Bill,
Just to add my $.02, I'm currently doing a 16"
f/4 BVC mirror--my first. I have polished it out completely, and have
made one false start on figuring, from which I have returned to a sphere. It
took only 9 hours to polish out with a full-sized lap. As I understand it,
this is quite a bit faster than Pyrex. Twice in this process, the figure has
been just about a perfect sphere. The second time, I could detect no deviation
from spherical whatsoever with a 100-line Ronchi screen. The Focault image
also went dark completely evenly. I took no special care in polishing other
than that I supported the mirror on a grass mat when I was working TOT.
I think I can say with confidence that there were no defects in the figure due
to the nature of the material, and that my success from here will depend
solely on my slow, painful acquisition of figuring skills. One drawback I
think is that the soft surface tends to get scratched more easily, but I don't
think this is a really big problem. I also highly reccommend this material.
It's a great deal.
Good luck,
Herb.
"Bill T." wrote:
Has anybody done a large BVC mirror lately?
I'm considering a BVC
project -- I'd appreciate hearing about
experiences with this rapidly
evolving material.
Bill Tondreau