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Equipment Discussions >> ATM, Optics and DIY Forum

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mconnelley
sage


Reged: 03/14/06

Loc: Hilo, HI
Re: Carbon fiber primary mirror new [Re: refractory]
      #3800646 - 05/12/10 02:08 AM

Hello:

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.

Cheers
Mike Connelley


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TeaDwarf
member


Reged: 02/20/10

Loc: Oxon, UK
Re: Carbon fiber primary mirror new [Re: mconnelley]
      #3801381 - 05/12/10 12:40 PM

There is on-going work in this field -- stiffness is the main concern, rather than outright weight (obviously the two are related).

e.g.;

http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=OPEGAR000046000009094003000001&idtype=cvips&gifs=yes&ref=no


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salies
member


Reged: 08/10/09

Loc: catalogne france
Re: Carbon fiber primary mirror new [Re: TeaDwarf]
      #3807570 - 05/15/10 06:31 PM

hello! I am in my 3e test on a hybrid mirror glass and Carbon.
the main challenges is to find a bubble-free bonding
for now, I pray for a lot of tinkering, but it allows me to power controller that the bonding does not move in time, it is as well!
http://forums.futura-sciences.com/materiel-astronomique-photos-damateurs/259283-construction-dun-600mm-transportable.html


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jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Carbon fiber primary mirror new [Re: salies]
      #3807836 - 05/15/10 09:29 PM

In bonding glass to graphite you now introduce the issue of differential coefficients of expansion (same principle that allows a bimetalic strip to measure temperature). If these coefficients are not identical - or very close - you would get optically significant distortion of the surface with temperature changes.
As for Dream blanks - these do seem to cut to the chase! I am toying with the idea of a cellular mirror for a fast 22"-24". What I am not clear on, however, is who figures their blanks? I wonder if the issue of print-through comes up for opticians who are used to monoliths?
One construction point: Cellular blanks, being lighter, also favor large dobs, where a shorter truss assembly yields a shorter lever to oppose the moment arm at the mirror box.
Joe


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Owen
professor emeritus


Reged: 06/21/07

Loc: New Zealand
Re: Carbon fiber primary mirror new [Re: jpcannavo]
      #3808230 - 05/16/10 04:32 AM

Hi Joseph,

I suspect print through is common, when the tool grinding is of significant weight - the thin unsupported glass deforms to 'avoid' the grinding, so slow and gentle, should avoid some of this...

Owen


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Reckedog
newbie
*****

Reged: 02/06/07

Loc: San Juan, PR, US
Re: Carbon fiber primary mirror new [Re: Owen]
      #3809092 - 05/16/10 04:51 PM

Hi,

Some one with no heavy lifting in mind works here:
http://www.compositemirrors.com/optics.html

Salvador


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polaraligned
professor emeritus


Reged: 12/26/08

Re: Carbon fiber primary mirror [Re: jpcannavo]
      #3828920 - 05/25/10 09:29 PM

Quote:


As for Dream blanks - these do seem to cut to the chase! I am toying with the idea of a cellular mirror for a fast 22"-24". What I am not clear on, however, is who figures their blanks? I wonder if the issue of print-through comes up for opticians who are used to monoliths?

Joe



Hi Joe,

Print-through is a moot issue when a Dream blank is in the hands of a competent optician. They are engineered for absolute minimal displacements when polishing. They use a complex series of sub-ribs of varying sizes that support the face between the main ribs. I think my .71m Dream mirror has 6 different size sub-ribs. Here is a link to a discussion on print-through: Print-Through Note Mike Lockwoods comments about Dream's cellular mirrors. Speaking of Mike, you asked who figures Dream's blanks, well, Mike Lockwood is the one optician that I would recommend. He does custom work, is reasonably priced and without a doubt the most talented optician for the amateur/pro market. Look on Mike's website also. He has some good things to say about Dream's blanks. Mike has completed a number of mirrors using Dream blanks and has the experience to give an honest opinion of them.
The other advantages of Dream cellular are huge, especially thermal tracking of the ambient. Take a look at their website. Thermal Tracking Shane is a great guy to deal with, friendly and helpful. Give him a call if you are serious about that 24". My experience with a .71m and a 9" m.a. elliptical diagonal left me with no regrets. Both were designed and cast in a matter of weeks, not months or years. Good Luck.

Scott


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