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Dielectric coatings on mirrors

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#1 azure1961p

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Posted 20 February 2013 - 11:22 AM

I'm told the 98% reflective coatings are dielectric and that while small mirrors like secondary and diagonals are ok for this, that large glass suffers from the coatings actually creating tension that distorts the glass ruining the figure. Slight but detrimental. Still at least one mirror coating fella sais that's not the case and its fine.

Would the 98% (assuming it s dielectric) distort my 8" mirror ?

Thanks in advance guys.

Pete

#2 mark cowan

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Posted 20 February 2013 - 02:43 PM

Dielectric coatings are not used on primaries, due to the stress they can impart and that they can't be stripped chemically. Dielectrics for secondaries are optimized to work at 45 degrees, and to work at 90 degrees requires different thicknesses on the multilayer stack making up the coating. It can be done but it's very expensive.

Enhanced coatings for primaries aren't dielectric, they enhance reflectivity over the underlying aluminum layer (which can be stripped) using multiple layers of various materials on top. In the past though some enhanced coatings used a chromium base coat which has to be physically removed to recoat.

No, such a coating, properly applied, won't distort any primary.

Best,
Mark

#3 David Knisely

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Posted 20 February 2013 - 03:10 PM

All of the high-reflectivity mirror coatings use thin multi-layer overcoats of specific dielectric materials on top of a metal coating to get enhanced reflectivity over a regular metal only coating. In the older coatings, the substrate had to be heated to a rather high temperature before the coatings were applied, and this could induce stress into the system. However, the newer coatings use ion-beam technology to deposit these coatings (no substrate heating needed), so they don't generally induce stress on the mirrors. Clear skies to you.

#4 denis0007dl

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Posted 20 February 2013 - 03:47 PM

Go ask Jeff Decker on info@majestic-coatings.com

I ak him once:
"I want to recoat my 10" f/5 telescope mirror, so how much it would cost?"

answe was:
"The cost to remove the old coating and recoat with 99.999% ultra-pure protected aluminum (secondary done at n/c) is $125.00, return shipping not included. Turn around is ~ 1 week. I cannot change the surface of the mirror.



Preferred payment method is a check or M.O. made out to Jeff Decker.



For shipping I would suggest a DOUBLE BOX method………wrap the glass in bubble wrap or any kind of packing material…….put the mirror(s) in a box with more packing material, seal the box and put it in a 2nd box surrounded by packing material so that the 1st box “floats” inside the 2nd box. I have used this method for years and never had a problem.

If you have any questions or need more info let me know.



Majestic Optical Coatings

152 Willow Way

Clark, NJ 07066"


Hope this helps!

#5 mark cowan

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Posted 20 February 2013 - 06:11 PM

All of the high-reflectivity mirror coatings use thin multi-layer overcoats of specific dielectric materials on top of a metal coating to get enhanced reflectivity over a regular metal only coating.


I assumed the OP was referring to pure-dielectric coatings, which can be many more layers, and much thicker as a result, than the several interference layers applied to an Al base for an "enhanced" coating.

See http://www.spectrum-...m/telescope.htm for a full explanation. :)

Best,
Mark

#6 azure1961p

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Posted 20 February 2013 - 09:57 PM

Thanks very much guys! A lot of clarification was had and Im grateful for the knowledge. I didn't kno if 98% was exclusively dielectric where by I had reservations . That they aren't dielectric primaries with warped figures is good news.

Again thanks.

Pete

#7 Starman1

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Posted 21 February 2013 - 02:06 AM

The reflectivity of dielectric coatings depends on angle of incidence.
Since the incoming wave hits the primary mirror at a variety of angles (flat in the center and angled at the periphery), the nature of the coatings would have to be continuously variable from center to edge, That's not beyond the possibility of technology, just beyond the size of our wallets.
Hence, primary mirrors will not have more than 3-5 transparent dielectric-coated layers on top of the aluminum, bringing 550nm reflectivity to from 93-96%. Even the standard dielectric layer of SiO on top of the raw aluminum brings 550nm to 91% reflectivity.
Secondaries have similar angles of incidence to star diagonals, though star diagonals are usually used on slower scopes with narrower light cones. hence, dielectric-coated mirrors are more appropriate in refractors and Cassegrains than as secondary coatings in a short f/ratio newtonian.
The issue with off-axis rays is that dielectric coatings have different spectra of reflectivity depending on angle of incidence.

#8 azure1961p

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Posted 21 February 2013 - 08:59 AM

Had no idea that was the case. Again my thanks - and help on the 32's.
We ll be talking.
Pete

#9 mark cowan

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Posted 21 February 2013 - 12:32 PM

Since the incoming wave hits the primary mirror at a variety of angles (flat in the center and angled at the periphery), the nature of the coatings would have to be continuously variable from center to edge, That's not beyond the possibility of technology, just beyond the size of our wallets.


Could you please cite a source or further documentation for this interesting argument?

Thanks,
Mark

#10 Starman1

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Posted 21 February 2013 - 03:28 PM

Mark:
1) http://en.wikipedia....Dichroic_filter
See "disadvantages"--the last note.
2) http://www.cvimelles...ence_Filters...
See "Wavelength Dependence on Angle of Incidence"

You can also see it quite well for yourself, and easily.
Take a nebula filter and look at the sky first straight through and then again at a 45 degree angle. Notice how the color changes radically with angle.
Now, the incidence angle change in a telescope is merely a couple degrees, but it does shift the bandpass in the spectrum and can admit other wavelengths not passed at a perpendicular angle of incidence.

So the reflectivity of dielectric-coated mirrors is optimum at one angle. The people at Lumicon once showed me that their dielectric star diagonal reflected a high percentage in the visual band at 45 degrees, but at 30 degrees had a huge peak in the yellow. That meant when you picked up one of these diagonals and looked through it, there was a lot of yellow light bouncing around inside the diagonal. Some people thought this meant the mirror had a bad coating. Except at night, the light didn't exhibit this yellow peak at all because light didn't enter the diagonal 15 degrees off axis.

Now on a primary mirror, the angle of incidence of the axial ray changes from perpendicular to the surface in the center of the primary to a tilt of several degrees at the edge of the mirror. That angle of incidence is equal to the angle of incidence from the focal point to the edge of the mirror, and obviously is less on a long f/ratio mirror than on a short f/ratio mirror.

But the interference nature of the dielectric coatings means a change to the spectrum of reflectance at the edge than in the center IF the coating is exactly the same in both places. At the very least it would mean a shift in the bandpass of reflectance which could, theoretically, chop a bit from one end of the visible spectrum or the other.

#11 mark cowan

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Posted 21 February 2013 - 03:59 PM

Yes, a pure dielectric coating on a old damaged secondary(*) I have actually passes light ranging from white at 45 degrees progressively from red to purple to blue at 90 degrees. The complement in reflection should be yellow.

And this effect should also apply to the dielectric interference layers on an enhanced coating, which may be another reason to only recommend standard coatings (which is what I do anyway). Any loss in coating reflectivity can easily be overcome by spending the enhanced coating costs on a slightly larger primary. :cool:

Certainly a poorly applied enhanced coating can show all kinds of color fringing as the thickness of the dielectric layers varies across its face.

It's not something I'd actually considered before, so I appreciate the info.

Best,
Mark

PS (*)And to the OP - that secondary was perfect when sent for coating, but the coating itself had blobs on it and the mirror acquired a couple of pits in the same area. The coater denied any mistakes, claiming it must have been that way originally. The stress that the coating set up on the mirror eventually caused it to crack internally. Whether there would be negligible strain with a ion-assist deposition dichroic coating is something I don't know, as I've never had another mirror coated that way and likely never will. I know of another optician who has a large collection of similarly damaged secondaries... :(


#12 azure1961p

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Posted 22 February 2013 - 12:31 AM

Well ok but I can get a 98% enhanced coating that's non dielectric yes? I'm not understanding why a guy would ruin secondaries if the same light reflectivity was had sans the stress.

Why is dielectric around at all if its so temperamental?

Thanks!

Pete

#13 Starman1

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Posted 22 February 2013 - 01:59 AM

Reflectivity at 550nm:
raw aluminum--89%
overcoated aluminum--91%
semi enhanced coatings--93%
full enhanced coatings--96%
dielectric coatings--98%+
Of course, reflectivity at 400nm and 750nm is much lower.

#14 azure1961p

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Posted 22 February 2013 - 12:43 PM

ahhhhh ok thanks for the clarification. For the record though Spectrum Coatings is claiming 98% for ion deposition.



Pete

#15 mark cowan

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Posted 22 February 2013 - 03:27 PM

Well ok but I can get a 98% enhanced coating that's non dielectric yes? I'm not understanding why a guy would ruin secondaries if the same light reflectivity was had sans the stress.

Why is dielectric around at all if its so temperamental?

Thanks!

Pete


Because the 98% coating is not pure dielectric with 50 plus layers, it's aluminum with maybe a half dozen dielectric layers on top. Also, as Don mentions, the stress from dielectric coatings may be due to the way they are/were applied and this may not be a factor with ion assisted deposition (where the energy of the ion beam is used to hammer down the atoms of the coating, rather than relying on thermal motion to lock them into place).

I wouldn't worry at all about having an enhanced coating applied by a knowledgeable coater. Although I don't recommend them to people it's what I prefer for my own 'scopes. ;)

Best,
Mark

#16 azure1961p

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Posted 22 February 2013 - 04:49 PM

Thanks Mark. I'm guessingy coating as they came from PARKS was the 91% variety. I just don't know. I recall a gentleman over the phone one day mentioning they weren't the most reflective but that they were very durable and long lived. I'm guessing it was standard coating of the 90s - whatever that percentage was. I kno it wasn't special or enhanced per se.

Pete

#17 Starman1

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Posted 23 February 2013 - 12:01 PM

What is generally known among filter producers and dielectric-coated mirror makers is that, despite the best efforts of the coaters, the perfect thickness of the coatings only occurs in the center of the coated surface and tends to roll off at the edges.

Since dielectric coatings function as interference filters, the angle and thickness of the coating makes a big difference in the coating quality.

Filter makers coat a number of filters at the same time, and the ones at the edge of the batch are never as uniform as those in the center of the coating chamber.

So, in a star diagonal, using an oversized mirror is a way around the non-uniformity of the coatings.

So why are dielectric coatings so popular?
--harder than typical front surface aluminum coatings so scratch less easily.
--resist oxidation better than aluminum or silver coatings, so last longer
--higher reflectivity than aluminum, and even silver if done right.
--scattering suppression at low angles of incidence. Significant wave cancellation occurs at low angles, reducing light scatter from environmental sources.






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