[ATM] Best locations for 4-Pt. Wiffletree edge support PLS.

["Bob Drew" <gofast3@hotmail.com>]

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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