Re: [ATM] please help with mirror cell design

[Dale Eason <atmpob@yahoo.com>]

There are so many issues with that design I don't know
where to start.

What is glass speculum?  Speculum is a metal that was
once used to make telescope mirrors until they
discovered glass with a reflective coating is much
better more than 100 years ago.

1. Pitch will flow and so will not stay put. Why
Pitch?  Just because it is a good substance for
polishing mirrors doesn't make a good candidate for a
mirror cell.  In fact just the opposite.

2. It will not provide the even support the mirror
needs even if pitch did not flow.

3. Wiffle trees are easy to make and design using
PLOP.  It is so much easier to make than the thing you
are talking about. 

4. The heavey cement is not any better than any other
flat structure you could make. So why bother?  Just
put it on a board and you will have the same quality.
 
5.  You already listed many reasons why it is not
good.

Dale Eason



Dale Eason

--- Ted Cohen <tcohen@blakeglobal.com> wrote:

> 
> 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.
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



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