ATM Back Support for Large, Thin Mirror

[PFAFF2@delphi.com]

>  
> From a purely theoretical viewpoint it would seem that an
> inexpensive way to support the rear of a large, thin mirror 
> would be to use an array of diaphragms.  The air pressure 
> would be regulated by a pendulum loaded regulator that 
> would automatically increase and decrease the pressure 
> (to balance the vector of the gravitational force that is 
> parallel to the axis of the telescope tube).  Small clips, or 
> an edge ring, would retain the mirror against falling forward 
> as the telescope approaches the horizon.  These would 
> also define the axial position of the mirror - the pressure 
> being regulated so there was always a small "upward" force 
> on the mirror - lightly holding it against the ring or clips.
> >I do believe such a system was used on one or more of the 
> large telescopes built during the 1960's.  It also used a 
> mercury filled tube to provide an automatically compensating 
> edge support.
> 
> It seems to me that this pneumatic rear support would be 
> much easier to for an amateur to build than the typical 
> multi-levered support system required to provide the large 
> number of supports typically required for a large, thin mirror.
> And the problems of stiction would be reduced if not eliminated.
> 
> Still - I would have each diaphragm support a pivoted triangle,
> so the number of diaphragms required would be 1/3 the number 
> of the support points.
> 
> And perhaps the edge support could also use a pneumatic tube 
> with a second pendulum actuated pressure regulator.  The 
> expense and danger of using mercury would be eliminated!
> 
> Del Stanton  - Burbank, California, USA (Near Los Angeles)
> 
> "sdl20@pacificnet.net"  (Lower case "SDL" followed by numeric "20")
>                         
It seems the first guy to use air cushions to support mirrors was 
Foucault. He did it manually by pumping the air bladder until mirror
would just touch the retaining ring.

An automatic system to adjust the pressure in a bladder support system
 was proposed by Lyle T. Johnson  Fluid Flotation Systems.
in Advanced Telescope Making Techniques Vol 2. It uses a liquid
filled column that tilts with the telescope to change pressure
in the mirror support bladder.

As for edge support to work the fluid in the support hose must be equal
or higher in specific gravity then Pyrex to float the mirror when it 
is tilted.  The specific gravity of mercury is about 6 times greater
then Pyrex so only a hose surface of about 1/6 the thickness of the mirror 
must touch the the edge of the mirror. The hose being a closed tube
system will cause no danger to people. Yeah the hose could break
but so can that fever thermometer you stick in your mouth. It happened
to my daughter about 30 years ago and she is still doing fine.

A pneumatic edge support I am sure would work if have an automatic way to have
increased pressure on the bottom edge of the mirror and decressed
pressure on the top edge. I am not sure this would be a simple
system for the ATM to construct and get working but go ahead
and prove me wrong. They once said thin mirrors would not work. <G>

Bob Pfaff