Re: ATM BVC Glass

["Bill Prewitt" <bprewitt@neo.rr.com>]

I did a 20" BVC 4 years ago and a 12.5 last winter.

The 20 blank had some wedge and an uneven back, but that was easily removed
with a piece of window glass and 80 grit on a turntable. I understand he can
blanchard them now.
The recent 12.5 was flat and true.
The blanks consist of a stack of disks, fused and slumped in a mold, so you
can see a few flat bubbles way down in there once you polish out. They're
apparently harmless and invisible once it's coated.

BVC is soft, so quick to grind and polish.
Both blanks turned into good mirrors....
It's very easy to figure because the image is bright, it stabilizes quickly
on the stand, and it polishes quickly.

I suggest you nail down an acceptable wedge spec and get the curve slumped.

Having said all that, I chose 7740 Pyrex for the current 30" project.
Larry Sayre and I had 30" disks waterjet-cut from opposite corners of a 40 x
60 slab for a good price locally. So I'll know more about pyrex vs bvc in a
few months.

Bill Prewitt

Webpage of Telescope Making and Caving
(My hobbies keep me in the dark)
www.astrocaver.com



> Has anyone had any good/bad experiance with BVC glass in the 14"-20" size.
> Any feedback is welcome.
> Sam Orawski
> Hamilton Canada
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