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Aluminum/silver protective film thickness?

Astrometry ATM DIY Equipment Lens Making Optics Eyepieces
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#26 JamesDuffey

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Posted 04 October 2021 - 02:05 PM

Pacific Cloth™ by Silverguard™ used for storage may put off the onset of tarnishing of silver mirrors.  

 

https://silverguard.com >

 

Has anyone tried this? I seem to recall someone, perhaps Zane?, had used it on his DIY silvered mirrors. 


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#27 Oregon-raybender

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Posted 04 October 2021 - 02:10 PM

Check the Oregon page, I believe they have tried using it with limited success. I found that

the best way to maintain silver is just keep it covered when not in use. But once it's out in the dew, insects, garage smog and such, it starts the clock for failure. I never give it a second thought about just re-silvering as needed, it's a simple process.

 

Starry Nightswaytogo.gif


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#28 Steve Dodds

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Posted 04 October 2021 - 03:31 PM

Resilvering once a year is no big deal.  Mike here in Utah with his 70" spends around $100 for chemicals, takes about 2-3 hours, a smaller mirror will take less.

.


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#29 Celestial825

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Posted 04 October 2021 - 11:53 PM

Check the Oregon page, I believe they have tried using it with limited success.

Did they try the conventional lacquering? I'm going to do it within the next two weeks on my 6.2" mirror. It's not generally a difficult process, but the issue is to manage the thickness of the film to be half the wavelength (200nm) to avoid a destructive interference. And certainly it's possible by considering the solution concentration, angular speed, evaporation time, .... Also viewing the interference (no apparatus needed and not very complicated) will help to find the exact thickness. If such a lacquer really prolong the silver life and is optically acceptable for visual uses, it will be helpful to many amateurs.

 

I will share the results here on CN in detail.

 

And thanks for the helpful information.waytogo.gif


Edited by Celestial825, 05 October 2021 - 01:38 AM.

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#30 Celestial825

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Posted 04 October 2021 - 11:57 PM

Pacific Cloth™ by Silverguard™ used for storage may put off the onset of tarnishing of silver mirrors.  

 

https://silverguard.com >

 

Has anyone tried this? I seem to recall someone, perhaps Zane?, had used it on his DIY silvered mirrors. 

To my knowledge such anti tarnish clothes / strips really work. The silvered mirror must be kept in a tight sealed compartment with the product when not in use.


Edited by Celestial825, 05 October 2021 - 12:51 AM.


#31 Celestial825

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Posted 05 October 2021 - 12:04 AM

Resilvering once a year is no big deal.  Mike here in Utah with his 70" spends around $100 for chemicals, takes about 2-3 hours, a smaller mirror will take less.

.

That sounds reasonable for large heavy mirrors. But doesn't it damage the mirror curve? I know it's really difficult to reshape a 70".



#32 Celestial825

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Posted 05 October 2021 - 01:17 AM

Pacific Cloth™ by Silverguard™ used for storage may put off the onset of tarnishing of silver mirrors.  

 

https://silverguard.com >

 

Has anyone tried this? I seem to recall someone, perhaps Zane?, had used it on his DIY silvered mirrors. 

Someone on a thread had claimed to use a silvered mirror with anti tarnish clothes for 3 years, until one night they forgot to put the hatches on the mirror and then some spots appeared. I guess the brand was 3M.


Edited by Celestial825, 05 October 2021 - 01:21 AM.


#33 Steve Dodds

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Posted 05 October 2021 - 08:31 AM

That sounds reasonable for large heavy mirrors. But doesn't it damage the mirror curve? I know it's really difficult to reshape a 70".

Why would it do that?  Silver and ferric chloride does nothing to glass.


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#34 Celestial825

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Posted 05 October 2021 - 09:40 AM

Why would it do that?  Silver and ferric chloride does nothing to glass.

Ferric chloride! I'll keep in mind your advice. Thanks  waytogo.gif



#35 Celestial825

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Posted 12 October 2021 - 07:55 AM

There are at least two kinds of diy coatings:

 

1) dipping the mirror in a solution that reacts with silver and forms an ultra thin film (e.g. 5nm). Benzotriazole and Midas are two common cases.

 

2) putting a lacquer such as nitrocellulose by a method called "spin coating" on the mirror. The solution is dripped on the mirror which is rotating on some device. Then the rotation speeds up to at least 500 rpm to spread the liquid evenly over the mirror. As the liquid evaporates, the lacquer deposits on the mirror.

 

I'm thinking doing the latter on my mirror.

According to an old ATM book, the appropriate speed for a 6" mirror is 3 round per second.



#36 MitchAlsup

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Posted 12 October 2021 - 10:04 AM

It's a dielectric coating, you can stack 25 layer pairs and get 99%.  Little fuzzy on how it works.

Lookup Bragg filters and imagine them working in transmission rather than in reflection--the filter prevents/reduces the absorption of the wave, thus enhancing the reflectivity.


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#37 ccaissie

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Posted 16 October 2021 - 02:03 PM

My friend out here with the 70" just started using the midas stuff, so we don't yet have any long term data yet, but previously he would coat in April and by October when the telescope was put away for the winter it was pretty orange, probably about 50% reflectance.  Hoping for the best.

Recent work by oregonscopewerks@groups.io  blog shows much benefit from the Midas overcoat.



#38 Celestial825

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Posted 16 October 2021 - 02:43 PM

Did they try the conventional lacquering? I'm going to do it within the next two weeks on my 6.2" mirror. It's not generally a difficult process, but the issue is to manage the thickness of the film to be half the wavelength (200nm) to avoid a destructive interference. And certainly it's possible by considering the solution concentration, angular speed, evaporation time, .... Also viewing the interference (no apparatus needed and not very complicated) will help to find the exact thickness. If such a lacquer really prolong the silver life and is optically acceptable for visual uses, it will be helpful to many amateurs.

 

I will share the results here on CN in detail.

 

And thanks for the helpful information.waytogo.gif

There are two conditions for a constructive interference: (1) the thickness (of the protective film) to be much less than the wavelengh (e.g. 30 nm), and (2) the thickness being half the wavelength (i.e. 200nm). In these two cases almost no light is lost owing to the protective film. However if the thickness is much more than the wavelength (e.g. tenfold), there will be a complex state of both constructive and destructive interference with no fringes visible on the mirror, resulting in a 4 or 5 percent loss in reflection. If such a thick layer does not deteriorate the definition (of the image) and provides an effective protection, the 4 or 5 percent loss is negligible. This was a conclusion from interference physics and what was said in old ATM books.


Edited by Celestial825, 16 October 2021 - 03:12 PM.



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