Re: ATM 3/4" Plate glass .. the Max..

["Malcolm Macdonald" <toliman@wave.co.nz>]

----- Original Message -----
From: "Russell Jocoy" <russjocoy@hotmail.com>
To: <atm@shore.net>
Sent: Wednesday, January 01, 2003 1:38 PM
Subject: ATM 3/4" Plate glass .. the Max..


>
>
> ("atm@shore.net")
>
>     O.K. guys I respect all of you input on how far someone can go with
> 3/4" plate glass when making
> a mirror. It seems that if you try a 16" diameter mirror this thin you are
> doing a "hat trick".

No, I would not say a 'hat trick'....just 'trickey.'

I have a 16'' F4 ...Yes...F4.   19mm /  3/4" thick and no slump.  I can't
remember the specks. off  hand but I got it to ''very good '' acccording to
Dave Row's Figure 45.  I am very happy with its performance. It's now my
favourite telescope that I use for Minor Planet Occultations.


 >I understand
> that support is all important, and that the demon astig.. is always
> looming..

This is true but not necessesarily the outcome.       FWIW   I use a piece
of ground sheet/mattress, that campers use, to support the glass. It is very
flat/éven, seems to have just enough resistence to preasure, and being
waterproof is easy to rinse off  between grits, it dosn't get soggy.

Take extra care with the fine grinding. Not too much preasure....softly
softly and plenty of side step round the barrel, moving the glass round
often. And be prepared for a little extra work and maybe some frustration
when figuring, but dont give up. The outcome is worth it.


Malcolm