditions in tests. The data I am reading suggests that under normal use from -10 degrees to 40 degrees celcius, the alloy dimensions would remain similar yet glass would have a higher deformation rate. Perhaps that is normal glass though, not low exp.

It may be just my experience, but a plate of alloy would appear easier to machine / shape as required. As apposed to only a few grinding techniques to achieve the porabolar, alloy's could be machined several ways. CNC, depth controlled trench cutting with an angle grinder, dremel routing with diamond bits, depth controlled drilling, etc. This list could go on and on.
But like I said, that may be just my experience.

Could you explain, the attracting dew faster? That interests me greatly. That would definatley be a neg for alloy's.


Although density is higher, would that not also allow us to manipulate it to suit our parameters? As in could we not cast, parabolas, re-inforced with steel frames for integrity? This would allow us to reduce the overall weight yet retaining a solid none flexing mirror.

I am guessing the alloy would tarnish not rust. Therefore not needing a "regrinding" more a polish. A polishing compound is all that need be applied... I know of a few alloy mirrors still in use in large scopes, in their cleaning proceedure, they polish it, not change / regrind the surface.
I wonder if a clear coat could be applied to inhibit tarnish... I guess the clear coat would distort the image...




Thanks for the input, you have given me much to think about...

[mental4astro (Alexander)]

Hi Lumen Miner,

Mate, do you have a background in metal working? You would be a good fellow to have on side, .

You make some really good points. Not dealing with alloy mirrors, your question is an interesting one for me.

On dimensional stability, the issue I think would be the conductivity of heat of the alloy creating uneven distortion to the figure. This is the main reason in using low expasion glass where in heating the mirror to remove dew, the figure tends not to distort. Remember we are talking the wavelength of light. These distortions are enough to create distructive interference, hence cancelling light out, something like the multicoloured patter you observe in a thing oil slick. Heating an alloy I think would be dificult to control the uniform expanision to avoid this problem.

Have you ever noticed how much colder a metal surface is in the cold compared to a non-conductive one? They therefore attract dew first- look at where dew settles first of an evening. Alloy mirrors would do the same.

Here the method of 'grinding' an alloy to produce a parabolic surface is my weakest area. I suppose that with the appropriate techniqes is no more or less difficult than figuring glass.

Tarnish, being the oxidation of a metal or alloy, 'rust' in steel, is removed chemically from a glass mirror, & reapplied chemically, all without disturbing the polished glass. An alloy, however, the chemical or mechanical removal of the oxide does affect the shade of the mirror. Polishing is grinding, only at a minute level, using very, very fine grade abrasive, hard or soft. Repolishing a mirror surface is not a task I would be doing too often, it would be inviting trouble. Applying a protective coating is possible, it's done to metal coated glass, but it only slows the process. Applied too thick, which would offer more protection, causes its own problem.

Lumen, I'll be up at Hargraves Lookout tonight with Hickny, join us if you can. We'll be there from about 5pm.

Cheers.

[Lumen Miner (Mitchell)]

Originally Posted by mental4astro

Hi Lumen Miner,

Mate, do you have a background in metal working? You would be a good fellow to have on side, .

You make some really good points. Not dealing with alloy mirrors, your question is an interesting one for me.

On dimensional stability, the issue I think would be the conductivity of heat of the alloy creating uneven distortion to the figure. This is the main reason in using low expasion glass where in heating the mirror to remove dew, the figure tends not to distort. Remember we are talking the wavelength of light. These distortions are enough to create distructive interference, hence cancelling light out, something like the multicoloured patter you observe in a thing oil slick. Heating an alloy I think would be dificult to control the uniform expanision to avoid this problem.

Have you ever noticed how much colder a metal surface is in the cold compared to a non-conductive one? They therefore attract dew first- look at where dew settles first of an evening. Alloy mirrors would do the same.

Here the method of 'grinding' an alloy to produce a parabolic surface is my weakest area. I suppose that with the appropriate techniqes is no more or less difficult than figuring glass.

Tarnish, being the oxidation of a metal or alloy, 'rust' in steel, is removed chemically from a glass mirror, & reapplied chemically, all without disturbing the polished glass. An alloy, however, the chemical or mechanical removal of the oxide does affect the shade of the mirror. Polishing is grinding, only at a minute level, using very, very fine grade abrasive, hard or soft. Repolishing a mirror surface is not a task I would be doing too often, it would be inviting trouble. Applying a protective coating is possible, it's done to metal coated glass, but it only slows the process. Applied too thick, which would offer more protection, causes its own problem.

Lumen, I'll be up at Hargraves Lookout tonight with Hickny, join us if you can. We'll be there from about 5pm.

Cheers.

Thanks for the info and the invite!! I don't finish work till 6pm (Damn late hours at Bunnings), don't think I'll be able to make it. But I will try. If you hear of anyone needing info on a bunnings product for DIY , send them my way. I'm like an 8th degree black belt in Bunnings products.... Plus i'm a x-BBC employee transferred to Bunnings. So I don't tell ppl I'll be back in a sec and never return.


Regardless of the overwhelming facts you have supplied, I will continue to experiment. I have 35cm porabolar in steel, I will polish it as well as I can (taken stainless to mirror quality before). I will see what type of results it gives...

You are right about the tarnishing... I suppose, if you could seal the front face of the steel mirror, with a 6mm pane of glass. Tap a valve in and apply a vacuum to the cavity/sandwich. Thus removing the oxygen and avoiding tarnishing all together..

[bojan]

This topic was discussed before..
have a look:
http://www.iceinspace.com.au/forum/s...light=speculum

http://www.iceinspace.com.au/forum/s...light=speculum

http://www.iceinspace.com.au/forum/s...light=speculum

All mirror manufacturers in the world can not be THAT wrong..
They would use recycled plastic for them, if only that were possible..

[Lumen Miner (Mitchell)]

Originally Posted by bojan

This topic was discussed before..
have a look:
http://www.iceinspace.com.au/forum/s...light=speculum

http://www.iceinspace.com.au/forum/s...light=speculum

http://www.iceinspace.com.au/forum/s...light=speculum

All mirror manufacturers in the world can not be THAT wrong..
They would use recycled plastic for them, if only that were possible..

I see the third thread contains a question about alloy mirrors. The other two are liquid and resin.

"All mirror manufacturers in the world can not be THAT wrong.."
I don't think, that is what I was saying. I don't think they are wrong, i'm just interested as to why it died out.

The answer of "thermal dynamics" does not seem to fly, as history suggests that speculum mirrors did not suffer, as dramatically or at all like some would make out.

I would put it to some people, that speculum was used in some wacked out parabola designs, that were way ahead of their time and destined to fail, giving it a bad rap... Quote "It seems like these bold and far-sighted experiments subsequantly backfired because of significant warping of the primaries,above all, to be expected in the astigmatic mode." This quote is in regards to a vase shaped alloy mirror.

Then aluminisation came along and speculum was forgotten, along with it the oppotunity to develope some parralel technology to pyrex.

Then again, maybe it was that bad, yet I have seen no written document to that affair.

I would like to see a document illustrating the negatives involved by someone whom has used/maintained one first hand.

[DavidU (Dave)]

Great thread Mitchell.
To machine or grind & polish a parabolic mirror for a telescope requires an accuracy of several millionths of an inch across the entire surface let alone the polish finish itself....
Any thing less will result in below defraction limited averages (useless mirror)
These mirrors are so accurate that a warm thumb on a metal mirror would screw up the figure.
Even if you could get this accuracy in a metal mirror covered in glass to seal out Oxygen the clear glass would have to be polished both sides to an even greater accuracy than the mirror it self. The glass would also have refractive index issues and also have both sides coated in an anti reflective mineral fluoride to stop back reflections. Put this glass under a vaccume and it will distort thus destroying the refraction to the mirror.
Glasses work well.

[bobson (Bob)]

I read a while ago about this material to be good for future cheap and light mirror making:

http://www.pmcguild.com/pmc.html

"Precious Metal Clay, PMC, is an exciting material developed and patented in the 1990s by Mitsubishi Materials of Japan. Microscopic particles of silver are mixed with a moist binder to create a material that has the feel and working properties of modeling clay. Using simple tools, objects are easily given shape, texture, and character. After air-drying, the objects are heated to temperatures approaching the melting point of the metal, where the particles fuse together to make a dense, fully metallic object. Fired PMC work can be polished, soldered, enameled, and enjoyed like any other silver item. PMC is available in three different versions of silver and in a 22k gold alloy."

bob

[bojan]

Originally Posted by bobson

I read a while ago about this material to be good for future cheap and light mirror making:

http://www.pmcguild.com/pmc.html

"Precious Metal Clay, PMC, is an exciting material developed and patented in the 1990s by Mitsubishi Materials of Japan. Microscopic particles of silver are mixed with a moist binder to create a material that has the feel and working properties of modeling clay. Using simple tools, objects are easily given shape, texture, and character. After air-drying, the objects are heated to temperatures approaching the melting point of the metal, where the particles fuse together to make a dense, fully metallic object. Fired PMC work can be polished, soldered, enameled, and enjoyed like any other silver item. PMC is available in three different versions of silver and in a 22k gold alloy."

bob

This is obviously OK for modelling of jewellery etc.
It is also questionable, how this compound (it is not new idea, though...) would behave when the size is larger (like telescope mirror). I think all the benefits of pre-shaping would be lost, as the firing time must me very long (to allow the heat to penetrate through the material and fuse the whole shape properly.. mind you. you do not want ANY inhomogenities in your future telescope mirror which will inevitably distort the shape of optical surface during thermal transients)
But you can not avoid figuring and polishing phase to obtain optical surfaces.. You can as well go straight to moulding metal if you like, then polish and figure (if this is possible.. the thing is, speculum was used because it is very brittle and thus can be figured and polished, while retaining the shape after the process. The other metals and alloys are not behaving like this, unfortunately).

The simple and inevitable fact is, glass (and fused silica and ceramic materials) are the name of the game in optics technologies these days, and this is not very likely to change significantly in the future.

[Lumen Miner (Mitchell)]

Originally Posted by bojan

The simple and inevitable fact is, glass (and fused silica and ceramic materials) are the name of the game in optics technologies these days, and this is not very likely to change significantly in the future.

That is 150% correct, glass is not only the name of the game, but will continue to be until someone has the inventivness to come up with an alternative. This won't happen soon, perhaps not at all.

On a purely theoretical note, as I should have stated in the beginning about this entire thread, what would you believe to be a future possiblity in the advancement of optics? Do you believe glass/pyrex developement to be the answer, or would you think that we may move to a new medium? I'm talking in the future here and purely theoretically.

[Lumen Miner (Mitchell)]

Originally Posted by bobson

I read a while ago about this material to be good for future cheap and light mirror making:

http://www.pmcguild.com/pmc.html

"Precious Metal Clay, PMC, is an exciting material developed and patented in the 1990s by Mitsubishi Materials of Japan. Microscopic particles of silver are mixed with a moist binder to create a material that has the feel and working properties of modeling clay. Using simple tools, objects are easily given shape, texture, and character. After air-drying, the objects are heated to temperatures approaching the melting point of the metal, where the particles fuse together to make a dense, fully metallic object. Fired PMC work can be polished, soldered, enameled, and enjoyed like any other silver item. PMC is available in three different versions of silver and in a 22k gold alloy."

bob

If it was 99.9% silver and 0.01% binder it may produce a resonable mirror. Yet the binding agent will always give you a fuzzy image.... Sounds like awesome stuff though... Heat and it's metallic, WoW!! thanks for the link.... Interesting reading.

[bojan]

Originally Posted by Lumen Miner

That is 150% correct, glass is not only the name of the game, but will continue to be until someone has the inventivness to come up with an alternative. This won't happen soon, perhaps not at all.

On a purely theoretical note, as I should have stated in the beginning about this entire thread, what would you believe to be a future possiblity in the advancement of optics? Do you believe glass/pyrex developement to be the answer, or would you think that we may move to a new medium? I'm talking in the future here and purely theoretically.

Glasses (borosilicate, pyrex etc) are good enough.
Perhaps, in the future some ceramics will be used, but only for very specific applications (UV, for example when even higher precision is needed due to shorter wavelength).
The technology moves into adaptive optics.. where galls again is adequate material.
I do not believe glass will be abandoned any time soon.

[bobson (Bob)]

Bojan

You're right about glass, but what about the glass that weights 10 times less:

http://www.corning.com/specialtymate...ror_blank.aspx

"Corning has a rich history in space exploration, producing mirrors for the Hubble, Gemini, and Suburu telescopes; along with providing window glass for all of NASA's manned spacecraft missions and the Internationl Space Station. These mirrors are made of Corning's ultra low expansion ULE® glass, a material that exhibits virtually no dimensional changes over extreme temperature variations and is generally considered the best in the world for astronomical optics.

The "lightweighting" techniques used in the Hubble mirror have been extended to other space program and ground-based astronomy applications. Reducing the weight of these large mirrors - sometimes down to one eighth the weight of an equal sized solid mirror - allows changes to the superstructure for ground-based telescopes, helping to reduce costs and extending functionality. Corning has continued to develop and expand the use of this technology.

Current and future space-based telescopes may contain Corning mirrors that are less than 10 percent of the weight of an equal-sized solid mirror. The technology has been adapted for use in the GOES Weather Satellites, which are key U.S. resources for weather, hydrologic and climate forecasts. "

cheers

bob

[Lumen Miner (Mitchell)]

Originally Posted by bojan

Glasses (borosilicate, pyrex etc) are good enough.
Perhaps, in the future some ceramics will be used, but only for very specific applications (UV, for example when even higher precision is needed due to shorter wavelength).
The technology moves into adaptive optics.. where galls again is adequate material.
I do not believe glass will be abandoned any time soon.

I concur. I would like to see some mullti-mirror adaptive optics, be incorporated into a Atm project. That would interest me.

[Wavytone]

Originally Posted by bobson

I read a while ago about this material to be good for future cheap and light mirror making:

http://www.pmcguild.com/pmc.html

Seen some parts made this way at work for an industrial application... the finished product is far too porous. It resembles sintered metal parts... it will take fine-grinding, then when polished the surface resembles polished basalt - it has what I would call a very high sheen that is visibly fine-grained - not exactly a high polish.

...basically you can forget making high quality mirrors from it.

A few years ago mirrors were being made of a type lightweight, black glass that was sintered from a fine powder, it was soft and ground and polished easily and polished well ... Satchmo will know what this was.

[bobson (Bob)]

Wavytone,

I can't find the link now where I read about it. But it stated mirrors could be built with this material in the future.

Just like this link where they say:

"Researchers at the Department of Energy's Sandia National Laboratories and the University of Kentucky are developing enabling technologies for a new thin-film, ultralight deployable mirror that may be the future of space telescopes and surveillance satellites."

http://www.sciencedaily.com/releases...0519064524.htm

cheers

bob

[DavidU (Dave)]

Originally Posted by wavytone

seen some parts made this way at work for an industrial application... The finished product is far too porous. It resembles sintered metal parts... It will take fine-grinding, then when polished the surface resembles polished basalt - it has what i would call a very high sheen that is visibly fine-grained - not exactly a high polish.

...basically you can forget making high quality mirrors from it.

A few years ago mirrors were being made of a type lightweight, black glass that was sintered from a fine powder, it was soft and ground and polished easily and polished well ... Satchmo will know what this was.

bvc?

[cristian abarca]

BVC glass http://www.ajstelescopeworkshop.ca/

Cristian

[A P Walker (Andrew)]

Hi there,

Dont know if you have seen this site http://frank.bol.ucla.edu/almirrors.htm
but it describes very well the process of making an aluminium mirror. Dosent sound any harder than making a standard mirror from glass. I would imagine the same rules apply and a mirror of reasonable apature and focal length would be pretty straight forward for the ATM'er wanting to give this ago. It sure would make a "different" project!