Re: [ATM] Ball bearing for triangles

[Mark Suchting <masuch@cia.com.au>]

At 07:46 PM 9/2/2004, Raphaël GUINAMARD wrote:


>What do you mean by "the extra triangle boundary area  for a larger support "


Hi Raphael

On a triangle that is not equilateral but scalene ,the balance point 
changes from the theoretical computed point very rapidly  when you add 
extra even width around w the three support points ( as is normally done )

On the 36" I was involved in , the large bottom triangles needed about 1/2 
kilo of weight on outside  edge to balance them otherwise the inner three 
triangles on the upper layer will be pressing   more or less than the edge 
triangles.

I would be much more suspicious of the cell problems than temperature 
effects. 40mm is really not that thin for a 24" . I have worked 45mm  thick 
and found no special problems.  I am guessing that David Vernet has done 
your mirror ?  He has made quite a reputation in making high quality fast 
mirrors.

  The mirrors you give example of effects due to different edge/center 
thickness were probably plate glass.  I remember someone very experienced 
with experimenting with large thin plate glass mirrors told me of the 5/8 
rule. Once the center thickness is less than 5/8 the edge thickness, 
thermal problems will make the mirror mostly impossible to use.    I don't 
think you will have so much problem with a Pyrex mirror. Do you have a fan 
fitted to help cool the mirror more evenly .

Mark Suchting




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