Re: [ATM] Grinding Machine Mishap!

[Michael Lindner <mikell@optonline.net>]

On Saturday 24 April 2004 12:18 pm, Mark Holm wrote:
> Also, I am not sure I believe that having the mirror a little off center
> on the table would necessarily cause astigmatism.  You are still
> stroking and rotating, and the only shape that will remain in contact is
> a sphere.

When you grind by hand, you are working "centerless". If you are grinding by 
machine, however, the action is defined not by the stroke across center, but 
by the offset of the tool (which is how you can dig a hole in the mirror even 
though the tool is on top).

What you get when you move the mirror off center is the grinding of a second 
sphere with a different center. Another way to think of it is more grinding 
action on a particular radius, as if you were grinding by hand and didn't 
rotate the work.

Anyway, I too find it odd that 15 minutes' work with 25 micron wasn't 
completely removed by 60 minutes' work on 220 (followed by 20 minutes each of 
25 micron, 12 micron, 5 micron, and 1 hour of polishing), but the astigmatism 
is real, unless you want to hypothesize another way it got there, and say 
that this is a coincidence. I'm open to ideas. This is the first time I've 
seen astigmatism in a mirror I've made, and the blank is fairly thick - 
18"x2".

Clear skies.
-- 
Michael Lindner
http://www.starastronomy.org *** http://home.att.net/~mikel
http://www.atmsite.org *** http://www.atmlist.net

_______________________________________________
ATM mailing list http://www.atmlist.net/