Re: ATM Foucault data question

["Bob May" <bobmay@nethere.com>]

You are running into an interesting problem with testing.  What is happening
is either/both of two major things.  You are moving the mirror when rotating
it and thus, you can't get to exactly the same distance between the back of
the mirror support and the front of the mirror.  Supporting the mirror on
the front of the mirror would take care of this but you then run into the
problem of looking up in order to be able to see the mirror although a flat
mirror in the path can cure this problem for your particular application
(not all that easy to do for many different sized mirrors).  The second
problem that you can be having is astigmitism in the mirror.  This one is a
lot easier to take care of because you don't rotate the mirror by 90deg. but
rather a smaller angle and then you look at the way that the shadows come
across the mirror.  If the shadows always go side to side without looking
like they are moving up/down then you are pretty clear of astigmitism.  The
Ronchi test is also very sensitive to the astigmitism as it will produce S
shaped lines at the worst angles on the mirror and is often easier as you
don't need to rotate the mirror but just rotate the grating (assuming that
the light is shining through the grating method of illumination of the
mirror) and the straightness can be easily detected.
Bob May
My new web space address is http://webu.wigloo.com/bobmay/ or
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and my new email address is bobmay@nethere.com
Bob May