Re: [ATM] Mirror Cell suggestions

[Dale Eason <atmpob@yahoo.com>]

No your biggest problem is one you and others keep
ignoring.  Pitch flows it will not keep its shape. 
Outgassing is the least of your problems.

For example a quater placed on top of pitch will sink
into it and the pitch will flow around it. Given time
it will finally end up at the bottom of the pitch. 
The time it takes depends on the temperature.  At 65
deg it will take a few weeks or months.  At 80 degress
it may take a day or two.  For example a small paper
back book fell onto my pitch lap and sat there for 1
week in my 65 deg basement.  It smashed the pitch down
by an 1/2 inch.

When your scope is stored with the mirror pointing up
most all of the pitch will get squeezed out.  When it
is stored with the mirror horizontal the pitch will
flow to the bottom and push the bottom of the mirror
out.



Dale Eason

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

> Thanks for the suggestions and ideas. It seems the
> biggest three problems
> will be the weight, the thermal sink created by the
> heavy support disk, and
> the tendency for the pitch to dry out over time. I
> think I can minimize the
> weight by using aluminum bars partially sunk into
> the cement and extending
> out from the bottom and sides for support and
> attachment to the tube. I have
> partially addressed the out-gassing problem with the
> pitch, by lubricating
> the surface with a layer of oil (which softens the
> pitch considerably);
> however, additional steps in this regard are
> warranted (such as sealing the
> pitch layer). The design is similar to the bubble
> wrap design; however, the
> pitch will stick to the glass and provide most of
> the lateral support with
> its distributed, viscous properties, unlike the
> bubble wrap. Whereas, in the
> bubble wrap approach, a requirement is a very flat
> supporting surface - the
> pitch design guarantees a flat surface due to the
> pitch's ability to conform
> to the exact shape of the underside of the glass.  
> 
> Note that the finite analysis to calculate the
> support points is not
> necessary in my design; since, the pitch provides
> continuous support of the
> glass. For example, in the finite element analysis,
> if you take the limit,
> as the number of support points goes to infinity,
> then you have a continuous
> support system, like that which I'm proposing. The
> benefit of a continuous
> support system is that there are no moving parts,
> minimal edge support (due
> to the viscosity of the support system), thermal
> continuity along the back
> (in other words the contact points are uniform and
> continuous, with only one
> coefficient of thermal conductivity to worry about. 
> 
> Another problem which I overlooked previously is:
> the messiness of working
> with pitch, although it cleans up pretty well with
> gasoline - as I'm sure
> you all know.
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



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