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General questions about glass - FL, ED, HT, APO

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Old Monday 18th November 2019, 17:34   #1
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General questions about glass - FL, ED, HT, APO

Did a little research online last night out of curiosity about my newly acquired FL 8x32 and also out of some boredom.

Why does the Zeiss FL appear to reduce CA better than the Ultravid and EL when these models also contain FL glass in the objective lens?
Does the FL have higher quality FL lens (more Fluorite in it) or is it the 3 piece APO objective lens that helps further reduce CA?

Is ED glass only used in 2 piece (achromatic) objective lenses ? Zeiss FL has only FL glass and no ED glass I believe.
Can you have both?

It seems ED and FL glass serve the same purpose to reduce CA, but is FL considered a bit better , higher quality?

I always thought HT glass was in the objectives, but apparently it's prism glass. When I read up on the Zeiss HT when it was first introduced it just mentioned using Bak4 glass, so I didn't make the connection. It's actually different grade called Bak-4HT glass which produces higher transmission in blue-violet color spectrum. Do I have that right?

Just needing some clarification here.

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Old Monday 18th November 2019, 18:03   #2
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Originally Posted by Gilmore Girl View Post
Did a little research online last night out of curiosity about my newly acquired FL 8x32 and also out of some boredom.

Why does the Zeiss FL appear to reduce CA better than the Ultravid and EL when these models also contain FL glass in the objective lens?
Does the FL have higher quality FL lens (more Fluorite in it) or is it the 3 piece APO objective lens that helps further reduce CA?

Is ED glass only used in 2 piece (achromatic) objective lenses ? Zeiss FL has only FL glass and no ED glass I believe.
Can you have both?

It seems ED and FL glass serve the same purpose to reduce CA, but is FL considered a bit better , higher quality?

I always thought HT glass was in the objectives, but apparently it's prism glass. When I read up on the Zeiss HT when it was first introduced it just mentioned using Bak4 glass, so I didn't make the connection. It's actually different grade called Bak-4HT glass which produces higher transmission in blue-violet color spectrum. Do I have that right?

Just needing some clarification here.
There are SO many things related to optical performance laid at the feet of anti-reflective coatings, glass types, prism types, etc., even when the most elementary answer takes more than a dozen other factors into consideration: number of elements in the objective, air-spaced or not, number of elements in the eyepiece, size and location of the field stop, focal length of the objective, aperture of the objective, and on and on. Some questions can't be answered simply, others simply can’t be answered ... at least when presented in such a way as to thoroughly answer the question.

When Einstein taught: “If you can’t explain it simply, you don’t understand it well enough,” he was speaking to HIS audience, whose understanding was miles beyond our own.

In short, the headbone is connected to the footbone ... with a bunch of stuff in between, without which the headbone couldn’t tell the footbone to move, making walking difficult to impossible.

Bill
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Old Monday 18th November 2019, 18:33   #3
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There are SO many things related to optical performance laid at the feet of anti-reflective coatings, glass types, prism types, etc., even when the most elementary answer takes more than a dozen other factors into consideration: number of elements in the objective, air-spaced or not, number of elements in the eyepiece, size and location of the field stop, focal length of the objective, aperture of the objective, and on and on. Some questions can't be answered simply, others simply can’t be answered ... at least when presented in such a way as to thoroughly answer the question.

When Einstein taught: “If you can’t explain it simply, you don’t understand it well enough,” he was speaking to HIS audience, whose understanding was miles beyond our own.

In short, the headbone is connected to the footbone ... with a bunch of stuff in between, without which the headbone couldn’t tell the footbone to move, making walking difficult to impossible.

Bill
Ok, throw out the question about FL vs ED glass and which performs better.

I'm also trying to learn the basics about what goes where, etc:

Is ED glass only used in 2 piece (achromatic) objective lenses ?
Or, can it be added in the 3 element APO obj lens design ?
Can you have FL lens element and also ED lens element in the same compound lens?

Is the intent of APO compound objective lens to reduce color fringing?
Or, does it have other/multiple purposes ?

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Old Monday 18th November 2019, 18:55   #4
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I got answer to couple of my questions already from this site.


Both achromatic and apochromatic lenses often contain at least one element that uses a special low-dispersion glass. For more take a look at my complete guide to Extra Low Dispersion Glass (ED Glass).

https://www.bestbinocularsreviews.co...binoculars-03/


It appears the APO design is used primarily to reduce CA. I'll read more about it later.

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Old Monday 18th November 2019, 19:09   #5
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The refractive index of glass varies depending on the composition and also differently with different colours. The optical designer can use these to design their systems.
https://en.m.wikipedia.org/wiki/Abbe_number
Chromatic aberration is caused by the different colours getting focussed at different locations, design and choice of glass can help. The following shows the index and dispersion (index variation with wavelength) for the mainly available glass types. ED just refers to glass with a lower dispersion. HT might infer higher transmission, but could be marketing speak, the main losses will be from reflections, unless the glass is highly doped and very thick. FL could refer to fluorite (one possible type of ED glass). The designer is trying to do the best they can with the materials and design options they have available and the budget of the customer. You could make all your optical surfaces horribly non spherical and achieve all sorts of clever designs, but making them to the required tolerances would be challenging. The Nikon WX show what optical perfection looks like, but the cost and the small issue of weight means they have a limited following. Seeing that some designs from 70yrs back still provide pretty decent views, we are well into the land of seriously diminishing returns.... even coatings are pretty close to perfect.

Cheers

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Old Monday 18th November 2019, 19:50   #6
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GG,

While this article is about telescopes I think it describes some of the various objective options also used in binoculars as quite well.
http://www.ianmorison.com/everything...ective-lenses/

David
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Old Monday 18th November 2019, 20:05   #7
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Originally Posted by Gilmore Girl View Post
I got answer to couple of my questions already from this site.


Both achromatic and apochromatic lenses often contain at least one element that uses a special low-dispersion glass. For more take a look at my complete guide to Extra Low Dispersion Glass (ED Glass).

https://www.bestbinocularsreviews.co...binoculars-03/


It appears the APO design is used primarily to reduce CA. I'll read more about it later.
Don’t throw out FL or ED. BUT, consider what each designation means to the observer at-large.

Any of the “glass” types you mention CAN be used in any combination of elements. AND they CAN be used to some extent to correct other aberrations—if the designer chooses to do so. But with the headbone connected to the footbone, all those decision have CONSEQUENCES. In reducing CA to a level accepted by ONE observer—although the magnitude of the aberration might not even be noticed by others—the change of curvatures, spacings, glass types, etc. to make that possible MIGHT adversely affect two other aberrations severely and another a bit.

The overarching use of the apochromatic objective is to reduce lateral color. But please, understand the manufacturer is trying to produce a product to satisfy a target audience. More often than not, those audiences don’t get wrapped up in the quagmire of meaningless questions we do on BF and CN; they just use their bino to heighten the enjoyment of their hobby or craft.

A careful reader of BF or CN can come away with the notion that every month or so there is a development that raises optical quality enough for the “average”—whoever he/she is—observer to notice. If that were really the case, a color-free, astigmatism-free, field curvature-free, binocular would cost $2.18, would weigh half an ounce, although it had a 60mm objective.

The questions for which you seek answers are intelligent, humble, and honorable; you are to be commended.

I believe the improvements made over the last 100 years—that that “average” observer can see, and pinpoint some single cause of the improvement—is more on the order of a half dozen than dozens and dozens—advertising hype and Internet opinions notwithstanding. “Auto-focus,” military, zoom binoculars, anyone?

Good advertising need not be accurate or even meaningful; it need only be believed.

Attached is a drawing from a military study of binoculars. Is it over the top for you? It is certainly over the top for me. Sometimes optical wizards can overlook the most basic of problems with their projects. This is the major reason the US Army’s M-19 bino had millions spent on development but only lasted a very few years. The second graphic is from one of those books I can’t name.

Different tests performed at different times with different subjects having different ranges of accommodation for different visual acuities under different conditions will produce different results.

Bill

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Old Monday 18th November 2019, 20:43   #8
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GG,

While this article is about telescopes I think it describes some of the various objective options also used in binoculars as quite well.
http://www.ianmorison.com/everything...ective-lenses/

David
Thanks David. This was super helpful ! I wonder if the Zeiss Fl's use the synthetic Fluorite ED glass and not the actual Fluorite crystal.
According to allbinos the 8x32 FL uses the 3 piece APO which according to your article above is called a 'true' Apochromatic.

Anyway, as Bill pointed out there seems to be a lot of other factors involved to achieve the desired results.

I'm thinking most binoculars use air spacing between the lens elements these days since I've never heard of oil spacing until reading this ... just a guess there.

Thanks Peter and Bill for your helpful posts.

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Old Monday 18th November 2019, 21:17   #9
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Hi,

ok, many questions here... let me try to answer a few...

ED vs FL vs HT (vs XD vs some other marketing term) glass: The correct term is ED glass (for extra-low dispersion glass). All the other names are just what the marketing department elected to call their kind of ED glass (in most cases the actual type and maker of the glasses used is not discerned for binoculars).
For an idea what ED glass is, have a look at Fig 148 on https://www.telescope-optics.net/apo_refractor.htm - ED glass can be found in the lower left part of the Abbe diagram - please note that CaF2 (calcium fluorite) crystal can also be found in that area although it is not really a glass but a crystal.
All the different ED glasses gain their performance close to CaF2 from large amounts of fluoride ions in the melt...

Is ED glass only used in 2 piece (achromatic) objective lenses ? Zeiss FL has only FL glass and no ED glass I believe.
Can you have both?

ED glass or fluorite crystal is used in both doublet and triplet designs.
Doublet designs with an ED or fluorite element are usually not called achromat any more but ED or fluorite doublet.
Building a doublet objective with two kinds of ED glass is not going to work well - one wants to use a partner glass which has the same (or very close) relative partial dispersion as your ED glass of choice and as large as possible difference in Abbe number (see the green horizontal lines in Fig 148 on https://www.telescope-optics.net/apo_refractor.htm). Two different ED glasses might be on the same line but the difference in Abbe number is small.
Triplet and multiplet designs can be and are being built with multiple ED and/or fluorite elements.

Joachim

Last edited by jring : Tuesday 19th November 2019 at 13:40. Reason: borken grammar
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Old Tuesday 19th November 2019, 06:13   #10
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Hey GiGi - you've had good information in this thread from all contributors. I'm no Einstein but I will try and answer your questions by eliminating most of them :)

As Bill said we are faffing around at the margins here. It is thought that observers are unable to detect differences of 3~6% in practice, sometimes more. Importantly though - the more losses we are able to reduce (ie. the closer we get to 100% transmittance) the less is the amount of 'stray' light (reflected, absorbed, transmuted[wavelength shifted]) that is flying around contributing to glare and reducing contrast. So even though we may be stacking BB's - getting that little frog to take another jump halfway to the wall is worthwhile even though we are now looking at improvements circa ~1% or so at a time.

Optical glass has a variety of properties (check the links peviously provided in the thread). Two important ones are Abbe No. and Refractive Index. These variously specified materials are used in Crown and Flint combinations of almost infinite different design - as Bill said the "headbone is connected to the footbone" .......

Among the various makers of glass they each have catalogue codes relating to the material composition and performance and this (along with Television Display terminology) is where a lot of these marketing terms have come from. Things like ED, ELD, SLD, XD, XLD, HD, HD+, UHD, FL, UFL, HT, UHT, and more !
The Abbe diagram in the Wiki article that Peter linked shows that rather than being distinct (and my dad is better than your dad ! :) type things, they are just glass products on a continuum. They still need to be used properly for the allocated design element they are assigned to.

One of the issues in trying to raise the Abbe No. is that the different atomic elements used in the glass formula can reduce bluish light transmittance more or less depending on manufacture method - cue the group of products called HT glass which can improve performance in that part of the light spectrum by a few % ! This is a noteworthy goal that industry is striving for since the brightness transmission standards have been updated to include slightly more weighting to the bluish part of the spectrum. See the 2005 update to the 1931 CIE standard here: https://en.m.wikipedia.org/wiki/Luminosity_function

Also particular to the Schott HT glass range is that the transmittance values are guaranteed minimums - hence the rather farcical claims for the Zeiss HT binoculars of "up to and more than 95% transmission" etc (although I'm sure it seemed perfectly Germanically Logical to them at the time ! or maybe it was the American marketers ) . Other makers which use HT glass such as Leica UVHD+, NV, and the Tract Toric HT, among others could also make these "up to and more than" claims .......
There are currently 13 Schott HT products:
https://www.schott.com/advanced_opti...ce&lang=french

Proper Fluorite Crystals (CaF2) are very difficult to make and work with especially as the diameter increases. Canon use it, as do a few select Telescope makers. Mostly any "FL" models use the highest grade of synthetic fluorite glass - ie. optical glass 'doped' with fluorite. Some top level Schott (or perhaps even O'Hara FPL-53 or equivalent) FL glass is what your 8x32FL and other FL's use, along with the SF's and other high end 'alphas'. Other budget models use very ordinary ED glass indeed. Not all 'ED' glass is created equal !

This pdf on 'transmittance' is also a good read which goes into 'HT' type glasses.
https://www.google.com.au/url?sa=t&s...qTMDTTy-UxOipk

If not short I hope this was at least somewhat simple and helpful :)





Chosun

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Old Tuesday 19th November 2019, 10:30   #11
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.....

I'm thinking most binoculars use air spacing between the lens elements these days since I've never heard of oil spacing until reading this ... just a guess there.

......
GG,

I've not heard of oil being used in binocular designs either. It is more practical with smallet lenses to cement them instead, but others may know differently.

Joachim,

I don't think Zeiss have ever divulged exactly what they used in the FL, but Gerold Dobler confirmed it has an ED element, and we can be pretty sure it contains a lot of fluorine (Fl) ions as one compound or another.

Thanks to you and the others for some interesting links.

David
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Old Tuesday 19th November 2019, 10:51   #12
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GG,

I've not heard of oil being used in binocular designs either. It is more practical with smallet lenses to cement them instead, but others may know differently......

David
David,

I was reading this, and it occurred to me that the air spacing is as a result of slight differences with the individual lens designs, but with the advent of aspherics what if the adjoining surfaces could be perfectly matched and finished so precisely that they cold be 'cold welded'* with the asymmetric side taking care of the lens function - this could eliminate a few surface to air gaps and might be worth maybe ~0.5% or more increase in light transmission.

* Cold Welding more often used with metal surfaces: https://en.m.wikipedia.org/wiki/Cold_welding
But can also potentially used for Optical Glass Contact: https://en.m.wikipedia.org/wiki/Optical_contact_bonding

Perhaps some of our Physicists, Gurus, and Optical Masters could comment ..... ?





Chosun

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Old Tuesday 19th November 2019, 13:18   #13
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Thanks Chosun, that's very good info. I got some things clarified and it's all a bit clearer now.
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Old Tuesday 19th November 2019, 13:30   #14
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Beth,

If you want to see just how far answers to your questions can go try wading through all three pages of this thread about recent developments in ED glass for high end telescopes:

https://www.cloudynights.com/topic/6...100-and-fdl55/

If we assume that Zeiss used Schott glass in the FL binoculars it's unlikely that the so called "FL" glass has an Abbe# any higher than about 84, since there was no glass type with a higher value in the Schott catalogue in 2004 when the FL binoculars were introduced.

The current Schott catalogue lists only one near Fluorite equivalent glass, N-FK58 (Vd = 90.80). It might be what Zeiss marketing calls "Ultra FL".

The Schott marketing material about N-FK58 in the link below actually has some excellent general information about the role glass types play in correcting longitudinal chromatic aberration.

https://www.us.schott.com/d/advanced...y-2014-neu.pdf

Henry

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Old Tuesday 19th November 2019, 14:00   #15
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I was reading this, and it occurred to me that the air spacing is as a result of slight differences with the individual lens designs, but with the advent of aspherics what if the adjoining surfaces could be perfectly matched and finished so precisely that they cold be 'cold welded'* with the asymmetric side taking care of the lens function - this could eliminate a few surface to air gaps and might be worth maybe ~0.5% or more increase in light transmission.
Hi,

- the term air spaced is usually used to mark designs with a significant air gap between elements (providing the designer with an additional degree of freedom and giving the production engineer a headache due to having to mount the elements to tight tolerances permanently). Stuff like a classic Fraunhofer doublet where there might be a tiny bit of air between elements due to different radii are not usually called that way.

- for small objective apertures two elements with identical (or nearly so) radii can be cemented with either canada balsam or UV cured epoxy. For larger apertures oil is used. In any case this takes away a degree of freedom from the designer since two radii must be identical or very close.

- aspheric lenses in glass are still rare and usually found in high priced refractors and aspherized by hand by a master optician. For plastic lenses, aspheric shapes can be pressed.

- I have not heard about cold-welding lenses.

Joachim
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Old Tuesday 19th November 2019, 14:43   #16
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GG,

I've not heard of oil being used in binocular designs either. It is more practical with smallet lenses to cement them instead, but others may know differently.

Joachim,

I don't think Zeiss have ever divulged exactly what they used in the FL, but Gerold Dobler confirmed it has an ED element, and we can be pretty sure it contains a lot of fluorine (Fl) ions as one compound or another.

Thanks to you and the others for some interesting links.

David
The FL is a glass containing fluorine ions, not the fluorite crystal (CaF2). It appears on this forum in a number of posts.
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Old Tuesday 19th November 2019, 17:07   #17
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Beth,

If you want to see just how far answers to your questions can go try wading through all three pages of this thread about recent developments in ED glass for high end telescopes:

https://www.cloudynights.com/topic/6...100-and-fdl55/

If we assume that Zeiss used Schott glass in the FL binoculars it's unlikely that the so called "FL" glass has an Abbe# any higher than about 83, since there was no glass type with a higher value in the Schott catalogue in 2004 when the FL binoculars were introduced. Schott only recently developed a true Fluorite equivalent glass, N-FK56. That one might be what Zeiss calls "Ultra FL".

Henry
That's interesting. The FL still has a great image. I know they have been updating the coatings over the years. Would upgraded coatings alone change the image from a slightly warm to a more neutral/cooler image?
Early FL's are reported to have slightly warmer tone.
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Old Tuesday 19th November 2019, 17:25   #18
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That's interesting. The FL still has a great image. I know they have been updating the coatings over the years. Would upgraded coatings alone change the image from a slightly warm to a more neutral/cooler image?
Early FL's are reported to have slightly warmer tone.
Hi,

differences in coatings are the main reason for different transmission curves and thus color renditions.

Joachim
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Old Tuesday 19th November 2019, 17:43   #19
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Zeiss found long ago that some of their doublets for photo lenses were so exactly made that the two elements stuck together and could not be easily separated. This is I suppose cold welding. They did not cement these doublets.
But with extreme temperatures I suppose they could become unstuck because of different expansion.

Achromat means two colours brought to the same focus. Presumably red and blue.
With telescope objectives a 3 inch f/9, 4 inch f/12 and 5 inch f/15 are considered achromatic using simple classic glass types.
However Conrady had higher tolerances and for him a 3 inch had to be f/15, 4 inch f/20 and 5 inch f/25 to be considered an achromat. Or longer still.

The large refractors such as the U.S. 36 inch and 40 inch apertures have red and blue focus about 5 inches apart. Yet they still produce outstanding planetary detail. Observers just ignore the CA or false colour.

With ED glass the focal ratio and focal length can be reduced to bring red and blue to the same focus.

Horace Dall's camera obscura objective had two elements of fine glass selected and specified by him.
It was 108mm aperture but f/30 i.e 30 times the aperture focal length. It brought 4 colours to the same focus. In use it was absolutely colour free at 135x with the magnifier on the white viewing table in his darkened loft.
The views at 1 mile were outstanding.

Lichtenknecker used to make 4 element objectives to bring 4 colours to the same focus. But making these is very difficult.

An Apochromat brings three colours to the same focus.
Super Apochromat maybe 4 colours to the same focus.
However, it is doubtful if some modern objectives are true to description.

Some photo lenses had 3 or even 4 cemented elements in one group. These must have been a nightmare to make.

The c.1920 Zeiss Aero triplets (Cooke triplets) eventually had one element split into two, as the three element lens was extremely difficult to make.
There were few skilled enough technicians to make them.
I got the 3 element 170mm plus aperture 1200mm f/7 for Harold Ridley.
It is possible these were aspherised.

I have a truly awful 135mm aperture refractor.
One of the problems is terrible ghosting from the two uncemented elements with the same internal curve.
One of the uses of oil between elements was to eliminate internal ghosting with identical internal curves. Mine has air separation.

With binoculars the objectives are very fast, perhaps f/3.8 and this causes problems.
But the magnification is so low that basically a binocular is two poor telescopes side by side.

Regards,
B.

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Old Tuesday 19th November 2019, 17:43   #20
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Hi,

differences in coatings are the main reason for different transmission curves and thus color renditions.

Joachim
I always assumed this, but never had it confirmed. thanks Joachim.
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Old Tuesday 19th November 2019, 21:55   #21
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You can “optically contact” bits of glass together and it will hold very well if done properly, this is what holds etalons together in solar telescope filters amongst other things. Getting it to work is a “black art”, the surfaces need to be very flat and smooth (same shape) and perfectly clean and then you press/slide/wiggle them together. You can see interference fringes from very closely spaced glass surfaces which then disappear at the contact area.
Had my colleague stick some old “crappy” flat bits together for a test recently. Need very flat (better than lambda/10) and very smooth. I doubt you could polish two non flat surfaces this good reliably. If the composition was different then the thermal expansion would likely mismatch and the joint fail. Mine were fused silica on both sides.

Peter
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general bird iding questions Sinaloa Birds & Birding 9 Tuesday 15th February 2011 00:26
Two general questions. UtahnBirder Information Wanted 3 Thursday 22nd October 2009 01:59
general hello markobrien Say Hello 2 Wednesday 2nd June 2004 13:33

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