Re: ATM concentric hills: Correction!

["Bill T." <twentiethwave@hotmail.com>]

Ryan,

My sincerest apologies!  I misread your previous post, I though the hill was
centered at 70%.  If it's INSIDE of 70%, you may not be in such bad shape.
Those long TOT, COC polishing strokes probably did a fair amount of
"parabolization" on the outer parts of the mirror, or at least made the
outer portions long compared to the center.

You might try some short W strokes bringing the lap just barely past the
edge of mirror (1" or less).  The idea is to selectively knock down the
center, so that it conforms to the longer focal length already established
at the edge.  First do a reference Foucault test, then give it about 10 or
20 minutes of the short W's, TOT, and see if the hill diminishes.  You can
bring the length of the W strokes slightly in or out as needed to control
the size of the "knock-down" area.  This will go a long way towards blending
the inner and outer portions of the mirror, but may also leave a smaller
flat area in the center.

To finally get the center deep enough you may need to go to Mirror On Top,
and try very brief spells, bringing the center of the mirror up to within
about at inch of the edge of the lap.  The should last 2 minutes max, with
an integer number of turns.  The center will deepen very fast, go slow.
This will eat up your remaining pitch thickness at the edges very fast.

If your edge was over corrected to start with, you may then need to deal
with the over correction with classic W polishing strokes, once the center
has been brought proportionally into line with the edge.

Bill T.