Re: [ATM] [ ] making Foucault tester

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

Several years ago (I took the tester that I was using and did
some show and tell u8p at RTMC of what testing with a camera is
like.  The people were amazed that I had the image displayed in
real time on a 30"  TV that wasas there at the front of the hall.
I even had a guy bring a mirror of his own that he wanted to see
what the results werre.  He saw things that he had never really
been able to see by putting his eye behind the KE.
The little micro cameras with their ounce or t3wo of weight don't
upset the balqnce of the tester at all and the whole crowd that
was there were able to kibbitz on the quality of the mirror under
test.   I operateed the test stage and had the crowd evaluate the
quality of the zone measurements for eveness of the graying of
the zones of interest  and all came to the conclusion, I think,
that the camera was an excellent way to do the test.
The camera was mounted on the moving stage so that as the stage
tilted, the camera tilted with the KE which ends up being that
you don't see any tilt in the surface shape of the mirror.  The
only thing that tilted wsas the Couder mask in the image and for
a few degrees of rotation that wasn't objectioooable.  It do4es
help that you do align the tester so that it doesn't move much
for where the returned light is cutoff as you move the stage but,
for the Foucault test, that is relatively unimportant.
The most important thing I see that the tester needs to have is a
smooth action free from jerks as this will allow for the tester
to smoothly cuto9ff the light from the mirror.  Second is to be
able to readout the distance you have moved the tester from one
zone of the mirror to the next with good accuracy.
The only modification to the Stellafane tester that I've done is
to use blocks that I've drilled through and run those on the rod
rather than using the arrow shaped Vee cuts that the Stellafane
tester has.  I prefer to have the tester stay in one pi4ece
rather than easily come apart in that way.  I'll also note that
the rod doesn'6t need to bbe of any great precision as I have
used wooden dowels in place of metal rods for that piece and
gotten good results from the tester.
My first tester  was built up with precision steel plates with
cigarette paper for spacings to allow the pieces to have the
clearance to slide and it wsas quite compact but I used a fine
threaded rod that made it hard to move the stage very far
quickly.  There was a perpendicular stage on top of that that
allowed the side movement of the KE and light source and that was
the big problem as the upper stage had no spring to keep it
against the screw but rather you havd to move the screw back the
other way to get the KE to move back.  This made it hard to
easily adjust for the best cutoff point.  I quickly learned to
get close to the cutoff and then bend the table as necessary to
see the final image I desired.  With the tilting stage of the
Stellafane design, the action of the KE is much finer and the
stage naturally allows the very fine movements necessary for the
pleasant viewing of the graying out of a zone.  The tilting
action also naturally induces a good perpendicular of the rod on
the movement of the KE foor the cutoff
All in all, most of the basic design of the Stellafane tester is
of the most excellent for what is needed for testing.  Go look at
the University Optics tester for a much more vrude design .  Also
look at the design that is in the earlier books where you have a
block supporting the KE qandd you move it aroudn the table top
and make marks on the table for where you have gotten the
readings you desire.  We have come a long way in tester design!
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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