Re: ATM Newbie with a problem (flat spot on tool)

["Bob May" <bobmay@nethere.com>]

A lot of people have a wrong idea about the Sharpie Test.  They seem to
think that the elimination of the lines is the result of the test.  In
truth, all of the lines can easily be removed with enough grinding of the
mirror and tool.  Thus, you have no idea of where the grinding is mostly
done.
In truth, the Sharpie Test is an inspection of where the grinding is
happening and thus, you need to see the fading of the lines and evaluate
from there.  On a perfect match between two surfaces, the lines should fade
equally all over while a typical surface will show a ring where the lines
are tending to be fainter than the rest of the surface.
This said, do the test again and only grind enough to see the lines starting
to fade on the mirror and tool.  Evaluate the surface match from there.
Odds are that you will find the ring wear pattern on the mirror and/or the
tool.  This tends to indicate that the surfaces aren't in good match.  When
skipping through a number of grits like you have done, there will indeed be
a bit of mismatch at the beginning as the surface starts out hyperbolic and
gradually attains the spherical surface as the grits get finer.  It takes
some dedicated short stroke work to get a piece of glass really even to the
parabolic curve at 80 grit so don't really worry that much until you are at
the fine grits.  I wouldn't consider the step from 220 grit to 25 micron to
be that large of a step so as to be difficult to work.  Pros tend to use
only 2 or 3 grits to go from the coarse work on a piece of glass to the last
5 micron or so grit.  I've started pregenerated curve surfaces at 320 and
gone down from there without problems.
For a solution, first take and do an inspection of the surfaces and see if
the pits tend to be about the same all over.  I like the glancing light test
for this as it tends very easily to show patterns of the coarseness of the
surface as opposed to trying to find and evaluate the size of the pits in
the glass.  What is especially nice about the test is that it becomes more
sensitive as the pit size gets smaller.
You will probably see that the center part of the mirror will be a darker
orange than a ring area around the surface.  If so, then you have a deep
hole in the middle that needs to be cleaned up before going to the next grit
down the road.  This work is to be done by short strokes with only a little
bit of a W to the stroke.  1/4 to 1/3 strokes with a W of about 1/6 to 1/4
of the diameter are what you are going to be wanting to do.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net