Hi Richard,
Well, first of all, we dont use a closed tube for star testing! There is
no tube. The mirror sits in a mount open to the air, with the diagonal
supported by a truss. There is no more "cool down" time than plopping it
on a foucault test stand. The positioning is perfectly repeatable because
the whole system is permanently aligned on the artificial star. No
collimation and no currents. In fact, I can read the out-of-focus rings in
a fraction of the time it takes to estimate the shadows and measure the
micrometer readings on 5 or 7 zones.
And, you dont need to put your head in the optical path, because we use a
1/10+ wave diagonal and its exactly what will be in the final telescope,
so it is in fact quite direct. You are actually measuring the final
result.
The ring patterns are not difficult to interpret. Suiter's book has a
comprehensive discussion, but in an nutshell, bright outer rings
outside focus are overcorrected zones, and inside focus are undercorrected
zones. Opposite for the center. This test is more sensitive than what can
be measured with knife edge shadows.
The foucault test is useful, but has limitations. It is very
difficult to detect astigmatism, for example. For faster mirrors the zonal
readings wont give you exactly the theoretical expected result. For
example, an f/4 figured "perfectly" according to the r^2/2R formula
will not show a uniformly corrected star test.
So, JD and I feel that the properly conducted star test IS the most rapid,
inexpensive and accurate final determinant of your mirror.
Mike
Received on Wed Apr 6 22:33:29 2005