[ATM] Ultra thin torture

[Dale Eason <atmpob@yahoo.com>]

I don't know if I'm torturing it or it is torturing
me.

I tried supporting it with 16 points just at the edge
all around the mirror. On my wooden test stand tilted
back 45 deg.  Tape lateral support.  I used 1.5 inch
long pieces of weather strip 1/2 inch thick aligned to
each diameter every 22.5 degrees.  The strips were
placed so that 3/4 inch would be under the edge of the
mirror.

I thought I might see the spherical error improve
since it has only 25% correction.  I thought gravity
might pull the center down.  I also thought that most
of the deformation would be a figure of revolution.

I was wrong.  The center stayed high,  the correction
did not change.  Instead I got 7 waves PV on the
wavefront of X astigmatism.

Possible causes:
1. stress in the glass.  I can't measure that because
the mirror is coated.

2. My test stand has that much x astig.  Don't think
so.  That much never showed up in the 3 support points
or when using the drawer liner tests.

3. Mirror has to be centered on the support points
better than I did.  Mirror is within 3/8 inch of being
centered on the points. 

I believe I know why the correction did not chage.
Since mirror is a slumped spherical shape it has a lot
of strength in the sphere shape.  It would require
much force to make the sphere smaller or larger.  It
would require a stretch or compressions in the
surface. Glass is strong in tension.  However it is
very easy to deform an edge by pressing on it.  I
think that would be a shear force.  Much like trying
to crush a paper cup when it is upside down on a
table.  It take much more force than pinching the
opening closed.

Dale Eason


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