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Hi Geoff,
I have a 12 inch F5 and observe often with a friend who has an F6. The main difference is I sometimes need a a couple of steps to reach the eyepiece in the F6 (I'm 5 ft 10.). My feet are always on the ground at F5. If the mirror is well made it will perform well no matter the F ratio but a well made short focus mirror is likely to be expensive. I don't use a coma corrector at F5 but some people do.
Rod.
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