Re: ATM Glass Advice

[ATS <drummer@austin.rr.com>]

http://www.jacksonville.net/~dcass/
Dan can answer all the curiosities about making a 20" scope from 7/8" glass.

mike coslo wrote:

>         Hi Peter,
>
>         My own opinion is that if you want to make a 20 inch mirror out of 7/8
> inch glass, be sure you are doing it for the right reasons. Money
> probably isn't the best reason for doing this.
>
>         Now if you want the pride of having done a real challenge of a mirror,
> *that* is a good reason. Lightness could also be a fair reason.
>
>         But when we go to such a thin mirror, expenses crop up in other areas.
> There is a good chance you would want to slump the mirror. Your saggita
> is going to make that puppy *really* thin in the middle. But slumping a
> 20 inch mirror will have it's costs. I'm not sure who would do one that size.
>
>         Your cell will need to be excellent.
>
>         Now if you are looking for a bargain in glass, the keyword is patience.
> Subscribe to Astromart. There is the occasional piece of glass that has
> been partially worked and the person is getting out of the hobby. I've
> seen the occasional really good deal there.
>
>         Good luck!      - mike -
>
>
> > Peter wrote:
> >
> > > Hi,
> > >
> > > I'd like to get some advice regarding purchasing glass for a 20" scope which
> > > I'm considering to build. I found a company on the Internet which sells
> > > cheap plate glass but it is only 7/8" thick.
> > >
> > > I'd like to know if it a bad idea to pursue making a 20" f4.5 out of 7/8"
> > > plate glass or should I not be so cheap and cough up the cash for thicker
> > > Pyrex? The Sagitta works out to approx. 0.278" leaving about 0.600" of glass
> > > thickness in the center of the mirror. The cost difference is huge.
> > >
> > > I've already made a few mirrors from scratch (12.5" f5 is the latest) so I
> > > do have some experience but I'm certainly not an expert.
> > >
> > > Thanks in advance for your input.
> > >
> > > Peter
> > >
> > > P.S. I'd appreciate receiving links to other Internet sources of cheap glass
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