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Laminated mirrors in use, anyone?

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#1 sopticals

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Posted 15 January 2013 - 09:18 PM

Anyone out :bow: there, that has successfully made a decent mirror, using two or more discs of glass laminated :question: together? If so, what bonding material was used? Thanks in advance.

Stephen. :cool:(44deg.S)

#2 sopticals

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Posted 16 January 2013 - 07:55 PM

Must be someone :question: out there? Im sure on a similar thread in the past there was at least one person that claimed to be using a successful laminated mirror.

#3 starman345

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Posted 16 January 2013 - 08:06 PM

Stephen, are you thinking of adding a layer to your 25" mirror, or is this a new project you are thinking about?

#4 careysub

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Posted 16 January 2013 - 10:12 PM

This thread:
http://www.cloudynig...3828920/Main...

Has this:
"One epoxy I've had my eye on is Masterbond EP30LTE-LO. This is a low CTE epoxy with low shrinkage and low outgassing. The CTE is advertised to be 12e-6 /C and the shrinkage is supposed to be 0.02%. I'm interested in it to bond two zerodur pieces together for our secondary mirror. "

#5 sopticals

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Posted 17 January 2013 - 02:13 AM

Stephen, are you thinking of adding a layer to your 25" mirror, or is this a new project you are thinking about?


Hi Brian, :bow:

Looking at a new :jump: project. Have finished the 25". Will be silvering it myself in a few weeks time, when we in the Southern Hemisphere, get some dark sky :roflmao:back, when summers gone. May actually silver one of my 22"ers first though as it will be easier to manage as a first time (for :grin: me) silvering.

Stephen.(44deg.S.)

#6 sopticals

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Posted 17 January 2013 - 05:22 PM

This thread:
http://www.cloudynig...3828920/Main...

Has this:
"One epoxy I've had my eye on is Masterbond EP30LTE-LO. This is a low CTE epoxy with low shrinkage


Hi Careysub, :bow:

Thanks for the input. Have been in contact with Masterbond, and looking globally for suitable glass bond substances with the right CTE. Not concerned about the out gassing issue, as I will be :)silvering.

Stephen.(44deg.S.)

#7 Ajohn

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Posted 23 January 2013 - 06:26 AM

There used to be some small cemented apo refractors about but I understand that the size was limited to under 3ins due to shrinkage of the cement deforming the glass. Also as sizes go up differential expansion rates breaking the bond.

It's a subject that interests me as I like the idea of a glass disc that is more or less self supporting. One idea I have which may work out is to grind a series of pockets into 2 discs made out of the same glass and bond them together with "pegs" leaving a space in between the discs. It might be possible to make the pegs out of borosilicate laboratory tubing for instance. Problem - what to use to bore the holes in the glass discs and how thick could the the bonding be?

I feel that this could result in a very rigid light weight structure that could reach the same levels of performance as old style full thickness blanks.

John
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#8 careysub

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Posted 23 January 2013 - 11:35 AM

There used to be some small cemented apo refractors about but I understand that the size was limited to under 3ins due to shrinkage of the cement deforming the glass. Also as sizes go up differential expansion rates breaking the bond.

It's a subject that interests me as I like the idea of a glass disc that is more or less self supporting. One idea I have which may work out is to grind a series of pockets into 2 discs made out of the same glass and bond them together with "pegs" leaving a space in between the discs. It might be possible to make the pegs out of borosilicate laboratory tubing for instance. Problem - what to use to bore the holes in the glass discs and how thick could the the bonding be?

I feel that this could result in a very rigid light weight structure that could reach the same levels of performance as old style full thickness blanks.

John
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How about simply making the entire back out of a big bundle of tubing fused/cemented together?

#9 careysub

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Posted 23 January 2013 - 12:51 PM

How about this idea to provide the entire backing of the thin mirror:

Stack Pyrex tubes in a hexagonal grid touching each other with an adhesive binder (epoxy say) that either does not change dimensions at all or shrinks slightly as it sets. Epoxy normally shrinks as it sets/cures, if the usual amount is too much this can be controlled and no-shrink epoxy is available. Other resins/binders can be used of course.

As the mirror cools below the binder curing temperature the Pyrex (CTE 3.3x10^-6/K) will not shrink much; the binder (if regular epoxy CTE 55x10^-6 or thereabouts) will shrink much more and put itself under tension.

But the tubing is in a maximum density space-filling configuration so it cannot pack more tightly together, there is no change in area of the grid other than that due to the glass tubing contraction, matching that of the mirror optical surface.







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