Re: [ATM] The 15.5 Ultrathin Project, Astigmatism discussion

[Dale Eason <atmpob@yahoo.com>]

For testing I think supporting this mirror by the edge
will always induce more error than you will want. No
matter what the method is (sling or pegs.)  Your
mirror is light enough that you may be able to support
it by one point at the center on the back.  Try some
sort of sticky tape or Velcro about 1 inch in diameter
in the center. Maybe a small dot of pitch that
attaches to a handle.  This will not eliminate sag all
together but it may reduce it a lot.

Putting it in a cell will not solve the problem
because the cell is meant to support it when there is
little edge force.  Attaching it to many points on the
cell in order to avoid edge support will also induce
astig because the cell is not meant to equalize
lateral forces when held in the horizontal position.

Testing from above is problematic since it is hard to
get that much vertical head room.  Then there is the
possibility of dropping something from the tester onto
the mirror.

This is an issue that ATM's have struggled with or
avoided by using thicker mirrors.  They make sure
there is no astig by using the proper grinding and
polishing techniques.  The thick substrates help them
in this regard.  Then they use the Foucault test that
is blind to astigmatism.  That is OK as long as the
mirror does not have astigmatism.  But for the cases
were it is not easy to manufacture the mirror without
astigmatism then you will have trouble measuring it
and determining if it is really the mirror or your
method of holding it during testing.  The star test
can help here.  But it has it's own issues like steady
seeing, completed scope to try it in, etc.

You have come upon what I think is one of hard issues
in thin mirror making.  I have been struggling with
that issue for the last year while making my 16 x 1
inch mirror.  I had no way to measure the amount of
astig to know when it was too small to matter.  When
it is too large it can easily be seen in Ronchi and
when it is way too large it can be detected with the
knife test. Neither test can tell you the magnitude.
So I built an interferometer and saw it and measured
its magnitude.  Next I needed to know if it was in the
mirror or just a test stand artifact.  That became
even harder to determine and I'm still trying to
figure that one out for all cases.  


Dale Eason

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