From: "Kreig McBride" 
To: "Bob Walker" 
Cc: 
Reply-To: "Kreig McBride" 


Bob,

Anything is possible.  If you slump the glass and carefully and properly
anneal the glass, then you are on your way.  Support is the big issue, both
in fabrication and in use.   This is a big project requires enormous
amounts
of commitment and probably a group effort.   I have a 38" x 3/4"
and a 48" x
1" glass for such projects but found the problems and solutions beyond
my capabilities and time.  ATMs are often too far apart to make a group
effort successfull.

In my case i needed to invest money into making a large kiln for slumping.
testing is required and i slumped three blanks.

14"    which slumped fine but the form/ mold was distorted.

14"    which cracked in half when the glass was being brought up to anneal
temperature.

26"    slumped and annealed perfectly.  Examination showed no stress, but
the blank cracked at a later time, possibly from a very brief exposure to
sunshine during transportation.

The risks in such projects is very high.  The failure rate also quite high.

Making up 5 - 12" scopes is about the same amount of effort and work
but little risk and you have 5 telscopes.

Or you can push the limits, and accomplish something very special.

Kreig McBride
telescope optics workshop
Bellingham, WA
www.wwu.edu/~skywise/optics.html



-----Original Message-----
From: Bob Walker 
To: ATM Group 
Date: Tuesday, February 25, 2003 3:41 PM
Subject: ATM Too big, too thin?


>
>I have just obtained an interesting piece of glass.  It's nice plate glass
>36" in diameter and 3/4" thick with a well finished edge ...
okay, it's a
>table top.  The question is, what should I do with it???  The obvious first
>choice would be to make a 36" mirror out of it.  From my readings on the
>List and elsewhere, I tend to think this is impossible.  But, the hope that
>springs eternal and all that impels me to ask, has anyone tried it?  What's
>the biggest 3/4" mirror you know of? (Successful one, that is!  Mel?)
>Another intriguing thought is grinding four 14" or so off-axis mirrors out
>of it.  That has the advantage of being feasible, at least marginally so.
>Then, of course, it could just be chopped up into a number of small
>blanks -- seven 11 inchers or five 12 inchers plus a 10" ... etc etc.  Any
>suggestions on practical ways to proceed will be welcome.  I've got plenty
>of time to mull this over; I don't expect to have my current projects
>completed for a year!
>
> *  Best regards, Bob
>*
>         *           *   *
>
>            *         *
>
>

--- BBBS/NT v4.00 MP