Re: [ATM] Astigmatism

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Bob is absolutely right about this. I didn't move my 7/8 inch thick 16.5 diam blank around enough when polishing, and ended up polishing some absolutely horrible astigmatism. 
I tried rotating it much more frequently on the absorbent backing on my turntable, and making my strokes more systematic, and it looked better, but it still seemed like there might be astigmatism when I placed it on a standard sling. Either that or just potato-chipping, couldn't tell which
However, once I finally built an 18-point cell plus sling, tipped back as far as I could comfortably make it (tester at about 4 feet off the ground, mirror cell ON the ground about 151 inches away), I could see that I had indeed whipped that astigmatism monster. Still got sort of a turned edge, but compared to astigmatism, that's a cinch to fix.

Guy

PS - Bob: I am so glad that you have started quoting the emails you are responding to. Your advice is generally excellent, but a lot of times, before this, nobody but you knew what, exactly, you were responding to.
Thanks!
GFB
Bob May <rmay@nethere.com> wrote: All during polishing you will be wanting to regularly
turn the glass on the turntable so that astigmatism doesn't show
its ugly head. Another thing with thin glass is that you have to properly
support it when testing or you will be seeing a lot of bending of
the glass which you will misinterpet as being actually in the
glass surface and thus will try to get rid of it, finding that
either you can't (because it isn't in the surface) or polishing
in the error!




Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

=============================
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