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When did aluminum replace silver on mirrors?

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#1 dddhgg

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Posted 28 June 2010 - 01:04 PM

Of course not all new technologies are adopted immediately by everyone, but I'd like to know around which time aluminum replaced silver as a coating material on reflecting telescope mirrors. I've read somewhere that silver-coated glass mirrors began replacing speculum metal ones around the 1860s to 1880s. But unfortunately it didn't say when aluminum replaced silver.

Many thanks!

#2 clamchip

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Posted 28 June 2010 - 01:27 PM

The early 1930's, vacuum deposited aluminum. I think Meade and Celestron both tried silvered secondaries in the 80's and it didn't go over to well.
I have many old amateur telescope making books up into the 1940's with recipes for silvering mirrors, apparently this process can be done at home, it's a chemically applied coating.

Robert

#3 dddhgg

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Posted 28 June 2010 - 01:34 PM

Thanks for your reply! Do you also happen to know why silver is less desirable than aluminum? The book said that silver tarnishes when exposed to air, just like speculum metal does, only slower. But would that also be much of a problem in closed tube scopes?

Also, I'm curious now as to what inspired Meade and Celestron to try what they did.

#4 highfnum

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Posted 28 June 2010 - 01:48 PM

silver tarnishes because of sulfur compounds in trace amounts in atmosphere they intern mix with moisture
that attacks silver and turns its a dark brown or black.

when silver s fresh it has very good reflectivity
if you live in a desert area it may last longer

if you have a closed scope and fill it with dry nitrogen
your idea may work -- remember all holes including where
you put eyepiece must be sealed --

majestic coatings will do a silver coating - it a bit more money

#5 gnowellsct

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Posted 28 June 2010 - 01:52 PM

There are coatings for silvered mirrors these days which apparently make the mirror last indefinitely.

But I don't know much about them. Had a conversation with a professional mirror place once, maybe an outfit in Newport CA, if memory serves, but nothing ever came of it.

Greg N

#6 clamchip

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Posted 28 June 2010 - 01:55 PM

Years ago I looked at a C8 with a badly tarnished secondary, even in a closed tube. I doubt you will see many of these or the silvered Meade's.

Robert

#7 highfnum

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Posted 28 June 2010 - 02:01 PM

they are considered better for infrared
but then again gold does this better and last a very long time

call majestic -- in NJ they may have a hard coat that perserves it for longer period

#8 dddhgg

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Posted 28 June 2010 - 02:06 PM

If it's easier to coat a mirror with silver (from a DIY point of view), you could also consider re-coating it yourself every few months or so. Or would that be objectionable in some way?

#9 teast

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Posted 28 June 2010 - 05:03 PM

I vaguely remember reading somewhere about the chemical process for silvering a mirror. I think it was in Amateur Telescope Making - Vol. 1 . IIRC there was a warning about the possibility of unstable or explosive conditions arising if the proper precautions were not taken. I know that's a pretty vague and generic warning but that's what stuck in my mind.

-Tom

#10 rmollise

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Posted 28 June 2010 - 06:54 PM

Thanks for your reply! Do you also happen to know why silver is less desirable than aluminum? The book said that silver tarnishes when exposed to air, just like speculum metal does, only slower. But would that also be much of a problem in closed tube scopes?

Also, I'm curious now as to what inspired Meade and Celestron to try what they did.


It's a problem even in closed tube scopes, but not for a while. Yes, the mirrors are overcoated, but nothing is perfect. Under optimum conditions (no salt are, minimum air pollutants, etc.) you may get ten years or MORE out of a silvered, overcoated mirror.

#11 rmollise

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Posted 28 June 2010 - 06:56 PM

If it's easier to coat a mirror with silver (from a DIY point of view), you could also consider re-coating it yourself every few months or so. Or would that be objectionable in some way?


Every few months? That would be objectionable to me... :lol:

With the availability of modern coatings, there's no reason to think "silver" for most applications.

#12 Greg Keys

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Posted 28 June 2010 - 07:01 PM

The caution is that allowing your silvering bath to rise in temp above 64 degrees (18C) will cause the formation of silver fulminate - a very explosive material. Looking over the formula for the silvering solution, I would elect to send my mirror to a pro. Risky stuff.

#13 clamchip

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Posted 28 June 2010 - 08:56 PM

1984 Celestron used Starbright protected silver mirror coatings, quickly switched to aluminum after a high number of tarnished mirrors.
They may have tried it because from what I can find silver has a higher reflectivity than alum., at least back then.

Robert

#14 Greg Keys

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Posted 28 June 2010 - 09:09 PM

According to the Amateur Telescope Making Book One, tarnishing on a silvered mirror has little effect on the reflectivity of the mirror. It reads a lot like current discussions on cleaning mirrors (Don't do it) I wonder how much the descision by Celestron and such was based on users assumptions that the tarnishing was bad when perhaps it made no real difference? I wonder how much is too much? It seems that there should be some research out there somewhere on reflectivity of mirrors with different levels of tarnishing. I am curious mainly because of the idea of what to do if I wind up with an older silvered mirror... Get it redone with modern coatings, resilver it, or leave it alone..... Things that make you go hmmmmmmmm.....

#15 Datapanic

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Posted 28 June 2010 - 09:42 PM

...It seems that there should be some research out there somewhere on reflectivity of mirrors with different levels of tarnishing...


I bet this info may be found when the S&T DVD set comes out. (Can't Wait!)

#16 DAVIDG

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Posted 28 June 2010 - 10:31 PM

Aluminizing mirrors was developed by John Strong, in the early 1930's. One can read about it in his book "Procedures in Experimental Physics"
Silvering can be done chemically and also by vacuum, methods like aluminium and other metals. Chemical silvering leaves a rougher coating then vacuum methods. Also chemical silvering somethings developes a white tranish as soon as it is deposited. This is removed by very lightly polishing the surface. The result is many very fine sleeks that scatter light. I believe when Strong aluminized the 100" Hooker telescope for the first which up to that point had been silvered every 6 month, to a year, the result was a gain of a magnitude because the reduced scattered light.
There are a number of modern methods to vacuum depositing other layers over silver to protect so the coating last as long as standard aluminium but with an increase in reflectivity of 97% for overcoated silver vs about 90% for aluminium.
I have both chemically and vacuum deposited silver in my lab many times. As an experiment I silvered the field mirror on my first Schupmann refractor. The mirror was mounted so it had a door to cover the surface and I lined inside of the door with a 3M "anti tranish" strips. The strips are used to stop silver and other metals from tranishing and sold to put in jewelry boxes and silverware boxes. The silver coating on my mirror stay bright for about 2 1/2 years until I forget to close the cover over the mirror during a few weeks in the summer.

- Dave

#17 Bill Griffith

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Posted 29 June 2010 - 12:58 AM

I have a 1986 2080 LX3 s/n 810720. I have owned this scope since 1989 it is from the silvered optical group. The primary is in great shape. The secondary I recoated approx.two years ago due to minor dark pinholes, these were a result in exactly what highfnum described. The Ag tarnishes due to pinholes in the dielectric coatings.

Silvered optics should have typical reflectance of 97+% from 700nm to 400nm. A good enhanced aluminum ( dielectric aluminum)typically runs in the low 90% reflectance with the exception out in the UV range (outside visible)Al moves to 98+% reflectance.
Bill

#18 rmollise

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Posted 29 June 2010 - 06:29 AM

According to the Amateur Telescope Making Book One, tarnishing on a silvered mirror has little effect on the reflectivity of the mirror. It reads a lot like current discussions on cleaning mirrors (Don't do it) I wonder how much the descision by Celestron and such was based on users assumptions that the tarnishing was bad when perhaps it made no real difference? I wonder how much is too much? It seems that there should be some research out there somewhere on reflectivity of mirrors with different levels of tarnishing. I am curious mainly because of the idea of what to do if I wind up with an older silvered mirror... Get it redone with modern coatings, resilver it, or leave it alone..... Things that make you go hmmmmmmmm.....


Tarnishing is not the only problem. The Meade LX3s with MCSOG secondaries I've seen are far worse off that that...with the coating actually being pitted/missing in spots.

#19 Dick Parker

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Posted 29 June 2010 - 12:51 PM

dddhg -

I am surprised no one has mentioned the cost. As a DIY project, chemical silvering requires that you obtain the chemicals, which today, is not as easy as it was when I was a kid. I have chemically silvered a couple mirrors. The silver nitrate is not cheap. And if you think you are going to do it right first try, think again. Many times the coating is blotchy or impregnated with a bloom that will not polish out. The last time I silvered (within 10 years) I had the help of a professional glass artist who silvered Christmas tree ornaments as part of his work. Even with his experience, we had to do it 3 times and came out with an even, but not very reflective coating. On one occasion, though, I did end up with a very good coating and wow, was it nice.

Dick Parker

#20 dddhgg

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Posted 29 June 2010 - 05:39 PM

dddhg -

I am surprised no one has mentioned the cost. As a DIY project, chemical silvering requires that you obtain the chemicals, which today, is not as easy as it was when I was a kid. I have chemically silvered a couple mirrors. The silver nitrate is not cheap. And if you think you are going to do it right first try, think again. Many times the coating is blotchy or impregnated with a bloom that will not polish out. The last time I silvered (within 10 years) I had the help of a professional glass artist who silvered Christmas tree ornaments as part of his work. Even with his experience, we had to do it 3 times and came out with an even, but not very reflective coating. On one occasion, though, I did end up with a very good coating and wow, was it nice.

Dick Parker


Very good point; thanks! Somewhere in the back of my head I'd already guessed that the stuff must be expensive, and that one might not succeed the first time. Still, if one insists on doing it oneself it might be an option.

By the way, does anyone know how difficult/expensive it is to build or purchase the proper gear (vacuum chamber, etc.) to aluminize mirrors and whether it's a difficult process once you have the necessary materials? Also consider the possibility of earning back some of the money invested by offering aluminizing as a service to other amateur astronomers.


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