Re: [ATM] 40" mirror some ideas

[Jeff Anderson-Lee <jonah@eecs.berkeley.edu>]

Jean-Guy Moreau wrote:

> Like this ?
>
> http://www.cablevision.qc.ca/astro/alex-36ins.jpg
>
> 36" dia. x 5" thick, 120 lbs, faceplate to f/4.5, flat back, homemade.

Jerry,

I'd seriously suggest following up with Jean-Guy.  He's done some work 
that seems to me to be very substantial on how to make fused blanks.  
While he uses plate glass I'm sure the same techniques could be applied 
to pyrex.  If I recall correctly, the face plate on his 36 inch mirror 
was only 1/2 inch thick and yet the whole mirror blank had the stiffness 
of a full thickness mirror with far less weight and no print through 
from the ribs.  (He can support it on a single central post for testing 
with no sign of deformation!)  By slumping the front plate first then 
adding the posts and back plates you avoid many of the problems of 
casting a thick blank with honeycomb inserts.  Plus you have the 
opportunity to force air between the surfaces for more rapid cooling and 
better thermal tracking.

If I every do something larger than my 16, I'm seriously considering 
this approach, using glass table tops for the initial round glass.  I 
know you've been burned by thin plate glass, but by adding the support 
posts it actually becomes a whole new ball game engineering wise.  This 
might not appeal to you as is, but (a) it can be way to prototype your 
large mirror and (b) the same techniques can be applied to thicker pyrex.

Jeff

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