Re: [ATM] Ain't got no shperometer

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

Assuming a Newtonian telescope, the testing is really pretty
simple.
The Foucault test may be used.  For this test, just make a mask
smaller than the mirror so that when you are looking at the
mirror at the ROC, the mask fills the mirror. The shadows are
what is important and the mask is just there to assist your eye
in seeing what is of interest!  Assume that the center is perfect
when doing the data reduction and you wil have something quite
close to perfect.  Also, make the first zone the diameter of the
secondary so you can cover the rest of the mirror with the rest
of the zones.
The Ronchi test may also bbe done just as easily with the same
caveat for the center of the mirror.
Finally, the KE test and the Ronchi test may be done on a bright
star.  The results of these tests should indicate a spherical
surface rather trhan a paraboloid as the source of the light is
at infinite distance compared to the ROC of the Foucault and
Ronchi tests done on the bench.  A good sharp edged Ronchi screen
may be used for the KE test on a star to good effect - the
SureSharp focuser is nothing more than a Ronchi screen that is
held at the focal plane distance from the end of the focuser for
a particular camera!  I've even done Ronchi tests on strange
scopes with nothing more than my comb, using the fine side of the
comb, to evaluate a person's scope!
Bob May
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