Re: Re[6]: ATM 1" thick glass at $450 for 13" plate!?

[spooner@page.az.net (Michael Spooner)]

Richard Schwartz wrote:
>If you go by the Toshimi Taki articles in S & T, 3/4" glass that size 
>is
>very close to the limit for a 9 point support.  It would be fine for an 
>18
>point support if  you want to buy into the extra complication.  If your
>figure is perfect, and your secondary is perfect, and your collimation 
>is
>perfect, a 9 point would be OK with 3/4 inch glass.


Then again, if the figure isn't perfect the 18-point won't make it 
better. ;-) Just finished grinding out an 11" by 5/8" thick (pyrex I 
think). I've been thinking of gluing it to a plywood disk with a ring 
of RTV about 4" in diameter. If I get the figure good enough and if 
the seeing allows problems to show, I'll try a better cell. The only 
mirror my wife ever broke was a 10" by 5/8" pyrex. It had a bunch of 
astigmatism in it. She couldn't figure out why I was happy. (I had 
ground and polished it twice). She's not the only one in the family 
to have broken a mirror, but I ain't admittin' to nothing! I'm hoping 
the astigmatism was from strain in the glass and not to process. 
Wasn't really worried about the mirror mount until I saw this post. 
BTW , how thick are those old Coulter mirrors?

Mike 

"Tenth wave optics, sub arcsec seeing, 50 miles from the nearest light 
and the eyepieces are HOME."