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Reflector coatings: Standard Aluminum the best?!?!

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#26 mark cowan

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Posted 13 July 2008 - 11:59 PM

Phase plate images are a good indication.

For those who didn't see it, Dale Eason just published a before and after coating phase contrast set for a 16" mirror - check out this post on the Zambutomirrorgroup (you may have to join the Yahoo group to access the files).

Just seems some people go for really really smooth surfaces, then just ship it to any old coater


No, not any old coater! Use a coater that understands what they're doing and does it correctly every time, otherwise you could end up with a mirror that "makes you look dead", as Wallie noted earlier...

Best,
Mark

#27 mark cowan

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Posted 14 July 2008 - 12:05 AM

Your assumption that most mirrors are tested before coating is probably incorrect

:question: :question: :question:

It's impossible to manufacture a mirror without testing before coating. Coating does whatever it does, oftentimes a mirror that is suspect will also have a deteriorated coating and it needs to be stripped before evaluation of the glass. In other words, no, the assumption is correct.

Best,
Mark

#28 mark cowan

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Posted 14 July 2008 - 12:23 AM

But I was wondering, if the above assertion that using thicker coatings can actually degrade high power images, wouldn't this idea show up with some consistency when they tested standard aluminum vs enhanced coated ones in their optical test numbers, RMS, P-V data and Strehl ratios? If true, as a rule of thumb wouldn't the enhanced coated mirrors show lower RMS, P-V data and Strehl numbers with some consistency?


I'll just say it again, none of the commonly used tests can distinguish the effects of any coating that's usable, let alone good. It may be that some coatings (particularly as applied in the past) could lower the contrast transfer of the best possible surfaces - and if you thought that was likely or you just wanted to avoid the possibility, you might refuse to apply any coatings except the simplest and most guaranteed possible. Coatings, OTOH, can always be done over, so it's not a totally sensible approach.

Best,
Mark

#29 Joe Cipriano

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Posted 14 July 2008 - 12:50 AM

OK, people - I've had to remove two posts for being disrespectful.

This is a good discussion - I don't want to have to lock it down.

Be respectful of other's opinions, please.

#30 sixela

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Posted 14 July 2008 - 02:49 AM

Phase plate images are a good indication.

For those who didn't see it, Dale Eason just published a before and after coating phase contrast set for a 16" mirror - check out this post on the Zambutomirrorgroup (you may have to join the Yahoo group to access the files).


For those that can't or won't join:
-the roughness features before and after are the same. The coating did not add roughness features of its own.
-the coating doesn't "smooth out" features either; in fact it may make them slightly deeper (that's not a surprise, because that's an effect I know from Al coatings on the simplest integrated circuits, which become "unsmooth" once you add Al layers and etch them away to make the conductive traces).
-(This is an opinion) From what I can see and the sensitivity of the test, the difference is really irrelevant, at least for this mirror - and one that the mirror maker himself rates as being less smooth than e.g. a Zambuto (but certainly smooth enough to make the differences have little relevance).

Contrary to some popular opinion, *very* small amplitude irregularities - too small to induce significant wave front phase shift - don't rob contrast (after all, in a tunnelling microscope, even a "perfectly smooth" mirror has bumps and troughs), and that applies for even some features you can see it in a phase contrast test. It's larger amplitude roughness that it dangerous because it makes some tests unreliable (some tests necessarily undersample the surface, and the smooth surface that fits the test samples may underestimate the phase shift that the real surface introduces).

#31 Mark Harry

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Posted 14 July 2008 - 07:02 AM

I see something about the post of the 16" mirror(s) that the images aren't the correct ones.(?)
********
FWIW, I've had primarily standard coatings, or those that amount to about 91% or less after having issues with a couple of enhanced mirrors about 10-15 years ago. I think that if applied correctly, the enhanced should work fine.
*******
But I have a penchant for doing very highly critical planetary viewing when conditions allow; and exploiting my meager apertures to their limits. So far, I haven't had issues at all with the standard coatings. And I -AM- seeing the stuff. But I have to admit not having much dealings with enhanced since I did some comparisons outlined above; so situations could have changed. (or not?)
I've also had discussions with coaters personally. One does a lot of anti-reflective lens work. Simple premise- the more steps/stages necessary, the more chances of scattering degradation. I also have the same response from one doing almost exclusively mirrors as well.
My personal thoughts, the fellows doing coatings for a living should know, right?
Actually, I don't really worry much about the coatings, as much as the smoothness of the figure and microfinish of the glass prior to a coat. That is the paramount thing!
Mark

#32 sixela

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Posted 14 July 2008 - 07:35 AM

I've also had discussions with coaters personally. One does a lot of anti-reflective lens work. Simple premise- the more steps/stages necessary, the more chances of scattering degradation.


So? That doesn't really amount to any quantitative statement, does it? It's a perfectly true statement but it makes no pronouncement, not even about how large the "chance" of degrading the views through scatter is.

Going from statements like that to any quantitative or even qualitative conclusion is fraught with danger - you might just as well rate the quality of an eyepiece merely by the number of air-to-glass transitions, and while it's certainly *an* aspect, it isn't the only one. I'll take a Pentax XW over any $5 Huygens any day, despite the larger number of lens groups.

Of course, for planetary work you don't need the enhanced coatings - light throughput is the least of your worries.

#33 rboe

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Posted 14 July 2008 - 09:23 AM

Your assumption that most mirrors are tested before coating is probably incorrect

:question: :question: :question:

It's impossible to manufacture a mirror without testing before coating. Coating does whatever it does, oftentimes a mirror that is suspect will also have a deteriorated coating and it needs to be stripped before evaluation of the glass. In other words, no, the assumption is correct.

Best,
Mark


Do they test for figure and smoothness? I understand there are lot's of tests - which ones would be useful for this discussion; that is to say which ones would indicate a prime surface for coating?

#34 Mitchell

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Posted 14 July 2008 - 09:52 AM

Just as a little side note in the zambuto mirror group some phase contrast images were posted of a mirror before and after coating. No noticable or significant difference. Lays to rest any thoughs of a coating possiable changing surface smoothness.

#35 mark cowan

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Posted 14 July 2008 - 12:28 PM

I see something about the post of the 16" mirror(s) that the images aren't the correct ones.(?)


Go to the Files section under Dale Eason and look for the image with the latest date, it's the correct one (there's no way to edit Yahoo posts). The uncoated image had a longer exposure time and suffers a bit of turbulence that varies by direction, as near as I can tell. Otherwise the images are faithful to each other at the scale shown, which is the point.

Best,
Mark

#36 jg3

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Posted 14 July 2008 - 12:51 PM

For all the progress and rising standards of mirror making, I'm surprised so little quantitative verifiable information is available on mirror coatings, leaving a mirror buyer with little basis by which to choose a type of coating.

Within the past couple years I encountered statements of opinion, I think even by some mirror makers, that enhanced coatings introduce roughness and scatter, and it might not be measurable the way mirror form is measured. One explained that the ion deposition process makes a spot with just a little too much coating even more attractive to the next ions - in other words, more coating goes to where it already sticks out. It's similar to how frost forms.

Some coaters now claim they use additional ion processes to "hammer down" the deposition, to smooth it out before high spots form, and make them dense rather than "frosty".

Without clear information, a somewhat defensive shopping strategy might be to get the simple "protected aluminum" on mirrors intended for planetary use (where you don't need the light gain), but go ahead with enhanced coatings if the mirror is intended for visual deep-space observation.

You can get a brighter view with more aperture instead of enhanced coatings, *but* only until you reach maximum exit pupil. More aperture thus limits your field size and minimum magnification. Furthermore, many of our eyes throw their own aberrations into the mix at wide exit pupils.

#37 mark cowan

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Posted 14 July 2008 - 02:05 PM

Ron,

Do they test for figure and smoothness?



Absolutely - but this is a seperate subject and has had its own threads (though another wouldn't hurt).

I understand there are lot's of tests - which ones would be useful for this discussion; that is to say which ones would indicate a prime surface for coating?


I'm not sure that any commonly applied test makes any difference at all in terms of "being ready for coating." The mirror is ready for coating when it's done.

The only thing IME that matters re the coating is that a better micro-surface (lacking sleeks, scratches, residual pits and micro-fractures from grinding) holds less contaminents (polishing agents mostly, but also redeposition silicates from polishing itself) and these are seeds for the coating to fail sooner and show pinholes.

But the effect on contrast performance (for a fresh coating) is probably negligible until such defects are quite noticeable. Whether that's true as the coating ages is another thing, the pinhole defects grow with dew/dry cycles and exposure to oxidants, which now can get under the protective layers and cause "blooming," a common cause of premature failure. The coater can't do anything about that, as the fault, SFAIK, lies with inadequate surface prep on the mirror itself. But of course the coater gets the blame...

Best,
Mark

#38 rboe

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Posted 14 July 2008 - 02:16 PM

Thanks Mark. My skill set with testing mirrors is limit to looking for scratches, dust, cracks and finger prints.

Any more questions on mirrors that I would have actually involve grinding one - a task I'm more than willing to avoid for now. :)

But surface prep - is that cleaning? Wouldn't the coater do the final cleaning to insure the mirror will accept and hold a coating?

Thanks again for the clarification - lot's to learn.

#39 rwiederrich

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Posted 14 July 2008 - 03:42 PM

So...apart form all this very interesting theory...stuff.

Mark....do you guys over there in Salem...do mirror coating?

It would be nice to send my mirror to someone who will pick it to death, and evaluate it for its many inconsistancies, errors, and manufacture defects. :grin:

After the mirror has been thoroughly evaluated, and the mirror maker identified as a *flake*.....you will aluminize the mirror...hence increasing its value 100% :lol:

Actually...do you aluminize mirrors there in Salem? :help: :grin:

Rob

#40 mark cowan

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Posted 14 July 2008 - 04:05 PM

Me aluminize? No. Pick mirrors to death? Only if I have to. ;)

Best,
Mark

#41 mark cowan

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Posted 14 July 2008 - 04:11 PM

But surface prep - is that cleaning? Wouldn't the coater do the final cleaning to insure the mirror will accept and hold a coating?


I guess I wasn't clear. The coater will clean the already pristine mirror with a succession of nasty compounds, generally ending with a methanol rinse. After which in the vacuum chamber they typically do ion bombardment to get rid of any trace organics. If the mirror has residual cerium from polishing or anything like that they do not take it off, and in fact will not touch the surface with anything other than solvents.

But what I meant by "surface prep" is all the steps involved in making that pristine mirror in the first place - grinding, polishing, figuring. My final cleaning is with dish soap and a sponge, followed by warm tap water, then cold tap water, then distilled water and dry. That's the same sequence used all through figuring as well.

Best,
Mark

#42 rwiederrich

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Posted 14 July 2008 - 04:33 PM

Me aluminize? No. Pick mirrors to death? Only if I have to. ;)

Best,
Mark



LOL :lol: shwooo..

I'm hoping to send it to someone who doesn't know who I am...so when they look at it...they can apply the appropriate descriptive language to an *unknown* person. :grin:

Any good local aluminizers around? Anyone you would recommend?

Rob(Did I mention I am close to testing the 18"....I have a nice polish on it now)

#43 mark cowan

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Posted 14 July 2008 - 07:19 PM

Try Steve Dodds at Nova Optical . Somebody in your neck of the woods, name starts with Z, he's been using Nova exclusively for a while. Plus Steve makes (or made) mirrors and knows what's important in coating. Close enough in Utah, I think.

Best,
Mark

#44 wallie x

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Posted 14 July 2008 - 09:42 PM

Are aluminum molecules "polar molecules"? Of coarse most metals are, but aluminum is nonmagnetic. I'm not a physicist but one would think that evaporated metal films would have a tendency to line up magnetically to a relative degree if the film being deposited consisted of a polar molecules. If such were true, the falling cordwood idea of multilayers randomly deposting upon one another in a disorganized heap suffers somewhat. The very nature of just what is being deposited chemically comes into play rather bigtime (I would assume).

#45 mark cowan

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Posted 14 July 2008 - 10:20 PM

Al is not a molecule, though. And anything made up of the same stuff, like, Al2, would not be polar.

But the answer is no. Al is paramagnetic in bulk. And individual atoms of ferromagnetic metals, like Fe for instance, are not in themselves magnetic, it's a bulk property of magnetic domains, for which there are minimum sizes required. So no.

Regarding thin film coating in general, I suggest you read through this carefully. It's pretty much all there. :)

Spectrum coating description.

Best,
Mark

#46 David L

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Posted 14 July 2008 - 10:51 PM

Okay, I have a question. Meade says that it used MgF2 overcoating on the mirrors in Lightbridge telescopes. I had never heard of using such on reflecting surfaces. If this is indeed the coating on the primary and secondary mirrors, just how good is it for durability?

#47 Cary

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Posted 14 July 2008 - 11:22 PM

Okay, I have a question. Meade says that it used MgF2 overcoating on the mirrors in Lightbridge telescopes. I had never heard of using such on reflecting surfaces. If this is indeed the coating on the primary and secondary mirrors, just how good is it for durability?


Magnesium flouride is used as a transmission coating. I've never heard of nor seen a prescription for Magnesium Flouride over Aluminum. I suspect this is a misprint. Hopefully in another month or two I will have my thin film spectraphotomerter and I can analyze the Meade coatings.

#48 rboe

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Posted 14 July 2008 - 11:41 PM

If memory serves, magnetic material - iron in this case - have their atoms behaving like dipoles. Little magnets. In the bulk material you can encourage most of them to line up to give the whole bulk a dipole - the more atoms aligned the stronger the magnet. Add heat, which juggles the lot up, you create more chaos and destroy the magnet - but not the dipole property of the atom. I want to say it all has to do with the electrons but I'd have dig out the books again which means I have find my reading glasses. :p

#49 mark cowan

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Posted 15 July 2008 - 01:17 AM

Yes, but you can't make a magnetic domain with just one atom. The atoms indeed get their dipole from their electron spin configuration like you say, but from there it goes OT...

See Magnetism on the Iron Range for why magnetite (Fe3O4) is strongly ferromagnetic and hematite (Fe2O3) is only weakly magnetic - magnetism is a bulk property.

Best,
Mark

#50 Mark Harry

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Posted 15 July 2008 - 06:24 AM

"So? That doesn't really amount to any quantitative statement, does it? It's a perfectly true statement but it makes no pronouncement, not even about how large the "chance" of degrading the views through scatter is."
**********
It wasn't meant to be quantitative, Alexis. Just means that your intended use may not fit in with mega-multicoated mirrors providing fuzzy blobs at mid-high powers. 10-15 years ago when I made my own comparisons based on qualitative premises fitting in with -MY- particular use, I noted it wasn't worth the expense when that was kept in mind. And I believe that enhanced coatings are more dependent on the expertise of the one doing it.
As to eyepiece surfaces- well, that's been settled before at least to my satisfaction, but I'm not going into that subject in this thread.
If you -HAVE- some quantitative information you'd like to post, it would be interesting to read about.
Regards, M.


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