Re: [ATM] practical limits of 1 inch mirror

["Olivier Biot" <olivier.biot@gmail.com>]

Fellow ATMers in Belgium use borofloat on a regular basis for larger
mirrors since regular float can only be found in thicknesses up to
18-19mm. We're using 18-19mm float glass for 10" mirrors. Once you go
above 12" most prefer to go with borofloat due to its added thickness.
I know of at least three 20" mirrors currently under construction (one
still rough grinding, the other 2 already in polishing) that were made
from 25mm borofloat. The tool was made from two (much cheaper) 18-19mm
float glass disks glued together with a bonding agent based on RTV
(silicone).

All these mirrors I know of are ground and figured by hand.

Regarding annealing, it appears that we have access to very high
quality float and borofloat glass over here, nearly making a polarizer
obsolete. We don't need to anneal our blanks for quite some time now.

Cheers,

Olivier

On 1/1/08, David Weinshenker <daze39@earthlink.net> wrote:
> I note that Schott advertises "Borofloat" plate glass (fairly
> similar in characteristics to Pyrex, but made by a "micro-float"
> process) in large sheets - but only up to 1 inch thick.
>
> Other than that, it looks like it would be excellent telescope-mirror
> glass (and should certainly work quite nicely for mirrors at least
> in the 6 to 10 inch range).
>
> To what extent are larger diameter mirrors practical with 1 inch glass?
>
> -dave w
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