From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


When you get to the short FL mirrors, it is better to use the wire test or
the Caustic test as these handle the short FL surfaces a lot better.  The
Pin and such other things mentioned are more variations of how to run a
particular test and aren't tests in and of themselves. Astigmatism is
developed when the mirror and tool are not well rotated and thus cause a
saddle shape which will then tend to keep itself going. Remember that there
are a number of shapes that can move in relationship to each other but
there is only one shape that will also allow random rotation in addition,
the spherical shape.  You can also get astigmatism (and it happens in the
thinner mirrors) from improper support of the mirror which allows some
stress on the mirror while grinding and polishing.  There have been
articles on stressing the glass and then polishing in the surface a
particular shape other than the spherical shape after the stress has been
released.  In fact, the process of stressing spherical mirrors to the
parabolic shape can be used with the force in the opposite direction to
make a permanent parabolic surface by stressing the glass and polishing the
easy to do spherical shape on the stressed glass. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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