Re: [ATM] Help my 17.5 inch coulter mirror is astigmatic

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

I don't remember how thick the 17" mirrors that Coulter did were
but I think that they were fairly thin.
The problem with thin mirrors is that you can't just set them
down on something and expect good surfaces to result.  The
problem is the old three legged stool thing where the glass will
be supported at only three positions and thus it is easy to
polish in a strange surface.

First thing I would do is put the grinding powders away.  The
solution is a pollishing problem, not a grinding one unless you
have inches of astigmatism. focal shift from one axis to the
other.  Either put the mirror on the cell while doing figuring or
do the mirror on the top qnd astigmatism should go away.  I'll
note that a Ronchi screen is a good wqy of checking for
astigmatism as the lines will take on a S shape as you get close
to the ROOC of the glass.  The replicq Ronchi gratings from
Willmann-Bell do an excellent job at seeing this and they are
really dirt cheap.  You don't need the chrome on glass expensive
ones to do a good job with a Ronchi grating.  Old timers havve
even made them by wrapping a double string of wire around two
posts and soldering the wires to the post after removing one of
them and then removing the other side o fht eone wire.  Works
fine as long as you don't touch the grating!\So polish back to a
sphere and make sure that there's no astigmatism in it with a
Ronchi grating and then you can then reparabolize the surface.
Bob May

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


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