130 replies to this topic

[hbanich]

To all those interested, the Oregon Scope Werks spray silvering pages are posted at https://is.gd/silvering

 

There you'll find these articles:

 

A. Everything you need to know to successfully spray silver a mirror

B. The testing behind an anti-tarnishing coating 

C. A remarkably easy and surprisingly accurate way to measure the reflectivity of a coating - silver or aluminum

 

Although there's a lot of information here these pages are very much a work in progress. We're not the first to use the spray silvering technique, and we don't pretend to know everything about it. But we've had some notable successes, are learning more all the time, and have come up with two notable advancements with the anti-tarnish overcoating and the reflectivity test. It's time to share what we've learned so far.

 

Our hope is that others will join in the fun and help advance the state of the art over time. In the meantime, we'll update our pages as we learn more.

 

By the way, the Oregon Scope Werks is just a small group of amateur telescope makers who mostly communicate with each other through an email list. We're not a club, or any type of organized group for that matter, but we do share a desire to make homemade telescopes perform better, and preferably at a lower cost.

 

 

[Bob4BVM]

Thanks for sharing the great work your group has done on this Howard..

Can't wait to try if for myself.

You guys have given me the push I needed to go ahead with this, can't thank you enough !

CS
Bob

[ccaissie]

The analysis of the protective coating is most valuable.  The crux of silvering has always been the durability.  As a young amateur, I silvered my mirrors but in the industrial atmosphere of Worcester, MA, they quickly turned yellowish.

[totvos]

The analysis of the protective coating is most valuable.  The crux of silvering has always been the durability.  As a young amateur, I silvered my mirrors but in the industrial atmosphere of Worcester, MA, they quickly turned yellowish.

Note that the protective coating right now provides chemical protection only. We are still working on mechanical protection of the surface which, currently, is a bit fragile and susceptible to things like bug poo. Even thought it is easy and cheap to recoat annually, it would be nice to not have to. 

[tommm]

Have you done a cost/benefit analysis? I'm thinking cost over 10 year period compared to protected aluminum.  For example, I paid $300 plus $400 shipping to have a 22.4" mirror coated in 2002, 17 years ago. The coating is still in very good condition. Last year I paid $1200 for coating of the same size mirror, plus around $150 shipping (closer). If it lasts 10 years, which seems reasonable given the performance of the first one, that's $135/year.  Factor in the trouble of re-coating each year...

 

But then I may well try silvering a smaller mirror anyway just for the fun of it and the satisfaction of making the entire mirror including coating myself.  So I can see there are other reasons for silvering. And of course coming up with a way to increase its lifetime to 5 - 10 years which also permits cleaning would be a game changer.  I agree it is great work. Always fun to explore new frontiers.

[Steve Dodds]

If you just buy the chemicals and use your own spray bottles  it's around $100 and that will do several mirrors and lasts for years if you put them in the fridge. 

[Bob4BVM]

For me there are several big draws to silvering at home:

 

1-Avoid shipping risk.   When I was still working I got a lot of experience with shipping lots of sensitive, heavy instrumentation gear. I learned that truckers CAN destroy ANY object, no matter how well it is packed and protected. I also learned that trying to win real/replacement value settlements in shipping-insurance claims is a complete joke.  I do not dare to risk shipping my pair of large bino-scope mirrors.

 

2-Higher reflectivity of Si vs Al coats. Of course you have the Si tarnish issue, but that is what makes the OSW research results so interesting to me.

 

3-Cost. Now, having a PAIR of largish mirrors to recoat clearly pushes the $ equation towards the Si coating

 

4-Freedom/flexibility to recoat whenever I want to restore to a reflectivity better than Al.

 

CS

Bob

[mconnelley]

Hello:

  

   Here's a question for the chemists out there.  Why isn't there a process for the chemical deposition of an aluminum layer onto a mirror?  I know that aluminum and silver aren't in the same column on the periodic table, so their chemistry will be a bit different.  What's special about silver that allows it to be chemically deposited onto glass?  If such a process were out there, then that would seem to solve the durability issue.  

Cheers

Mike

[totvos]

Not a chemist but technically the silver is not attached to the glass. Tin is bonded to the glass, and the silver bonded to the tin. 

[tommm]

Chemical deposition of Al alone wouldn't seem to solve the durability issue, since it is the hard protective coating over the Al that gives long life and permits cleaning.  A way to chemically deposit those would also have to be found.

[dave brock]

Bare aluminium forms it's own protective layer. While not as good as the overcoats normally applied,

it still makes aluminium washable and typically lasts 4-5 years.

[Ed Jones]

 

Why isn't there a process for the chemical deposition of an aluminum layer onto a mirror?

Aluminum has a very high reactivity compared to silver and could never form in a water solution.  Aluminum was very expensive before they found a way to produce it in a high temperature electric furnace.

 

I never heard of using calcium carbonate to clean mirrors before, is there any risk to the mirror from impurities.

 

Great  post BTW!

[jpcannavo]

This is fascinating. One thing I wonder about is the smoothness of coating on a micro level. AFAIK aluminum does not add roughness. I have no idea about sprayed silver. Wonder if this has been considered. See discussion around aluminum below.

https://www.cloudyni...the-best/page-2

 

Joe

[hbanich]

Hi Joe,

 

I've been observing with a spray silver coating for over a year now, and given how much less scatter I've seen in the images through my 28-inch I'd say the micro roughness is probably less than aluminum. I haven't tried measuring it because the eyepiece views tells me all I need to know.

 

That said, spray silvering, like anything else, isn't for everyone. It's not really worth the time and expense for a mirror smaller than 12-inches, and if you live near the ocean or near an active volcano (i.e. the Big Island of Hawaii) then given what we currently know, a silver coating won't last nearly as long as aluminum.

 

But if you have a large mirror you don't want to ship, and compare the expense of aluminum vs spray silver over time, you come out ahead with the silver even with the current state of affairs of re-silvering every so often. We hope to push out the need for re-silvering even further, but even if we don't come up with an overcoat that physically protects the silver it still makes economic sense for me to re-silver rather than spend $2000 to put an enhanced aluminum coating on my 28-inch mirror and subject it to the risks, and cost, of shipping.

 

One other thing to share that isn't in the articles, is how bright and colorful the views are with a fresh silver coating. Star colors, the planets and even relatively faint nebulae show their subtle colors much more readily - quite awesome!

[DAVIDG]

Aluminum has a very high reactivity compared to silver and could never form in a water solution.  Aluminum was very expensive before they found a way to produce it in a high temperature electric furnace.

 

I never heard of using calcium carbonate to clean mirrors before, is there any risk to the mirror from impurities.

 

Great  post BTW!

 At this years Stellafane one of the authors was doing the silvering demo and Alan Ward his friend was aluminizing mirrors. My 8"  was one the first to be coated and Alan had me scrub the heck out of the surface with Calcium Carbonate and distilled water.  The mirror had a perfect polish without a sleek so I was bit skeptical about the Calcium Carbonate scrubbing and worried when it was coated it would look like it was polished with steel wool. When it emerged from the coating chamber it was perfect not a defect or sleek to be found !  

 

                - Dave 

[jpcannavo]

Wow, I have a 16” and a planning a 22”. Definitely have me intrigued!

[jpcannavo]

I’m also wondering if there would be a way to store the mirror in some sort of protected, purged storage box.

[Ed Jones]

 

 

I was bit skeptical about the Calcium Carbonate scrubbing and worried when it was coated it would look like it was polished with steel wool.

  Well me too but I just found on the Oregon Scope Werks web site that they used precipitated calcium carbonate and that's not going to have any hard contaminates.  It's a whole lot better than using old school concentrated nitric acid; really nasty stuff.  

  I'm curious what's in the Midas anti-tarnish.  I looked at the MSDS but none of the listed ingredients react with silver.  They did say that some proprietary non-hazardous ingredients may not be listed and I'm betting it's benzotriazole (BZT).  I know about 15 or 20 years ago I posted a pic on CN of a silvered microscope slide half of it dipped in BZT.  I had exposed it to hydrogen sulphide gas and the BZT really did protect it.  Not perfect but necessary.  I hope to see what their other anti-tarnish fix is.

  The other thing with silvered mirrors is to always make a mirror cover to keep dust and other nasties out.  

 

Ed

[Mike Spooner]

I’m also wondering if there would be a way to store the mirror in some sort of protected, purged storage box.

Hi Joe,

 

Should be relatively simple to put an N2 blanket on if the storage case can be made relatively airtight. This is done with many types of large transformers using 2 or 3" of H2O positive pressure supply coming from a cylinder of nitrogen to prevent moisture contamination in cooling oil, etc. If the seal is decently airtight it should last a long time. If silver and nitrogen don't play well together (I suspect they're fine) argon might work at a bit of a price premium. 

I may have to figure out a way to seal the mirror in the cell in the 36" belonging to a friend and me. The silvering was pretty easy but would be nice not to deal with it for an extended time. Might have to check eBay for a magnehelic (sp?) gauge. 

 

Mike Spooner

[KLWalsh]

I have an old homemade telescope mirror in storage I made as a teenager. I’ve been planning to regrind and repolish the mirror to a different f/ratio.
The mirror has a ‘Beral’ coating on it. I think Beral was just a tradename. I don’t think it’s beryllium-aluminum as the name implies. The coating was put on 40+ years ago.
Does anyone know if ferric chloride will remove this coating? Any precautions I’d need to take? Of course I would wear goggles and a mask, and do this outside.

[Bob4BVM]

I have an old homemade telescope mirror in storage I made as a teenager. I’ve been planning to regrind and repolish the mirror to a different f/ratio.
The mirror has a ‘Beral’ coating on it. I think Beral was just a tradename. I don’t think it’s beryllium-aluminum as the name implies. The coating was put on 40+ years ago.
Does anyone know if ferric chloride will remove this coating? Any precautions I’d need to take? Of course I would wear goggles and a mask, and do this outside.

 

I have an old homemade telescope mirror in storage I made as a teenager. I’ve been planning to regrind and repolish the mirror to a different f/ratio.
The mirror has a ‘Beral’ coating on it. I think Beral was just a tradename. I don’t think it’s beryllium-aluminum as the name implies. The coating was put on 40+ years ago.
Does anyone know if ferric chloride will remove this coating? Any precautions I’d need to take? Of course I would wear goggles and a mask, and do this outside.

 

I have used gallons of  FeCl3 over the years for PCB etching. It is somewhat corrosive and acidic, but if you don't drink it , it is pretty mundane stuff. It will remove your coating just fine.

CS
Bob

[Bob4BVM]

Hi Joe,

 

Should be relatively simple to put an N2 blanket on if the storage case can be made relatively airtight. This is done with many types of large transformers using 2 or 3" of H2O positive pressure supply coming from a cylinder of nitrogen to prevent moisture contamination in cooling oil, etc. If the seal is decently airtight it should last a long time. If silver and nitrogen don't play well together (I suspect they're fine) argon might work at a bit of a price premium. 

I may have to figure out a way to seal the mirror in the cell in the 36" belonging to a friend and me. The silvering was pretty easy but would be nice not to deal with it for an extended time. Might have to check eBay for a magnehelic (sp?) gauge. 

 

Mike Spooner

 

Another, simpler, option may be to used canned air (not really air, but inert gas mixture) to flood the mirror container and displace the oxygen. it is heavier than air so if injected at a high point, and you have an exhaust port at a higher point, it will displace the air in the box.  I have been doing this for years to preserve expensive finishes in their cans. They last for a very long time if you remove the air/oxygen in this manner.

CS

Bob

[Bob4BVM]

  Well me too but I just found on the Oregon Scope Werks web site that they used precipitated calcium carbonate and that's not going to have any hard contaminates.  It's a whole lot better than using old school concentrated nitric acid; really nasty stuff.  

  I'm curious what's in the Midas anti-tarnish.  I looked at the MSDS but none of the listed ingredients react with silver.  They did say that some proprietary non-hazardous ingredients may not be listed and I'm betting it's benzotriazole (BZT).  I know about 15 or 20 years ago I posted a pic on CN of a silvered microscope slide half of it dipped in BZT.  I had exposed it to hydrogen sulphide gas and the BZT really did protect it.  Not perfect but necessary.  I hope to see what their other anti-tarnish fix is.

  The other thing with silvered mirrors is to always make a mirror cover to keep dust and other nasties out.  

 

Ed

 

Whatever it is, the Midas stuff looks very promising according to the OSW tests.  It will be my choice when I silver my b-scope mirrors unless something better comes along.

 

I agree with your comment on the mirror covers too. I am always amazed to see uncovered exposed mirrors on some of the current crop of lightweight dobs.  For my b-scope I am copying the system Ed (Phonehome) used on Elvira, which goes a bit beyond a simple cover, it actually pressurizes the mirror box on the scope with HEPA-filtered cooling air, and while observing also provides the mirror with a constant boundary-layer wash of filtered air which effectively keeps dust at bay during use. Knowing now that I will be silvering my b-scope primaries make the 'Elvira system' all the more attractive, it will be a perfect match for the sensitive Si mirrors.

CS

Bob

[totvos]

The perfectionist in me needs to point out that Si is silicon, and Ag is silver. ;-)

[hbanich]

Thanks Tomo, you just beat me to it!

 

Also, the Midas anti-tarnish overcoat means you don't need to create an oxygen-free environment to slow down tarnishing - that's what the Midas is for. The act of observing with a silver coating on your mirror means other environmental factors will get to the coating no matter how you store the mirror before and after observing.

 

Dust, bugs, pollen and who knows what else floating around in the air will land on the silver coating no matter what. Our best guess is that water vapor in the air is soaked up by these particles while they're on the coating and stuff that degrades silver is dissolved onto the coating and creates small (1 to 4 mm diameter) spots over time. If you observed only in extremely low humidity, with no bugs, this might not be a problem, but who doesn't eventually get a dewy night and a few bugs?

 

Anyway, this is what we hope to prevent with a new overcoat, and if successful it could mean a spray silver coating could become nearly as durable as an aluminum coating. Stay tuned!