[ATM] The 15.5 Ultrathin project

[vorblesnak@peak.org]

Just a brief note.

I dusted off the original mirror cell that I made for the 16 inch
Ultrathin and put the 15.5 on it.  It is a 36 point design from plop and
the basic glass curve is close enough for some quick tests.

I ran some basic ronchi tests with it set at a 45 degree angle.  To do
this I set the mirror and cell at the bottom of a stairway and worked my
up the stairs till I was at the ROC.  I see the same wild curves I saw
before.  Next I placed the mirror flat on the floor and went up the stairs
to the second floor and looked straight down.  Same astigmatic curves as
when it is vertical.

I still see a strong central hill, and the glass is not polished out by
any means, so I will continue to polish and see if I can get rid of the
hill at very least.  The figure may improve but I have my doubts.

The second issue would be even if I get it figured out, setting it in a
working cell will be difficult at best.  The bottom line being 3/16ths is
probably too thin for a slumped mirror 15.5 inches in diameter.  I have a
working 16 inch made with 3/8ths inch slumped plate glass that has a good
figure.  So barring any sudden epiphany or correction I am going to say
3/8ths is about the minimum thickness for a slumped 16 inch.  This fits
with the pie plates I did.  They were essentially 8 inches, 7.5 working
diameter, in 1/8th inch thickness.  The next test will be 21 inches in
3/8ths glass, BUT, I have polishing left to do on this 15.5 before I
surrender.

A test I may try is to cut out a 12 inch disk from the 15.5 and see if it
can be figured.  That would fit within the success scale of thickness over
diameter that has been established.

SO! This will probably be the last post on the 15.5, though I will
probably post a final ronchi set when it is fully polished.  With my work
schedule that will be two or three weeks off now.

Thanks to everyone who pitched in many great ideas.

David Davis
Toledo, Or 97391

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