[ATM] Supporting those pesky ultrathin mirrors

[vorblesnak@peak.org]

I sent my cell to Dale yesterday.  Unfortunately coming from the middle of
no where it will be Friday of next week before he gets it.

When I first tried to work out the support for the 16 inch, I tried a
number of ideas.  I found the mechanical supports printed through,
regardless of how well I smoothed the bearings.  They would never set
right against the curved back.  Pie Plates are held by the edge by virtue
of their construction.  I tried that idea using a bicycle rim siliconed to
the edge of the mirror.  Astigmatism!  Next I went with three then six
points at the 70% zone.  Distortion!  I tried using the ATM1 calculations
for central and annular zones and was no better off than a simple 18 point
mechanical cell.

The glass seemed to do well, or better perhaps, just laying there or
standing on edge.  That was whe I got the idea for a soft point cell.  The
cell would need to match the curve of the back of the mirror and allow the
mirror to just lay there.  I have a large bowl lathe and I turned up a
disk with a curve that was pretty close to the curve of the back of the
mirror.  The prototype used mini marshmellows as the soft points.  It
worked!  I layed out the supports from the plop program, but only used the
outer two rows.  The central 12 inches or so just hang there.

Later I did a second design that used a disk and a ring in the outer 20%
or so.  This was easier to make and achieved the same end.  You could do
it with a simple disk and supports of different thickness.

I find that my testing is very poor and I may have missed some cell
induced distortion, but even with my primitive tests the results were
markedly better using the soft fixed point.

Dell will have the cell in a few days and we will see some good accurate
work done on it.  Ad hopefully some better answers.

David Davis
Toledo, OR 97391

BTB:  When I figured the 16 I thought I was too deep.  I also saw some
minor  hairyness to the very middle, which I attributed to the ronchi grid
I was using.

D.

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