From: Guy Brandenburg 
To: scottythefiddler 
CC: Ray Desmarais , Kreig McBride ,
 atm{at}shore.net
Reply-To: Guy Brandenburg 




scottythefiddler wrote:
> That does it....I have long had the notion of building a large thin mirror
> and supporting the bulk of it's weight on a bladder of air.  i.e. a
> semi-deflated 'balloon', with a reservoir of extra air in a bellows being
> pressurized by a suspended pendulum type weight.  As the scope is tipped
> toward the horizon, the pendulum puts less weight on the bellows so that the
> mirror is not lifted off the collimation points....anyways, thanks for the
> encouragement.  The time has come to try to make this idea work.

The time has come? It's already been tried, quite successfully, by none
other than Leon Foucault in the late 1850's and early 1860's. His mirrors
were very, very thin, also - but not as big as the monsters being made
today. He made a rubber air-filled bladder that supported the primary
mirror from behind, with a valve and a long stem that the observer
(himself, usually) could blow into and inflate, or else let the air out of
an deflate, as conditions warranted and as the angle of observation
changed. Apparently it helped a lot.

I learned this from the book on Foucault that was written by the English
bloke who is living in New Zealand and has been translated into French and
published, so far, only in that language. Good book.

> There is the possibility of developing a system which offers very nearly
> equal support over the entire surface of the back of the mirror when one
> considers floating the glass on air.  It seems to me that minimum of 3 rigid
> support points are required for the purpose of collimation.
> Further, some allowance must be made for the obvious changes in the volume
> of air within the support bladder, with changes in temperature.  I think
> that  it is possible to support perhaps 95% of the mirrors' weight on air,
> and the remainder on rigid collimation points.  If the weight of the mirror
> is known, then a weight with a known moment of force can be adjusted to bear
> on a bellows type reservoir to provide the correct amount of air pressure to
> maintain a level of support which changes as the altitude of the scope
> changes.   It is possible...is it workable?   I think that a persistent
> person (though, not overly talented) could make it work.   (I just happen to
> know such a person...)
>
> see ASCII schematic below  (not drawn to 1/4 wave)    Top of scope tilts to
> right to view horizon, and weight (at left side of drawing) provides less
> force on bellows, preventing mirror from lifting off of collimation points.
>
>
>
>                      weight
> __________________________
>                     -------
> |                mirror                         |
> +------------|         |
> |_________________________|
>                     -------
> ____________________________
>                           air supply to bladder
> (                                                        )
>                     <
>
>>--------------------------------------(
>
>                     )
>                     <
>
>>--------------------------------------(
>
>                   )
>                     
> (____________________________)
>                     bellows
>
>
> The question that remains in my mind is:  do I need to grind the back of the
> mirror, so that the glass has a meniscus shape, or can I leave it flat.  It
> appears that to me, that a bladder will exert an equal upward force on each
> part of the mirror?  If this is so, and if I leave the back of the mirror
> flat, will it not have the effect of raising the thinner center area?
>
> Input most welcome and requested
> .
> Thanks Ray, for a thought provoking and encouraging discussion.
>
> Scott Donaldson
>
>

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