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the longer focal length creates a tighter field of view so you are more 'zoomed' in with the longer focal length. you don't see many f/8 newts these days. if it were me i'd probably go with the f5 newt - in fact that was my first scope.
I don't know anything about the orbinar brand but the scope is f4 which means you will definitely need a coma corrector for viewing.
what are you looking to put it on top of?
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