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Light pollution does not matter for the Moon. Even transparency doesn't matter that much. What does matter is seeing, a.k.a. air turbulence. It is very rare that an exit pupil smaller than 0.5 mm is useful for anything - perhaps for some tight double stars, but that's about it. So take that as a hard lower limit. In terms of a "soft" limit, it depends. If ...


Since the magnification is determined by focal length of the objective/focal length of eyepiece it is difficult to have an eye-piece with the same f ratio as the objective. Assuming pupil aprature 0.25" then the ideal eye-piece aperature will be 0.25" for most effective light transmission of the eye.A generally acceptable f8 would have a focal length of ...

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