Re-silvering mirrors?

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mooge

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Hey fellas,

I've messed up again and I accidentally removed ~40% of the semi-silvering on my M2's beamsplitter. I put some water on the prisms and stuck them together to get an idea of how the gluing would go and I didn't remove the water and it just sort of dissolved the aluminum mirroring. bad news.

this is what it looks like:



but maybe it's not over yet-- I might be able to re-silver the prism.

so here's the question:
Have any of you guys done anything like this (silvering mirrors)? If so, any advice or suggestions?
would methods like this or this work (or why wouldn't they work?)? the first one says that a semi-silvering is possible with a diluted solution or a lesser time... so I guess if I could do that, it'll just be a ton of experimentation to find out how to get 50% transmission (but how would I test for that?). I expect the quality of the finished finder will suck at best but this is sort of the only hope I have now of getting my M2 back into operation for the summer.



cheers.

(I won't even attempt to make my own vapour deposition chamber but apparently it is possible to do it yourself.)
 
My M3 just came back fro DAG, where it was to have an RF overhaul. He said that he couldn't improve the RF much because in some previous repair (before I got the cam) someone had accidentally "wiped off" the silver coating in the process of re-cementing the prism. He said there was nothing more to be done. VERY disappointing!
 
For one, many companies can do vacuum deposit coatings - some amateur astronomer site is almost bound to have a link to some company reasonably near you. You'll have to prepare (lacquer the surfaces that are to remain transparent) it yourself, though.

And you don't need to vapour coat it for silvering - the good old way with silver nitrate works perfectly well for rear-coated mirrors like prisms, and is quite feasible DIY if you were at least average in college grade chemistry.
 
vic- CRR Luton (in the UK) offers a RF overhaul (including resilvering) service. I'd have my M2 fixed by them but I sort of spent all my money and more importantly, they're not accepting more repairs until next year (!).

sevo- I don't know anything about telescopes so I wouldn't know where to start... but maybe the university I go to has a vapour deposition thingy. I might ask around tomorrow (electronics department, right?).

But if they don't, silver nitrate it is. And in the meanwhile, 12mm heliar.
 
The online research I've done about optical fabrication and repair suggests that, as sevo has mentioned, the amateur astronomy crowd are very switched on about some of this stuff. Although much of the talk centres on mirrors they still have some need for prisms too. I actually loaned a book or two on fabricating your own telescopes from my library a few months ago and, although there wan't as much direct relevance to photography optics as I expected, it was still interesting stuff and some of it quite sophisticated. You could do worse than chase some leads in that area, if you're keen on trying to rejuvenate the prism yourself.

Here in Hobart we have a specialist optical fabricator who makes one-off or low volume items. I'm sure he could tackle this sort of task (at a fairly reasonable price, usually) if I needed it, and frankly, if I did, I'd probably farm such a job out to him.
Regards,
Brett
 
Okay...

so I went to the library (the other university's library, haha) and found a book on making telescopes and it discussed methods of silvering. It mentioned that the rochelle salt + silver nitrate method was particularly suited for semi-silvered mirrors because it's slow. Good news I guess. There was also some stuff on vacuum deposition but it even said there that it's not that easy to do...
Oh yeah and it also said that the chemical method gives an rough (relatively speaking) surface. Especially if the glass isn't clean. I wonder what impact this would have for a beamsplitter-- a bit of light diffusion, or heavier consequences? hmm.

So I guess the plan now is maybe call (er, email) some companies who might do this and email the guy who runs the mystery electronics lab at uni. Maybe. But it would be great if I could do this by myself and get an acceptable result, so I'm sort of leaning towards that idea.

Thanks again, guys!
 
Interesting to have a look at this thread, we both had the same idea at the same time.

I believe that this will work fine but you need to be ready to desilver the mirror if you make a mistake like overdepositing the silver, or a splotchy surface, and that will require nitric acid.

The one thing I am not sure of is if the increased brightness of the silver will be a good thing in the viewfinder. The process of focusing the M's is all about having good contrast between the two patches. I'm not sure if the better transmission of a real silver mirror will increase or reduce contrast.

You'll also need to be extremely careful because both nitric acid and silver nitrates are dangerous. Nitric acid fumes can attack the lungs and nitrogen dioxide (a biproduct of many kinds of nitric acid reactions, including with silver) is very lethal. Silver nitrate will also cause long-lasting skin stains and can be toxic in excessive amounts so do be careful in the handling.

As for having someone do it for you, I think that is the easiest, and any telescope resilvering place should be able to do it with aluminum vacuum deposition, but they'll probably also charge you a bundle for the job.
 
So today I put together the silvering solutions sort of according to the make-stuff website (link in the first post) and I threw in some pieces of glass (a glass-canada balsam sandwich, to be precise) to see what happens.

And have a look!



so apparently my solutions work. Good news. Bad news is that it doesn't really stick (or am I supposed to let it dry first?) and it's quite uneven (which is sort of a big deal).

Any suggestions on how to get an even surface? clean it well? clean it with what? pretreat it with something?

Or ideas on how to get the aluminum off the prism? I guess I should just be able to scrub it off like I did before (grr...) but I haven't got there yet.

cheers.
 
If you are going to silver a mirror it had to be really clean and any previous coating had to be completely removed. Yes chemical soldering is rougher than vacuum deposited aluminum but silver is good enough for telescopes magnifying hundreds of times.

Bill
 
I had done silvering for telescope mirrors in my high school days long time ago. I wasn;t too difficult if you know your high school chemistry. Cleanily is of utter importance---i.e., use distilled water, clean the mirror surface with nitric acid , etc. And if you made a mistake, the silver can be taken out with dilute nitric acid. BTW, you are using dilute nitric acid, so it will not be too dangerous if you know your chemistry.

but back to the part of having to coat to a s=cetrtain thinkness, I'd have not answer for you. You just have to experiment, I guess. The other complication is the remaining aluminum on the surface. I had never had that experience as I, and my colleagues back then, only use clean, freshly ground mirror surfaces.

I don't know how long a semi-concealed silver surface will last. as I recall, silvered mirror surfaces would need to be redone every year or so. This of course depends a lot on the degree of air pollution in your home town.

Good luck!

Tin
 
Tin said:
I had done silvering for telescope mirrors in my high school days long time ago. I wasn;t too difficult if you know your high school chemistry. Cleanily is of utter importance---i.e., use distilled water, clean the mirror surface with nitric acid , etc. And if you made a mistake, the silver can be taken out with dilute nitric acid. BTW, you are using dilute nitric acid, so it will not be too dangerous if you know your chemistry.

but back to the part of having to coat to a s=cetrtain thinkness, I'd have not answer for you. You just have to experiment, I guess. The other complication is the remaining aluminum on the surface. I had never had that experience as I, and my colleagues back then, only use clean, freshly ground mirror surfaces.

I don't know how long a semi-concealed silver surface will last. as I recall, silvered mirror surfaces would need to be redone every year or so. This of course depends a lot on the degree of air pollution in your home town.

Good luck!

Tin
Click to expand...

I agree, I am not sure how to strip an aluminum coating. Usually silvering was reserved for mirrors that had never been aluminized.

As for how long it will last? I would say at least a year; perhaps longer since unlike a Telescope mirror, the mirror here will be inside a camera and not exposed as directly to the elements. Also I suspect it can tolerate a larger loss of reflectivity.

--
Bill
 
Nitric acid?? I don't have anything like that. Would stop bath acid work? that's the pretty much all I have...

it better last more than a year! it's part of a beamsplitter so it'll be covered in cement in between two prisms, out of the reach of oxygen...

thanks for the suggestions, Tin and Bill!
 
mooge said:
Nitric acid?? I don't have anything like that. Would stop bath acid work? that's the pretty much all I have...
Click to expand...

Definitely NOT. Stop bath is made out of acetic acid.

Check the following link:

http://lcao.co.za/index.php/MirrorSilvering

(Do read the section on Cleaning the Mirror for a short description of cleaning a aluminum mirror.)

Before I read the above link, I did not realized that there are several method of silvering. What I recall from the old days seemed to a simpler process. Unfortunately, I do not remember the exact formulation. I do remember using glucose as the reducing agent, and of course silver nitrate and either potassium or sodium hydroxide.

What you are doing here is to use a reducing agent (glucose in my case) under a strong alkaline environment (the hydroxide) to reduce silver nitrate back to metallic silver, which would deposit onto the mirror surface. Chemically, it is similar to using the reducing agent in a developer to develop the silver nitrate embedded in the emulsion of a silver-based film. Cleaning a previously silvered mirror with nitric acid is just the reverse process
 
Eh. I know this is not well known to all, but metal coatings on mirrors or whatever are very easy to damage with even the slightest wipe. These days what companies do is give the metal a protective dielectric coating that actually increases the reflectivity in addition to some light scratch protection.

I wouldn't advise doing something like resilvering unless you can guarantee a clean room environment. The slightest dust on the surface might cause the silver to flake off, and more might follow. You actually have to wash off the silver, give the object a good clean in a sonic cleaner and then polish the surface just to be sure it is smooth, clean the surface again, and then deposit the silver.

Another thing though. Aluminium oxidizes with time. You will really want to stick with silver.

In theory though, you might be able to find a large enough dielectric broadband visible wavelength beamsplitter that might do the job. Something like this: http://www.newport.com/Broadband-Dielectric-Beamsplitters/141113/1033/info.aspx#tab_Overview
 
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