On Wed, Aug 27, 2008 at 10:57 AM, Jamie Dillon <ngc1023jd@No-Spam> wrote:
> There's that very handy rule of thumb, multiply your f/ratio by 7, and don't
> mess with an eyepiece with a longer focal length than that number.
Ooo, I sense a mystery! Where does the 7 come from? Why not 8? Or 6.02E+23?
> Yes this has to do with exit pupil, just a simple way to work the equation.
Yeah. And here, for the great unwashed (including me, before I knew
what you were referring to), here is the equation:
fr * Pex = FL
where
fr = focal ratio of objective
Pex = exit pupil
FL = focal length of objective
Generally, for minimum magnification, the idea is to have the exit
pupil roughly match the entrance pupil of the eye, which is between 5
and 7mm for most people. That's where the 7 comes from.
I believe the main reason you don't want a very large exit pupil is
because at very large values, the central obstruction (in an
obstructed telescope) starts to become visible in each star you see.
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Received on Wed Aug 27 14:32:10 2008
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