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Aluminum/silver protective film thickness?

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#1 Celestial825

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Posted 03 October 2021 - 04:28 AM

Hi.

 

I know that for an anti reflective film, the thickness must be one quarter the wavelength of light in the anti reflective substance in order to happen a destructive interference. It's because a 180° phase change takes place for light reflected from each surface.

But how about a protective film on a reflective (metal) surface?

Seemingly very thin protective films (e.g. 5nm) are applied to large telescope reflectors that means a 180° phase change must take place on the metal surface for a constructive interference to happen in this case.

 

Therefore the second option for the thickness of the film must be half the wavelength (200nm) for the desired constructive interference.

 

But I'm not sure about this. Anybody know the correct answer.

Thanks to all.


Edited by Celestial825, 03 October 2021 - 11:01 AM.


#2 Tangerman

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Posted 03 October 2021 - 06:50 AM

A metal such as aluminum or silver is reflective enough without protective films. If you are applying a film only for protection, I would think you would want it as thin as possible so as not to change how the light reflects from the metal surface. But, I've never done this or really studied it myself, so I could be wrong. You can also apply coatings to get higher reflectivity, but I would assume that if the only purpose is protection, then 5 mm would be good. 


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#3 Steve Dodds

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Posted 03 October 2021 - 08:59 AM

Generally Aluminum about 1500 angstroms, SiO 2000 Angstroms (1/2) wave


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#4 Celestial825

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Posted 03 October 2021 - 09:38 AM

Generally Aluminum about 1500 angstroms, SiO 2000 Angstroms (1/2) wave

Then my guess was right. Thank you waytogo.gif



#5 Celestial825

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Posted 03 October 2021 - 09:46 AM

If you are applying a film only for protection, I would think you would want it as thin as possible so as not to change how the light reflects from the metal surface.

You're right and a thinner coating is optically better than a thick one. But the problem with an ultra thin coating is the relatively weak protection due to the existence of pores and cracks in the coating, especially when one applies a diy lacquer. crazy.gif



#6 Steve Dodds

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Posted 03 October 2021 - 09:47 AM

Enhanced aluminum is 2 (or more) 1/4 wave layers.  One low index (SiO2) one high index (TiO2)  1/2 wave total


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#7 Celestial825

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Posted 03 October 2021 - 09:55 AM

Enhanced aluminum is 2 (or more) 1/4 wave layers.  One low index (SiO2) one high index (TiO2)  1/2 wave total

Interested on how this increases the total reflection of aluminum help.gif



#8 Steve Dodds

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Posted 03 October 2021 - 10:37 AM

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



#9 Celestial825

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Posted 03 October 2021 - 12:16 PM

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

The enhancement must be be due to the constructive interference from consecutive layers. But one issue remains: the aluminum surface absorbes more than 10% of incident light on it, then where the 99% reflection come from?



#10 Tangerman

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Posted 03 October 2021 - 12:55 PM

You're right and a thinner coating is optically better than a thick one. But the problem with an ultra thin coating is the relatively weak protection due to the existence of pores and cracks in the coating, especially when one applies a diy lacquer. crazy.gif

Certainly, there's a trade-off between protection and thickness. I'd like to know more about how these coatings are done myself. An optics lab may do it differently than I could. 


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#11 Steve Dodds

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Posted 03 October 2021 - 01:20 PM

The enhancement must be be due to the constructive interference from consecutive layers. But one issue remains: the aluminum surface absorbes more than 10% of incident light on it, then where the 99% reflection come from?

With a pure dielectric coating there is no aluminum. Just stacked overcoats, each pair provides about 5% reflectance.  This is mostly for small mirrors like diagonals.  On large mirrors so many layers changes the figure.


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#12 Celestial825

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Posted 03 October 2021 - 01:38 PM

Certainly, there's a trade-off between protection and thickness. I'd like to know more about how these coatings are done myself. An optics lab may do it differently than I could. 

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.



#13 Celestial825

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Posted 03 October 2021 - 01:43 PM

With a pure dielectric coating there is no aluminum. Just stacked overcoats, each pair provides about 5% reflectance.  This is mostly for small mirrors like diagonals.  On large mirrors so many layers changes the figure.

That was interesting, though not possible for diy smirk.gif



#14 Steve Dodds

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Posted 03 October 2021 - 01:56 PM

That was interesting, though not possible for diy smirk.gif

Oh yes, you need a pretty top end coater to do dielectrics.  To do a 20" it's going to set you back about $100,000


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#15 Steve Dodds

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Posted 03 October 2021 - 02:01 PM

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.

I have never heard of that type of coating, can't imagine it giving you any kind of quality optical surface.

Everybody doing it themselves nowadays is using this technique, which works pretty well, although you need to redo it every year.

 

https://sites.google...elescope-mirror


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#16 Celestial825

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Posted 03 October 2021 - 02:14 PM

Oh yes, you need a pretty top end coater to do dielectrics.  To do a 20" it's going to set you back about $100,000

10,000$!!! tongue2.gif blush.gif



#17 Celestial825

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Posted 03 October 2021 - 02:19 PM

I have never heard of that type of coating, can't imagine it giving you any kind of quality optical surface.

Everybody doing it themselves nowadays is using this technique, which works pretty well, although you need to redo it every year.

 

https://sites.google...elescope-mirror

This is an easy way of silvering the mirror, but protecting is a different and optional step. Midas is introduced here:

 

https://sites.google...ults?authuser=0



#18 Steve Dodds

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Posted 03 October 2021 - 02:25 PM

This is an easy way of silvering the mirror, but protecting is a different and optional step. Midas is introduced here:

 

https://sites.google...ults?authuser=0

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.


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#19 Steve Dodds

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Posted 03 October 2021 - 02:36 PM

10,000$!!! tongue2.gif blush.gif

For the coater, not per mirror.


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#20 Steve Dodds

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Posted 03 October 2021 - 02:54 PM

Here is a pic of the 70" taken yesterday, normally at this point it's bright orange, but we used the Midas stuff in the spring, and now it's still usable, just a little yellow.

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#21 Celestial825

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Posted 03 October 2021 - 03:13 PM

Here is a pic of the 70" taken yesterday, normally at this point it's bright orange, but we used the Midas stuff in the spring, and now it's still usable, just a little yellow.

70"??? That's great!!! As I recon, in addition to Midas, using anti tanish strips (e.g. 3M) or activatedcarbon packets in a sealed space around the mirror when it's not used, will prolong the life span of the coating. (Maybe tenfold). Activated carbon is capable of absorbing many compounds and provide a more fresh atmosphere for a silver coating. It must be much beneficial for such a big mirror.



#22 Celestial825

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Posted 03 October 2021 - 03:22 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.

Midas and benzotriazole just form an ultra thin film (maybe 2-5nm) that certainly cannot provide a highly effective protection for the sensitive silver surface. Then additional types of protection is essential.



#23 Steve Dodds

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Posted 03 October 2021 - 03:40 PM

Midas and benzotriazole just form an ultra thin film (maybe 2-5nm) that certainly cannot provide a highly effective protection for the sensitive silver surface. Then additional types of protection is essential.

No matter what you do to protect a mirror, you will probably be recoating it every year.  Before Midas we got 4-5 months in dry desert air. Now we can get a full year.


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#24 Celestial825

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Posted 03 October 2021 - 03:53 PM

No matter what you do to protect a mirror, you will probably be recoating it every year.  Before Midas we got 4-5 months in dry desert air. Now we can get a full year.

That's quite disappointing. But I'm still persistent to try some new methods. question.gif



#25 Oregon-raybender

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Posted 04 October 2021 - 01:59 PM

I still have a silvered mirror in my office with 2.5 year old silver coating, with and with out Midas (OTW)  It looks fine, it was kept in wrapped paper towel and zip-lock bag. But in the real world, silver lasts just about a year, with some care. It's just the nature of silver, it degrades slowly exposured to what is in the air. Believe me having been involved with the optical coatings for 50 years. Everyone has tried to solve the issues with silver as first surface coating, slowly getting there, but for spray silver there is limited action (none) on professional side, the coating is not tough enough, too soft. So any findings is on the ATMers shoulders

 

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