Re: [ATM] The 15.5 inch Ultrathin, more pictures

[vorblesnak@peak.org]

Not quite as hi tech as that but ..

While working on the 16 I got great tests on the stand and disasters at
anything put verticle.  Astigmatism jumped right in when the glass was
tilted to, oh say .. 45 degrees.  Right then I knew that for a more
rigorous testing regimen I would need the cell the mirror was going to
live in.

Mark Holm counceled me on various configs and using PLOP I came up with a
54 point design.  This was contrary to the KISS principle in me and I
immediately  started to pervert the design.  Sorry Mark. ;)

I layed out a disk of wood the diameter of the mirror and plotted all of
the support points on it.  I was unable to find the spongy material I
wanted to use, so I used Jet Puff mini marshmellows to prototype the cell.
A marshmellow was stuck to each support point on the board.  I did the two
outer rings of points and then layed the mirror on it and noticed the
marshmellows did not compress very much at all.

Now I put the cell with the mirror in it on the test stand and noticed no
astigmatism.  I expected that, the mirror always tested well on edge. 
Next I took the cell and mirror down to the bottom of the stairs and set
it on edge at different angles.  Each time I retreated up the stairs and
used the ronchi tester to test it.  No astignmatism.  That was how I came
up with the cell I used in the tube at OSP.  Ultimately I found a source
of medium density foam buttons that are the support points in the
permanant cell.

The cell works because the points allow the glass to float on a forgiving
grid.  Placement of the points is very important.  Bubble wrap did not
work.

BUT! The 15.5 is not even gound out yet, let alone polished.  There is at
least another 20 hours of tool work to get a polished piece of glass. 
That's if everything goes exactly right.  Then, I will have to see if the
glass will test on edge on the stand.  Then I will talk about a cell.


David Davis
Toledo, OR 97391

> Would it be possible to test the mirror horizontally with a flat
> above at 45 deg? This could shorten the floor space needed as
> well.
>
> Gary Fuchs
>


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