Re: [ATM] please help with mirror cell design

["Anthony Owens" <towens@shibumi-consulting.net>]

Ted,
There is lots of established practice in the mounting of large precision
optics and successful reinvention of the wheel is perhaps more effort than
it is worth.  The closest analogue to what you are proposing might be
attachment to a whiffletree using blobs of humble silicone sealer (aka RTV
silicone adhesive) which works fine if done to a specific design tailored to
the size and shape of your mirror.  The whiffletree can be simplified if you
like by replacing the joints between movable pieces with larger blobs of
silicone.
I would advise avoiding large thermal masses anywhere in an astronomical
telescope by minimising structure.

An alternative approach which avoids a surprising number of the issues
involved is hub-mounting which may still be possible for you depending on
how far your project has gone.  Bob Royce has some good material on his
commercial website about how this works.  Tom Krajci reviewed the issues on
an excellent web about scope engineering at
http://www.atmsite.org/contrib/Krajci/cell-bearing.htm.  I have some
relevant material too, at www.
shibumi-consulting.net/primary_mirror_mgmt_casestudy.html

Hopefully some of this material will be helpful

Tony Owens

-----Original Message-----
From: Ted Cohen [mailto:tcohen@blakeglobal.com] 
Sent: 29 March 2008 00:31
To: atm@atmlist.net
Subject: [ATM] please help with mirror cell design


Dear ATM List Members, 
 
I'm hoping you can provide some feedback on the following situation
regarding mirror cell design.  - Thanks, Ted.

I?m working on a 23? thin plate glass speculum and don?t think I have the
skill (or inclination) to make the required whiffletree, so I have been
brainstorming for alternatives. The most economical option, with a
reasonable chance for success, I believe, is the following. A steel (or
aluminum) reinforced concrete (or cement) circular disk 26? radius perhaps
2? ? 3? thick, with a 1? raised edge on one side. In order to mount the
glass onto/into the disk, I would first apply a thick layer of soft pitch,
into which an impression of the glass would be sunk. Finally, a little oil
would be rubbed onto the pitch for a bit of extra lubrication and softness.
The mirror could then be placed into the formed, oiled pitch. The mirror
would thus ?float? in the oiled-up pitch. I would then have to figure out
how to secure it down safely against accidental tip over. 
 
I can see two drawbacks. One is the lack of air flow around the underside of
the glass. However, I believe this is mitigated by the thinness of the
speculum, which is only ¾ - inch at the edge. Also, the pitch should provide
some measure of thermal insulation from the cement or concrete plate. The
other drawback is the weight of such a disk, which may be 80 to 100 lbs.
this is mitigated by the light weight of the glass, and a sturdy, segmented,
OTA. 
 
The benefit of this design, I think, is the rigidity of the reinforced
cement plate against flexure, the lack of thermal expansion/contraction
forces on the glass due to the soft pitch supporting structure, and the
maintenance of nice dynamical equilibrium between the glass and the pitch,
The ability to remove the glass from the pitch (with great messiness), and
the ability to add more or less oil to the pitch if considered necessary.
 
Does anyone have any feedback on whether this sounds reasonable?
 
Thanks, 
Ted.



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