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Mirror Coatings: Enhanced or not?

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#1 Paul R.

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Posted 18 June 2021 - 10:10 PM

Aside from cost, are there any disadvantages to coatings of higher reflectivity?

#2 Jim Waters

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Posted 18 June 2021 - 10:16 PM

Way back ~20 years ago enhanced coatings were not as smooth as lesser coatings.  Ion vapor deposition technology has evolved so this may no longer be the case.


Edited by Jim Waters, 18 June 2021 - 10:19 PM.


#3 photomagica

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Posted 18 June 2021 - 11:24 PM

Love the enhanced coatings on my Evolution C8 and on my 127mm ES EMD Triplet Objective. More light is good and both of these telescopes have met or exceeded my expectations on subtle planetary detail. I expect that today, any coating lack of smoothness is more than compensated by the lack of reflections and thus flare from the enhanced coatings.

Bill



#4 TOMDEY

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Posted 18 June 2021 - 11:49 PM

Folding flats comprising dielectric layers (this includes enhanced metal films) can be quite polarization sensitive. This rarely matters unless you are doing polarimetry or using equipment that needs unpolarized input to function properly.    Tom


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#5 MeridianStarGazer

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Posted 18 June 2021 - 11:57 PM

The question could easily be answered by shining a red laser at both kinds of coatings and seeing how much brighter the dot looks on one vs the other. If about the same, or if the dot is invisible due to super polished glass, then that means enhanced have no drawbacks.

 

 

One other concern is how easily it is to remove the coatings when or if they need a recoat. Might not be an issue at all, though I don't recall the final word on that.

 

96% vs 89%. That is like an 8% increase in aperture, almost an inch on some mirrors. Though some would say you need a 20% increase in brightness (only 16% here) to notice a difference. Maybe ever little bit helps, and some objects will show it more than others. But if it is not super noticeable, given the price difference and any question of quality, I'd stay with the simple one. But I could be living in the past.

 

Enhanced coatings add brightness without adding weight or size.



#6 MeridianStarGazer

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Posted 18 June 2021 - 11:59 PM

Folding flats comprising dielectric layers (this includes enhanced metal films) can be quite polarization sensitive. This rarely matters unless you are doing polarimetry or using equipment that needs unpolarized input to function properly.    Tom

Would that reduce the blue sky background when looking at Jupiter during the daytime, with the light from Jupiter staying just as bright?



#7 Jim Waters

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Posted 19 June 2021 - 12:43 AM

Call Optical Wave Labs and chat with Ben Fields about enhanced coatings.

https://opticwavelabs.com/



#8 TOMDEY

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Posted 19 June 2021 - 01:00 AM

Would that reduce the blue sky background when looking at Jupiter during the daytime, with the light from Jupiter staying just as bright?

Possibly. Thing is that sky polarization leans linear but high performance enhanced coatings tend to circularize... so the interactions are not necessarily obvious. It could help or hurt contrast, depending on the clocking of the mirror's surface normal vector projection to the  sun-field center vector. You'd be better off just inserting a linear polarizer in the chain to darken the sky as much as possible.

 

We tend to think of polarization as being plain linear with some clocking orientation. But the generalized case is a blend of elliptical and unpolarized... which is a 4-dimensional characteristic defined by e.g. Stokes Parameters (I, Q, U, V).    Tom


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#9 SandyHouTex

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Posted 19 June 2021 - 04:36 PM

It used to be that when “enhance” coating were used and needed to be replaced,  They had to be polished off, and the mirror refigured.

 

I believe that is why Carl Z. does his own “unenhanced” mirror coatings.



#10 Steve Dodds

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Posted 19 June 2021 - 04:43 PM

It used to be that when “enhance” coating were used and needed to be replaced,  They had to be polished off, and the mirror refigured.

 

I believe that is why Carl Z. does his own “unenhanced” mirror coatings.

That has never been true.  Carl doesn't do enhanced because most people cant tell between them, 8% just isn't enough difference to tell visually.


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#11 Mike Lockwood

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Posted 19 June 2021 - 05:50 PM

This topic has been hashed out time after time on these forums, and half of the information is still wrong.

 

Modern enhanced coatings are as easy to strip as overcoated aluminum (non-enhanced), and they are not rough on an optical scale.  My spectrometer measursed the difference in reflectivity at more like 4-5%.  I have measured a number of coatings.

 

See joint statement from Carl and I regarding mirror coatings, and my recommendations for coaters contained within:

  http://www.loptics.c...oatingrisk.html

 

As you can conclude from the article, Carl started doing his own coating because he got tired of getting poor coatings or having coaters damage mirrors.  The same has happened to me.  Nasty chemicals used means regrinding the mirror at the owner's cost.  It seems that on CN people fixate on cheap coatings and recommend the same coaters with inconsistent lead times and other well-documented issues over and over and over again.  Maybe someone will find some of the old threads and post links to them here.....


Edited by Mike Lockwood, 19 June 2021 - 07:05 PM.

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#12 CHASLX200

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Posted 19 June 2021 - 06:06 PM

This topic has been hashed out time after time on these forums, and half of the information is still wrong.

 

Modern enhanced coatings are as easy to strip as overcoated aluminum (non-enhanced), and they are not roughon an optical scale.  My spectrometer measursed the difference in reflectivity at more like 4-5%.  I have measured a number of coatings.

 

See joint statement from Carl and I regarding mirror coatings, and my recommendations for coaters contained within:

  http://www.loptics.c...oatingrisk.html

 

As you can conclude from the article, Carl started doing his own coating because he got tired of getting poor coatings or having coaters damage mirrors.  The same has happened to me.  Nasty chemicals used means regrinding the mirror at the owner's cost.  It seems that on CN people fixate on cheap coatings and recommend the same coaters with inconsistent lead times and other well-documented issues over and over and over again.  Maybe someone will find some of the old threads and post links to them here.....

I would buy what you and Carl say over anyone else. My 2002 made Galaxy mirror has 96% enhanced aluminum coatings.

 

I would think it has never been recoated but they still look fine after all these years. I know the scope sat inside for 10 years never used from the owner i bought it from. I live on the gulf and not sure how they will hold up where i live.  I would hate to have a mirror ruined due to a bad coater and need a refigure.


Edited by CHASLX200, 19 June 2021 - 06:07 PM.


#13 MeridianStarGazer

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Posted 20 June 2021 - 11:16 PM

This topic has been hashed out time after time on these forums, and half of the information is still wrong.

 

Modern enhanced coatings are as easy to strip as overcoated aluminum (non-enhanced), and they are not rough on an optical scale.  My spectrometer measursed the difference in reflectivity at more like 4-5%.  I have measured a number of coatings.

 

See joint statement from Carl and I regarding mirror coatings, and my recommendations for coaters contained within:

  http://www.loptics.c...oatingrisk.html

 

As you can conclude from the article, Carl started doing his own coating because he got tired of getting poor coatings or having coaters damage mirrors.  The same has happened to me.  Nasty chemicals used means regrinding the mirror at the owner's cost.  It seems that on CN people fixate on cheap coatings and recommend the same coaters with inconsistent lead times and other well-documented issues over and over and over again.  Maybe someone will find some of the old threads and post links to them here.....

Are coaters that are not on the list sporadically bad, or consistently mediocre? Do they only use harsh methods on mirrors with old coatings, or also on brand new mirrors awaiting a first coat?

Thank you.



#14 SandyHouTex

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Posted 21 June 2021 - 09:37 AM

That has never been true.  Carl doesn't do enhanced because most people cant tell between them, 8% just isn't enough difference to tell visually.

So quite a few years ago, I sent Carl a 16 inch mirror that I had rough ground and fine ground.  I asked him to polish and figure it.  I then asked him about an “enhanced” coating, and that’s when he told me that they had to be polished off and the mirror refigured.  He said he only provided easily removed aluminum.

 

I was also told the same thing about my 20 inch OMI mirror in my Obsession which has “enhanced” coatings.

 

So I’ll take Carl’s word over yours, because I don’t know who you are or what your expertise is.


Edited by SandyHouTex, 21 June 2021 - 09:42 AM.

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#15 happylimpet

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Posted 21 June 2021 - 09:42 AM

Enhanced aluminium coatings (if thats what we're discussing here) have lower reflectivity in the NIR - which may or may not matter to you. It matters to me! I do a lot of DSO imaging in the 700-1000nm range, and (for example) jupiter imaging at methane band at 889nm. IIRC, the difference for enhanced/standard at 889nm was something like 60%/85%.



#16 happylimpet

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Posted 21 June 2021 - 09:46 AM

fig-4-mmc.gif

 

with regard to my comments above....basically after R band longwards you're better off with standard aluminium. Similarly in the UV. I imagine the finer details of this are dependent on the specific coating.


Edited by happylimpet, 21 June 2021 - 09:49 AM.


#17 Mike Lockwood

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Posted 21 June 2021 - 11:07 AM

Are coaters that are not on the list sporadically bad, or consistently mediocre? Do they only use harsh methods on mirrors with old coatings, or also on brand new mirrors awaiting a first coat?

I can't answer the first question.  There are good coaters out there that I don't mention that I have not worked with.  There are coaters that have damaged my optics and those that have damaged others' optics.  I only can say, as I do in the article, that I recommend working with coaters who have some experience with optical work and how to deal with a polished surface.  I am skeptical of ultra-high reflectivity claims, but to be fair, I have not measured samples of those coatings.

 

I can also say that there is more danger when doing a recoat, particularly when the old coating is damaged or stubborn and will not strip easily.

 

I had a 45" mirror coated by Evaporated Metal Films, and to me it looked excellent.  I may add them to my list after I have more data points.

 

So quite a few years ago, I sent Carl a 16 inch mirror that I had rough ground and fine ground.  I asked him to polish and figure it.  I then asked him about an “enhanced” coating, and that’s when he told me that they had to be polished off and the mirror refigured.  He said he only provided easily removed aluminum.

I was also told the same thing about my 20 inch OMI mirror in my Obsession which has “enhanced” coatings.

So I’ll take Carl’s word over yours, because I don’t know who you are or what your expertise is.

Steve has made and coated a lot of mirrors, including some of mine.

 

The part that you mention about polishing off a coating is no longer true, and has not been true as long as I have been making mirrors commercially (~2007).  I have stripped and refigured a lot of mirrors, and I only ever had a few coatings that I couldn't strip.  None of those were on primary mirrors, all unstrippable coatings were on secondary mirrors, and were probably some form of dielectric coating.

 

OMI's own coatings were easily strippable, and they coated many optics for me over many years.  Their work was excellent.  I don't know who they used before they built their own coating system.
 

So I read your link, and the only joint part of it with you and Carl is the very short beginning.  All it does is recommend three companies that you jointly trust to coat your mirrors.  The rest of it is just “your” opinion.

Actually the green Ronchi images in "the rest" of the article are from Carl, provided by him.  I encourage you to ask him if he disagrees with any of it.

 

After talking to and working with ELEVEN coaters over my career, I believe that it is very good guidance, and it is intended to help my clients and Carl's clients to preserve their optics.
 

As for the Typical Reflectance Curves above, a typical curve is calculated, it is not an actual measured curve.  Similarly, the reflectance plots on many coaters' web sites is a theoretical calculated curve, not a measured curve.  Enhanced aluminum can be altered to move the peak reflectivity around for more UV or IR reflectivity.  No coating is typical.  All coaters do things a little bit differently.

 

My measurements, using a spectrometer with calibration standard, show a ~4-5% difference between the protected and enhanced Al coatings that I have measured, and the protected Al is over 90% reflectivity.  The difference is not huge, they are both easily strippable, and this is why I recommend both types of coatings.


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#18 mark cowan

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Posted 21 June 2021 - 11:30 AM

attachicon.giffig-4-mmc.gif

 

with regard to my comments above....basically after R band longwards you're better off with standard aluminium. Similarly in the UV. I imagine the finer details of this are dependent on the specific coating.

Yes, enhanced coatings tend to have a "busier" response curve over frequency.  I think it's pretty much inherent in the interference between layers that provides the enhancement in the first place...


Edited by mark cowan, 21 June 2021 - 11:32 AM.

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#19 Starman1

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Posted 21 June 2021 - 11:44 AM

Aside from cost, are there any disadvantages to coatings of higher reflectivity?

For visual use, probably not.

With your big mirror, I'd recommend Terry Ostahowski for enhanced coatings.



#20 MeridianStarGazer

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Posted 21 June 2021 - 06:01 PM

...

 

My measurements, using a spectrometer with calibration standard, show a ~4-5% difference between the protected and enhanced Al coatings that I have measured, and the protected Al is over 90% reflectivity.  The difference is not huge, they are both easily strippable, and this is why I recommend both types of coatings.

Have you measured the reflectivity of a super polished mirror vs a standard polished one?

 

I suspect that if a laser pointer can be seen on a standard polished one from every angle, that is a lot of light going out, and might be a noticeable fraction if the total, if not half. I'm not sure if the lost light could come at the correct angles to compete as noise. There are reports on CN of Zambuto mirrors having better large scale contrast that allows seeing galaxies better. I suspect a super polish is brighter than more layers of coatings on a regular polish.

 

I'm just hoping you did or can do the tests.

 

My own tests include shining a laser at the surface of water and getting a bright reflection that is not visible on the water. I also shined a laser at freshly split table glass and did not see the laser on the freshly exposed surface, even though it gave a bright reflection.

 

Thank you.



#21 Steve Dodds

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Posted 21 June 2021 - 06:18 PM

Super polishing is not just extra polishing.



#22 Paul R.

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Posted 21 June 2021 - 08:31 PM

For visual use, probably not.

With your big mirror, I'd recommend Terry Ostahowski for enhanced coatings.

Hi Don,

            Bought an eyepiece from you.  The price was very fair, and the shipping was super quick!  Thanks!  I thought about using Terry, he has a great reputation, but unfortunately, man, California is a LONG way from the north Chicago burbs...  Shipping is not only very risky these days, but the cost is prohibitive, especially if you factor in the 6K replacement insurance you'd want OF WHICH, they probably wouldn't honor in the event of a mishap..anyway.  The mirror I'm about to recoat is not only excellent, but the guy who made it no longer lives.  I will be taking myself by my car to a local coating facility..approximately a 90 minute drive from my home.


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#23 Paul R.

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Posted 21 June 2021 - 08:36 PM

So I read your link, and the only joint part of it with you and Carl is the very short beginning.  All it does is recommend three companies that you jointly trust to coat your mirrors.  The rest of it is just “your” opinion.

Sandy, are you aware of ML? lol, Apparently not. Suggest you do some research...


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#24 Steve Dodds

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Posted 21 June 2021 - 08:52 PM

Sandy, are you aware of ML? lol, Apparently not. Suggest you do some research...

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


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#25 mark cowan

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Posted 21 June 2021 - 11:11 PM

.  Shipping is not only very risky these days, but the cost is prohibitive, especially if you factor in the 6K replacement insurance you'd want OF WHICH, they probably wouldn't honor in the event of a mishap..anyway.  The mirror I'm about to recoat is not only excellent, but the guy who made it no longer lives.  I will be taking myself by my car to a local coating facility..approximately a 90 minute drive from my home.

 

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. 


Edited by mark cowan, 21 June 2021 - 11:11 PM.

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