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  1. #1
    PS Surfer's Avatar
    PS Surfer Guest

    Default Scope focal length vs Eyepiece focal length

    On my daily walk with my daughter, my thoughts began to wander thinking
    about building another scope, but the following riddle, so to speak, came
    to mind. Given two hypothetical scopes, say one 8" diameter, 750mm Focal
    length (scope A), and a 6" diameter 1500mm focal length (Scope B). If you
    take an 4mm focal length eyepiece and place it on scope A, and focus in on
    say Saturn, then an 8mm focal length eyepiece on scope B and focus in also
    on Saturn. Given that both of these scopes have equal magnification, and
    otherwise we will assume them to be of equal quality, and design, which
    would be the better setup?

    P Sorah A_T Mutual Data PERIOD COM

  2. #2
    Llanzlan Klazmon The 15th's Avatar
    Llanzlan Klazmon The 15th Guest

    Default Scope focal length vs Eyepiece focal length

    PS Surfer <> wrote in
    news:3f6f8698$0$51803$ m:

    Scope A Focal Ratio = 750/203 = 3.7

    Most eyepieces will not work well with such a fast scope. If the scope is
    a Newtonian it will also have severe coma.

    Scope B Focal Ratio = 1500/152 = 9.9

    If you are talking Newtonian, then this scope will have negligable coma
    and will work well with any eyepiece design.

    Scope B. Your scope A is just too fast to work well with most eyepieces.




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