[ATM] The 15.5 Ultrathin Project, more polishing notes

[vorblesnak@peak.org]

I have a couple of hours into the polish of the project and had enough
smooth glass to run a ronchi test.  Well ... it was not nice.  The lines
were reversed.  That is, what you would expext inside the radius of
curvature was outside, and outside was inside.  Those of you experienced
at ronchi will immediatly suspect a BIG HIGH central zone.  Also, as I
crossed the focal zone that nasty old astigmatic twist to the lines.  Why?

Well the basic cause of the central high zone is poor contact or short
strokes,  neither of which I was guilty of.  I considered that I pressed
too hard while tuning the lap and flexed the glass down depressing the
central zone of the lap.  However, contact looked very good over the
middle.  Then some memories of doing the 16 inch came to mind and may hold
the key to the problem.

While figuring the 16, I would see a very deep middle when first removing
the glass from the lap,  but after a few minutes on the stand the middle
would come up.  Just the opposit of what I would expect to see with
regular thickness glass.  There the middle should look high due to the
heat caused by the polishing, but it should fall as it cools.  The why
proved to be thermal expansion of the back of the mirror.  My hands warmed
the glass on the back and increased the curve, this pulled the middle in
and up making it look deep.  I ran a very simple test to prove this.  I
laid the mirror on my legs and just rotated it, warming the back but not
the front.  I then put the mirror on the stand and saw a nice deep curve
that flattened as the glass cooled.

This is what I suspect is going on.  As I polish I warm the back and pull
the glass away from the lap in the middle.  This diminishes contact and
generates the hill and maybe the astigmatism.

Back to the ronchi test and watch the end of the lines.  If they jump as I
cross the focus, the astigmatism is across the curve.  If they stay locked
and only the middle twists, the astigmatism is in the central zone.  Only
the middle twists.  I hung the mirror on the edge of the lap, center over
edge, and did about two minutes of polishing.  Put it on the test stand
and the lines are nasty but straighter.  Give it a few mintes and nice
straight lines, no twist crossing the focus.  The astigmatism is in the
central zone and can be polished out.  I rotated the mirror through a few
more tests and found some minor twisting at 90 degrees to the first test. 
Mel called this second level astigmatism, (I think that is what he called
it),  but I missed the explaination of what it was.

SO!  Lots of polishing left to do.  I will retune the lap and get back to
the basic need for the moment, POLISH THAT GLASS!  However I will watch
and attend to that center as necessary.  Also time for a pair of gloves I
think.

David Davis
Toledo, OR 97391

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