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Re: [ATM] Best locations for 4-Pt. Wiffletree edge support PLS.
Bob,
I think you are pushing the limits of knowledge and
experience ahead of what most on the atm list know.
There is another important list on yahoo groups called
atm_free. James Mulherin of OMI Torus watches and
replies on that list. He makes mirror for armatures
and big professional scopes up to about 32 inches. He
could probably answer your question much better. He
also has a yahoo group called Torusmirrorgroup.
Try those places.
Regarding the 45 degs. The theory is that places the
lateral support points at 90 deg to one another.
Supposedly for altaz mounts that helps keep the mirror
from folding over on itself much like adding two folds
90 deg apart on a piece of paper.
Besides keeping the mirror on the cell the lateral
support should not induce any deformation into the
mirror for side forces. One way to do that is to
apply the force at the center of gravity of the edge
of the mirror. If that is not done then the edge will
be pushed up or down compared to the center of the
mirror. It is dramatic for a thickness ratio mirror
like yours. Nils Olof Carlin has done some research
with PLOP in this regard. PLOP is not designed to do
lateral support simulation but Nils Olof was able to
push it a little.
The cell must be designed so that collimation does not
move the mirror in relation to the lateral supports.
For altaz a sling can do the job however it can be
difficult to position it exactly on the edge center of
gravity. A single slings don't work on a EQ mount
however.
Dale Eason
--- Bob Drew <gofast3@hotmail.com> wrote:
> Mike,
> Some time ago a ?conventional? mirror had a 6:1
> diameter thickness ratio.
> It was thought that the mirror would be inherently
> stiff enough to hold its
> own figure.
> I assume by ?standard? mount you mean three edge
> supports spaced at 120
> degrees? This also works for equatorial mounts where
> the tube orientation
> changes.
>
> Mirror diameters climbed, usually sourced from 2?
> sheet pyrex. So the
> thickness ratios where getting pretty large. Now, a
> 10 to 1 ratio is
> considered conservative. With atms using even
> thinner material for even
> larger mirrors, more attention was being directed
> to trying to control
> flexure.
>
> The center of that large mirror with a short focal
> ratio would also get
> quite thin in the middle. For example, the center
> would be ½ inch thinner
> on an F4 32? diameter primary.
>
> Good works have evolved for supporting the back of
> the mirror while at
> zenith. ( Plop etc. ).
>
> The challenge now is also to properly support the
> lower edge of a thin
> mirror as it approaches the horizon. The Gemini
> telescope uses 120
> actuators [ plus air pressure!] for the back, and
> 72 hydraulic actuators
> supporting the edge. ] Didn?t some observatories
> encircle their mirrors
> with a tube of mercury early in the last century?
>
> In your Alt/Az scopes amateurs have been using
> slings with good success,
> with some design evolution lately. There are some
> excellent posts with
> pretty esoteric math [over my head] explaining the
> results of forces acting
> on that poor, thin mirror, as well as selection of
> materials for the sling
> itself.
>
> With a 200 lb mirror, it must be very sturdy to
> locate the mirror, so I'll
> start conservatively. Given the required strength
> of the fabricated
> components, for now, I would prefer not to enter
> experimental territory
> with a sling for this weight of mirror. ( Altho I
> suspect that a solution
> may lie there ). Trying to avoid an expensive
> re-do.
>
> That is why I am seeking some input from those
> mechanical engineers and math
> genius? and atms on where they would place the 4
> support points on the 2
> pivoting arms?
> bob
>
>
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