[ATM] The ding, the inteferogram, and the analysis

[Dale Eason <atmpob@yahoo.com>]

If found an interesting surprise in this
interferometry analysis of my 16 inch F5 plate glass
mirror.  

http://home.comcast.net/~doeason/ding.jpg

Notice the spike in the 3d surface plot almost at the
12 O?Clock point.  That corresponds with a small ding
I put in the mirror during figuring.  It is one of
those clam shell fractures that does not break the
surface and is more visible from the edge than the
front. The actual crack inside the glass is about 1/8
inch inside the edge and about 1/2 inch long.  I think
I continued to polish the mirror after making the ding
but I don?t remember.

I was surprised to see it in my high resolution
analysis.  Also shown is the actual interferograms. 
You can see that 2 or 3 fringes are distorted by it. 
This 3D images also shows the noise my system adds to
the analysis.  It requires many igrams and mirror
rotations to remove that noise and I have not bothered
to do that here.  The second smaller spike is also
noise from my system I think.

The height of the spike is close to 1/2 wave on the
surface of the glass.  I don?t know how accurate that
is but I imagine it is at least that tall.

Some of you might be amused by how the ding happened. 
This is a thin mirror at 1 inch thickness.  It bends
over on the test stand when held vertical and creates
artificial test stand induced astigmatism.  I decided
it should be tested laying flat on its back.  The only
way I could do that is in the garage were I have the
height to build a vertical test bench.  My bench as an
aluminum ladder with the test stand clamped onto one
end of it and the ladder pivoted on a saw horse with
the other end up high in the air.  The whole thing was
was to limber and the testing gear too heavy to mount
on the top end. So I gave up and lowered the ladder
back to the horizontal.  But I had placed the mirror
on the test stand without the mirror safety block to
keep the mirror from falling forward.  It was not
needed when the ladder was close to vertical.  Now it
was needed and the mirror fell off the stand and onto
the ladder.  Ding!

Dale Eason



 
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