[ATM] The Ultrathin Project

[vorblesnak@peak.org]

The ultrathin project..

Just to bring the discussion up to speed.  Two pieces of glass were sent
to Dale Eason for testing.  One was known to be flawed.  Dale found that
both were flawed. This was news to me as I had viewed with the coated
mirror.  There was some discussion about what may have changed or been
missed by me or changed by me.  Then winter set in.

When I got the 16 inch mirror back from Dale the coating was quite
tattered.  I polished it off and sent it out to be coated.  It is
currently mounted in a full tube and I view with it.  The original cell I
sent with the mirror had two rings of supports.  One at about the 70% zone
and one at about 95%.  The view was murky.  I was confused.

Looking at Dale's analysis of the mirror it struck me that the glass was
so thin the supports were pressing through and distorting the front.  This
fits with what I see when making pie plate mirrors.  They show no
distortion when mounted.  That is because they are mounted by the rim and
the mirror, the bottom of the plate, is held by it's circumference. 
Perhaps this gives a sudo thickness to the mirror.  Regardless, removing
all but the outer ring of supports cleared the view and gives me good
images up to 150 power.  I know this is less than the perfect numbers, but
it was never that good of a mirror.

Bill Kelly ran an analysis on the 16 and said it would take about 2 pounds
of pull to fully correct it.  However he said the glass is far too thin
for that.  He is probably right, but I am going to try it anyway.

The 15.5 is another issue.  The distortion in that one is legendary.  The
question was, what had caused the distortion. Glass thickness was one
culprit.  Hand heat was another.  However, as thin as that piece is I
began to suspect that just the pressure of my hand on the glass was enough
to distort it and cause issues.  I needed to remove myself from the
equation.

About this time, late May 2006, a full size Hindle machine was offered to
whomever would pick it up and use it.  I went and got it.  Learning to use
the thing, (and I still don't really know what I am doing), has been an
adventure.I used it to produce a couple of pie plates.  I tried extended
polishing with no weight on the plate or lap, which ever was on top at the
time, to see if a polish could be attained without pressure.  If the 15.5
could just float with a light weight lap on top it might be do-able.

How to support the 15.5?  I mounted it on a wooden disk with RTV rubber. 
The rubber was at the support points shown by PLOP.  I mounted that
assembly on the Hindle and reground the 15.5 with 600 grit.  For a tool I
used a 1 pint wide mouth canning jar with water in it for weight.  I did
the micron wets, (AO) with a small pitch lap.  The disk looks good.  I
noticed some distortion when the wood had warmed up a bit, (solar
reflection looking for gross astigmatism), and that caused me some
concern. So I cut the glass off of the wood and remounted it on a rigid
Styrofoam disk.  I also mounted it using just the ring of supports at the
95% zone.  I hope to start polishing on it this week.

David Davis
Toledo, OR 97391



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