[ATM] rubber pad support of ultra thin mirror interferogram analysis

[Dale Eason <atmpob@yahoo.com>]

Ok for all of you who think a 3/4 in piece of plywood
and a foam pad will support a very thin mirror here is
what I have found for one example.

I used David Davis's 16 X 3/8 inch slumped mirror. 
>From previous analysis I know about what the surface
is at least to 1/4 wave PV surface error.  Thus I can
place that mirror on other supports, measure the new
surface deformation and then subtract the old mirror
surface leaving only the stand induced error.

I used my mirror test stand that is made out of 3/4
thick plywood.  It has a hole in the center 4.75
inches in diameter.  I use it to cool my mirror after
polishing.  The hole probably helps because the mirror
is slumped so the center is not pushed up too high by
the stand.  Over that surface I placed a piece of
non-skid drawer liner.  It is about 1/8 inch rubber.
It is made up of lots of little rubber beads fused
together to form a mat.

I tilted the stand back at 45 deg.  I used the tape
lateral support discussed earlier.

The box in the image is 1 wave high.  This is
wavefront error.  So the surface error would be 1/2
this.  Remember we would like the wavefront error to
be 1/4 wave or better.

http://home.comcast.net/~doeason/DDavis/draw_liner.jpg

The very edge part that turns sharply is an artifact
caused by diffraction at the edge of the mirror.  It
is not the real surface.  But about 5 lines in can be
believed.  The predominate error is astigmatism of
about .75 wave PV on the wavefront.

The rms wavefront error is 1/5 wave and the strehl of
the surface is .252.  This is what this support would
do to a perfect 16 x 3/8 mirror.  Most would consider
that unacceptable.

Dale Eason

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