From: "Bob May" 
To: , 
Reply-To: "Bob May" 


You need to go find the places that service the industrial/commercial glass
needs.  they will have glass in thicknesses up to 25mm.  for a 6",
13mm can be used if needed but thicker would be wise, especially for a
first mirror. "Full thickness" for a mirror is 1:6 for a
traditional ratio and about 1:10 for a more modern ratio.  After you get
used to the processes of grinding and polishing and do a good optic, you
can then try ratios of as much as 1:20 if you watch for astigmatism. 
Ratios even higher than that have been successfully done by slumping the
glass in a kiln to the approximate ROC of the mirror and then grinding out
the errors of the sphericity and then polishing the surface.  Astigmatism
is a real problem as well as deformation from pressure so this sort of
stuff is really in the pervue of those that really want a serious challenge
to their optician techniques. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

--- BBBS/NT v4.01 Flag-4