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Hi Bob
Longer focal length eyepieces, dobs and middle age eyes don't play very well together. They are a young observers eyepiece.
For example, if you use a 12" f5 and a 40mm eyepiece at around age 50, only half the light enters your eyes and you get the light transmission/object brightness equivalent to an 8" scope.
Not an issue if you are using a f10 SCT but more of an issue the faster the f ratio of your scope.
Joe
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