Re: [ATM] Persistent turned-up-edge in mirror

["Jerry" <wa4guu@verizon.net>]

Wait until you have it completely polished to worry about the TUE. Any edge
can be turned down in just a few minutes.

I would be worried why after over 23 hours it is not fully polished.

Either your fine grinding failed to remove pits from an earlier grade of
grit or your polishing is not very efficient. Even allowing for first time
inefficiency, 23 hours should have been enough.

You did not say if both the light source and knife move together or if one
is fixed.

If you disregard the 3/16" turn up, what is the shape of the rest of the
mirror. What are the mask dimensions and knife readings for all the zones?

Jerry

-----Original Message-----
From: Bern Kosicki
 
Hello,

I'm a new member of the ATM list, and am working on my first mirror (8",
f/5.3)

Currently, my mirror is well polished out (over 23 hours, only very small 
amount of pits left).

However, it has a VERY persistent turned-up-edge (TUE) that defies 
polishing out.

The TUE is limited to about 3/16" from the edge.  Typically, the difference 
in longitudinal focus from the Center zone to Zone 5 has been from about 
0.050" to 0.080" ( with the shorter focus of course from the TUE).  (I've 
used the Texerau standard mask for an 8" mirror, except that I've divided 
the standard Tex outer zone into two equal radial zones so I can measure 
the TUE.)

To see how persistent the TUE is,  most recently I've  polished for 6.5 
hours in four sessions (one, mirror on top; three, TOT) using a 
Texerau-standard stroke.  The longitudinal difference between the Center 
(Z-1) and the outside (Z-5) has remained at Z1-Z5=0.050"   showing 
practically no change during this series of four sessions!  The figure has 
become pretty smooth during this series except for the TUE (at least 
something is working)

Before each session before I've slightly warm the tool and mirror in warm 
water (80F) for about 1/2 hour, then press (14 lbs) for about 1/2 
hour.  The tool is in good contact.  I use CeO.  I've rebuilt my tool 
recently (the original was getting thin) with harder pitch (73F vs the 
original 65F), so the tool probably isn't the issue.  I've used a generous 
1/3 stroke for this series, and tried to keep a neutral hold on the tool or 
mirror during polishing.

So, I'm running out of things to do to correct this problem.  Has anyone 
else had a problem like this?

Thanks for your advice,

Bernie Kosicki




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