Re: ATM 16" blank thickness extended question

[Ricardo Dunna <rdunna@infolink.com.br>]

  I have a 16" F4.5 (40 cm) only 20 mm thick , a 20:1 ratio , plate
glass . Initialy it was floating over a wood disk and a layer of rubber
foam. No astig can be detected and it was performing very acceptably at
1/6 wave, according to the focault measurements.
  Now it's floating on a 12 point cell designed by Richard, and it's
really better. I'm planning to float my next 40"+ mirror on another 20mm
glass disk surface matched with  mirror's  back , with the rubber foam
between the two glasses.
  If you decide to go to that thick , the only suggestion  I have is to
to use small tools on the last figuring stages, because of unpredictable
behavior of this thin glass with a full sized tool - you can damage a
zone while fixing another.

   There was a thread some time ago with the subject "Encouraging plate
glass usage" - see archives.

Regards

> >...my question is now, how much is the =
> >increase of effort and the risk to end with a not so good mirror when =
> >using thin instead of thick material ?
> 
> My sixteen inch mirror is 1:10 thickness ratio.  That's a thin blank by old
> standards, but by new (at least with ATM'ers) standards that's not really
> very thin.
> 
> I had no problems with astigmatism in figuring the mirror, and it is
> supported in its cell on only six points, with no detectable image
> degradation.  (Based on that alone, maybe we should label 1:6 thickness
> ratio mirrors as "excessively thick."  ;-)