RE: ATM polishing help

["Mel Bartels" <mbartels@efn.org>]

>>>
I have attempted to polish five times now
and each time as the polish progresses (spherical ok at the early
stages) the centre third of the mirror takes on the appearance of a
"moon crater" and in trying to remove this I cause  problems at the
edge
and develop a biscuit appearance.
<<<

I suggest that you not check the figure during polishing.  If you do,
it will lead you down the path you are treading - correcting for this,
correcting for that, redoing this, redoing that...

Instead, your should fully polish out the mirror - that's the only
goal of polishing.  If the lap is well pressed and you have good
contact, the figure will not wander far.

The moon crater can be caused by several things such as thinner pitch
in the middle, and pitch that is too warm to start with (transfers
heat to the center of the mirror, swelling it, which then is polished
off, causing a crater as it cools).  It is easily fixed during
figuring.

The problems at the edge are occuring because you are messing with the
center.

The biscuit appearance occurs because of jerky contact - a sign that
you are compensating for a center problem with unequal pressure.

Just polish hard with simple even strokes, letting the polishing
compound thin out until you hear screeching, then adding a drop of
water to keep it from going completely dry.  Do this until there are
no more pits at the edge, and then do it 30% more time than what you
have expended to this point.

Then you can back off the stroke pressure, thin the compound, and get
started on figuring.

Mel Bartels