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Mirror Coatings: Enhanced or not?

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51 replies to this topic

#26 MeridianStarGazer

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Posted 22 June 2021 - 01:38 AM

Super polishing is not just extra polishing.

I knew that. Super polishing uses a pH near 4, temperature not too hot, much smaller cerium size, and lower concentration. It gets a much finer polish than any amount of regular polishing could cause. And the effects are the absence of scatter, invisible laser contact point. .

 

I'm just curious about the magnitude of normal scatter, and hoped someone with access to both kinds of mirrors and testing equipment would be able to give that info.


Edited by stargazer193857, 22 June 2021 - 08:15 AM.


#27 Paul R.

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Posted 22 June 2021 - 04:16 AM

FWIW it IS possible to ship mostly risk free, and damaged to a properly packed is statistically unlikely (one in hundreds or less). Unproperly packed damage is almost certain. The difference between the two is huge though and there are some decent guides to how to pack and ship large mirrors on line from the usual sources.

But if it were me and mine I'd drive that in a minute.

Yes, the mirror was packaged and shipped by me in '09 with success to Spectrum for it's first recoat. That though was a different time outside of a crippling pandemic. Today labor shortages and insanely increased demand through unique economic environments, have not only stressed the industry, but driven the prices up to ridiculous levels. Driving it myself to a local firm with decades of coating experience is a complete and total no brainer. I wished I'd of chose this route 12 years ago! Incidently, let me add that some of the coaters I contacted wanted nothing to do with my mirror because of where it had been previously coated.

Edited by Paul R., 22 June 2021 - 08:24 AM.


#28 vahe

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Posted 22 June 2021 - 02:09 PM

The 10” primary mirror of my TEC Mak was originally coated with enhanced coatings but it had to be removed due to coating problems and recoated, the removal of enhanced coatings damaged the substrate requiring refiguring the primary before it could be recoated again.

.
Later when I discussed this issue in another post Roland had the following reply:

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“I suspect that it was not the removal of the enhanced coating that caused the problem. Rather it was the use of an adhesion layer underneath the actual aluminum that was the problem. This adhesion layer is usually not fused properly to the glass surface, so when the aluminum was etched away, some of the adhesion layer also came off in bits and pieces.
When you have your mirror coated and wish to have an enhanced reflectivity, ask your coater to NOT use an adhesion layer underneath the aluminum. That way, if you ever need to remove the coating, it will come off clean and leave the glass in a pristine state. Otherwise the adhesion layer will need to be stripped off using a very harsh acid etch, and this can damage the glass. It opens up any faint sleeks and turns them into wide broad scratches.
Some coaters even use an adhesion layer under a normal protected aluminum coating, Again, ask them not coat the mirror that way”

.

Vahe


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#29 Steve Dodds

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Posted 22 June 2021 - 02:14 PM

Unless there is a undercoat of chromium, there is nothing in a enhanced coating that is any more difficult to remove than plain aluminum.


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#30 Paul R.

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Posted 22 June 2021 - 02:55 PM

Dropped the 20" mirror off at HL Clausing in Skokie, Illinois  today.  Decided to get their enhanced *Magnabright* coatings. After research and considerable conversation with Howard himself, I feel very confident they will provide me an excellent coating. I'm looking forward to many more years of exciting views! Thanks to all whom participated in this thread.


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#31 SandyHouTex

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Posted 22 June 2021 - 05:47 PM

There is a discussion that has been started in this thread that talks about "enhanced" coatings that can't be stripped with simple chemical means:

 

https://www.cloudyni.../#entry11184290

 

Post #6 specifically.

 

Like I said.


Edited by SandyHouTex, 22 June 2021 - 05:49 PM.


#32 mark cowan

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Posted 22 June 2021 - 06:30 PM

Any coating with a chromium base layer can cause problems in stripping, or so I've heard.  ;)  They all date from long ago and I've never seen one nor had to strip one. :shrug:


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#33 EJN

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Posted 22 June 2021 - 06:37 PM

There is a discussion that has been started in this thread that talks about "enhanced" coatings that can't be stripped with simple chemical means:

 

https://www.cloudyni.../#entry11184290

 

Post #6 specifically.

 

Like I said.

 

That is referring to an all dielectric coating of the same type often used on refractor diagonal mirrors. It is not

the same thing as enhanced aluminum.


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#34 Steve Dodds

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Posted 22 June 2021 - 06:44 PM

Any coating with a chromium base layer can cause problems in stripping, or so I've heard.  wink.gif  They all date from long ago and I've never seen one nor had to strip one. shrug.gif

When I was doing coatings I had a couple that were fairly old.  Nothing would touch it.  A friend who is a chemist said there is a acid that would remove it, but was super expensive and super dangerous.  I just sent the mirrors back and said I couldn't coat it.


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#35 Steve Dodds

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Posted 22 June 2021 - 07:28 PM

CN moderators are usually pretty on top of stuff like that, I once mentioned something I had for sale on the classifieds and they shut me down real quick.


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#36 CHASLX200

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Posted 22 June 2021 - 07:40 PM

FWIW it IS possible to ship mostly risk free, and damaged to a properly packed is statistically unlikely (one in hundreds or less).  Unproperly packed damage is almost certain.  The difference between the two is huge though and there are some decent guides to how to pack and ship large mirrors on line from the usual sources.

 

But if it were me and mine I'd drive that in a minute. 

It just scares me tooooo death to ship my 18" Mirror out if i do get it refigured.  Mike Lockwood would even build me a shipping box done right so that i would trust. But with UPS they have to pack it for it to be insured.

 

Not sure what shipper Mike thinks is best to deal with. But if something gets lost or broke well you are on your own many times.  As for recoating i don't know who is best. My coatings seem fine and i think they are the same coatings since the mirror was made in 2002.


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#37 CHASLX200

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Posted 22 June 2021 - 07:42 PM

Mike Lockwood has extensive experience with every coating company in the US, he knows what he is talking about. 

I would trust him more than most anyone else.


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#38 Moravianus

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Posted 23 June 2021 - 12:34 AM

Ready to pack

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#39 SandyHouTex

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Posted 23 June 2021 - 09:18 AM

My 20 inch Obsession is from 2008, and has a great OMI mirror.  For some reason that escapes me, I had to clean it a couple times when I first got it.  Never scratched it at all, which led me to believe that it had dielectrics on top.  It has an “enhanced” mirror.



#40 Steve Dodds

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Posted 23 June 2021 - 10:52 AM

Frequently people who do “enhanced” coatings won’t discuss what they are doing specifically because it’s a trade secret.

 

For an “enhanced” reflective coating, something other than aluminum must be used.  Aluminum’s reflectivity is only 88%.  Off the top of my head, the only things that come to mind are silver and dielectrics.

When I was doing coatings my formula for enhanced was:  Aluminum base with 1/4 wave thickness SiO2 and 1/4 wave TiO2, overcoat is a dielectric and adds 7% reflectivity.

.


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#41 EJN

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Posted 23 June 2021 - 11:22 AM

For an “enhanced” reflective coating, something other than aluminum must be used. Aluminum’s reflectivity is only 88%. Off the top of my head, the only things that come to mind are silver and dielectrics.



Uh, no. Aluminum reflectivity can be enhanced with certain types of overcoat.

See preceding post, and here: https://www.edmundop...irror-coatings/

Edited by EJN, 23 June 2021 - 01:32 PM.

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#42 Starman1

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Posted 23 June 2021 - 12:31 PM

Technically, any transparent reflectivity-enhancing overcoat IS a dielectric coating.

Even SiO can be thought of as one.

In enhanced aluminum coatings, 3-5 additional transparent layers are added, and these behave exactly like dielectric coatings on a mirror to enhance

ranges of reflectivity in the spectrum by constructive interference with the wavefront.


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#43 mark cowan

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Posted 23 June 2021 - 01:42 PM

My 20 inch Obsession is from 2008, and has a great OMI mirror.  For some reason that escapes me, I had to clean it a couple times when I first got it.  Never scratched it at all, which led me to believe that it had dielectrics on top.  It has an “enhanced” mirror.

That's just typical of an enhanced coating.  Above the aluminum will be several fractional-wave layers of materials like titanium dioxide, silicon dioxide, and sometimes other transparent layers which mostly escape me at the moment.  Through interference they improve the reflectivity, but they also are harder than the aluminum and so protect it quite well.

 

A pure dielectric coating has no metal at all and works solely by interference.  There can be dozens of layers in a pure dielectric coating, and they can create significant stress on the glass as well, which is one reason dielectric coatings are generally only used on secondaries, with the coating designed for 45 degrees reflection.


Edited by mark cowan, 23 June 2021 - 02:09 PM.

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#44 Sean Cunneen

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Posted 23 June 2021 - 03:49 PM

I spent quite a bit of time cleaning up this thread, lets stay on topic from here on out. This thread is not about vendors, its about mirror coatings, so lets keep the conjecture, conspiracy theories, and questions of bias off CN. If anyone sees any post by anyone they find questionable, please hit the "Notify Moderator" and we are happy to take a look... 

 

Thanks,

Sean


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#45 SandyHouTex

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Posted 23 June 2021 - 05:47 PM

Uh, no. Aluminum reflectivity can be enhanced with certain types of overcoat.

See preceding post, and here: https://www.edmundop...irror-coatings/

That would fall under, "...,something other than aluminum must be used."  I did not imply that it had to be mixed with the aluminum.



#46 Steve Dodds

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Posted 23 June 2021 - 06:01 PM

Something other than aluminum is almost never used.  For visible spectrum mirrors aluminum is 99.9% of all mirrors.  Overcoated silver is a poor choice because the overcoat is not just a impenetrable sheet of glass.  It is pourus and crumbly allowing oxygen to get through and oxidizing the silver, overcoated silver only lasts a couple of years.  Aluminum will last 10-20 years if taken care of.



#47 Peter Nance

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Posted 24 June 2021 - 01:23 AM

Some information:

Chrome as a base layer was sometimes used, it has the most stress of any material. So if you want your 1/20th wave mirror turned into a potatoe chip coat it with Chrome.

They say it's for adhesion, not true, aluminum sticks to glass just fine.

 

As for stripping chrome, easy, potassium-ferric chloride and sodium hydroxide with water. It's a dip and the chrome is gone very quickly.

 

As for the "secret formula" yes some people hold their designs as trade secrets. Steve Dodds said it above, Aluminum, 1/4 wave low index, 1/4 wave high index material, you can add more layers but you don't get something for nothing, the reflectance goes up but the bandwidth gets shorter.

 

If anyone wants more information just ask, I learn something every day. Been doing coatings since 1979, and no I did NOT have anything to do with the Hubble grin.gif I worked at the poor sister company on the west coast. waytogo.gif


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#48 SandyHouTex

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Posted 24 June 2021 - 09:21 AM

Something other than aluminum is almost never used.  For visible spectrum mirrors aluminum is 99.9% of all mirrors.  Overcoated silver is a poor choice because the overcoat is not just a impenetrable sheet of glass.  It is pourus and crumbly allowing oxygen to get through and oxidizing the silver, overcoated silver only lasts a couple of years.  Aluminum will last 10-20 years if taken care of.

My Baader diagonal has silver, as does my Quartz mirror Questar.  It’s a 1968.  I thought Celestron used silver on some of their OTAs.  I just don’t remember which ones.

 

Also one of the telescopes on Mauna Kea, Subaru I think, used silver.  They vacuum deposit it and overrcoat it as I recall.

 

A small minority for sure.



#49 SandyHouTex

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Posted 24 June 2021 - 09:40 AM

I found this link from 2016, which may be why I think “enhanced” coating removal may have issues.  The mirror in my 20 inch Obsession is an OMI from 2008.  The speculation there is that Chromium may be the culpret.

 

https://www.cloudyni...ating-problems/



#50 mark cowan

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Posted 24 June 2021 - 04:17 PM

I would not have considered OMI in those days for coating if they had used chromium base layers.  So, no. 

 

From that thread, by RayD the OP:

 

I just talked with James Mulherin at OMI- he was very interested. I'm just going to have to get an optician involved at this point. He did say that they don't use an adhesion layer and that was the process in '04.

 

 

Also the symptomology is wrong, the defects are more likely what I posted in 2016 in that thread - contamination of some sort.




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