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Dave,
That's my conclusion ("for
instruments with focal lengths longer than 2 meters...") with my Cass at
about 116" focal length, my star diameters are
dependent on the seeing almost exclusively. If I can see the bright stars "blob
out" in the camera, it's pointless to shoot at these high powers...instead it
becomes a short refractor night or piggyback. Martin Germano always goes up with
two lists... good seeing and bad seeing.... if the seeing is bad, he switches to
the 8" f/4.5 Newt. Bottom line in all of this theory... rare is the night
when the seeing will allow theoretical performance... unless you are the HST.
Tony
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