ATM Curious Observation

[Anthony Stillman <atmer@flash.net>]

Following the numerous scratches my 16 inch Walters' blank incurred at the
public demo and lacking enthusiasm to start another refractor, I took the
16 inch back to 600 this last Friday as the old Chabot.  Wisely, I hadn't
covered the tile tool and used it as the pitch lap and hence had is in
storage.  So far I've put in about an hour and a half at 600 and an hour at
17 micron.  In the process, I've removed all the scratches and made a
curious observation.

Starting with 600, TOT, 1/3 W I quickly removed the polish, but only at the
center.  This spread slowly toward the edge, which up until the last 20
minutes continued to show polish.  I've been toying with how this could be
and while I think I have a notion I'm open to conjecture.

The facts are; I polished out with a 12.5 inch lap TOT, concentrating about
6:1 on the edge as compared to the center.  Polish arose uniformly. That
is, no part of the mirror appeared more polished than another, more or
less, during the 10 hours of polishing it took for me to feel it was
complete.  I did not overhang the tool particularly, perhaps as much as two
or three inches and this was less common than the typical inch or so.
Though I misinterpreted the tests initially, having video-ed them I can say
that the mirror was overcorrected from the start.

The implication of how the grind developed in re-grinding the polished
surface is that I pushed down the edge and extended the radius of curvature
while polishing.  In effect I wasn't over corrected, but rather severely
undercorrected.  In all my years of pushing glass I've never before seen
this effect, and I have re-polished more surfaces than I care to count, so
the fact is I'm guessing.  I curious if anyone else has seen this before or
has an alternative explanation.  If so, I'de like to hear it (read it).

With any luck I'll be back polishing this glass by the end of the week, as
at this time I've nearly cleared away enough room between the washer and
dryer to work at home.

Anthony