From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


=looks like a paraboloidal type shadow curve with a high center.  or
alternately, a sphere with a low zone at about 70%,  If you draw straight
lines along the curved ones, you see the spacing is about the same at the
edge and the center....same radius.  Inside COC, the lines spread out near
the 70% zone.  The rays from that area are closer to focus than the other
zones. The 70% zone is low, or shorter radius.

To go to sphere, I'd say make a ring of aluminum foil about the diameter of
the zone and an inch wide.  Press it into a superficially warm lap (quick
dunk in hot water) with the mirror for a few minutes, then peel the foil
out.  Press for a couple minutes, then do some very (1/5, 1/4) short
strokes for 15 minutes. Re-press and smooth stroke it.

To go more towards paraboloid, you could try deepening the center.  What
are your Foucault readings like?

what is your stroke like?  It seems you may be letting the lap edge dwell
too long at the 70% zone.  Try some variation, and don't let your strokes
get too regular.  Have a friend do 15 -20 minutes to cancel out your
habitual strokes.

Looks real good, except you say it's not polished at the edge.  Go TOT for
a couple more hours, before your next ronchi test.

colin

-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Scott
Berfield
Sent: Sunday, April 27, 2003 1:01 AM To: atm{at}shore.net
Subject: ATM Tips on polishing to sphere




while I seem to finally be making progress on the polishing of my 12"
- the laser is noticeably less visible on the surface, though it is still
pretty bright at he outer edges. Unfortunately I now have a pretty not
spherical shape according to the ronchi test. My camera is not working so i
did a crude sketch. Can I get some pointers on taking it back to a sphere
as I get rid of the rest of the pits? The mirror is a 12.375" f4.55
(at present). The screen is 150 CPI. Drawing is at
http://www.berfield.com/ronchi.jpg

Thanks in advance.
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