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ED80 is the best allround bang for your buck widefield scope. I think its F7 or F8 but you can get a reducer.
Its also good for astrophotography.
I am not aware of any F5 or F4 refractors except FSQ106ED F5.
The lower the f ratio the higher quality the glass and optics need to be otherwise aberrations of various types will kill it. Even high end APO refractors are rarely faster than F7. You can get reducers to bring them down to mid F5 but those reducers are usually worth more than you are probably considering for a scope.
If you want F5 best go with a Newt. They are cheap, light, collect a lot of light and have cheap accessories like reducers and flatteners and are also good for widefield astrophotography.
F4/5 in a cheap refractor is probably an unreal prospect.
Greg.
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