[ATM] The Ultrathin project a brief report

[vorblesnak@peak.org]

I had attached a pie plate to the back of the mirror to use as a handle to
control the glass on the lap.  Attachment was made with RTV, 6 blobs in a
circle that would be covered by the secondary.  About 3 inches in diameter
in the middle of the back.  I made a full diameter lap and my thought was
to polish with just the weight of the glass.  With a full lap there should
in theory be no distortion and the figure should be good.

On the machine the RTV lasted for about 5 minutes and then it ripped off
of the pie plate.  I reglued with more blobs and tried hand polishing.  As
soon as I got good drag between the glass and the pitch the RTV began to
stretch.  The rubber looked like it was going to give up so I stopped.

Having used hard adhesives before, pitch included, and having found
distortion due to uneven expansion because of the adhesive, I am at a
standstill.  I have not explored the partical bed idea yet.  But I have
some other glass to get out of the way so I am putting the ultrathin away
for awhile.

One piece I have to do is a 14.5 inch port hole, 1/2 inch thick.  I think
I will slump that one to an f7.  Another is a second 14.5 inch port hole
but this one is 3/4 inches thick and I think I will try a traditional
mirror on that one.

I also have a couple of platters I got at a thrift store.  I had one
polishing and found it badly astigmatised.  Checking with polarizing
lenses showed some bad strain near the edge.  Fortunatly I now have a kiln
and I will anneal the glass and regrind.  These should yield 9.5 inch f4.5
mirrors.  I would like to do another pie plate but I don't think I have
time.

If anyone comes up with an idea for the ultrathin let me know.  I will
probably pick it back up in the fall.  I think it is doable if I can
figure out how to control the glass without distorting it.

David Davis
Toledo, OR 97391



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