Peter Santangeli wrote:
>
> That seems maybe a teensy bit drastic (though it clearly depends
> on your eyes). I regularly use my 35 pano in my F4 SN. Maybe the
> exact number is personal though (for me it might be 8).
Yes, it's personal, and it varies with degree of dark adaptation
and with age, perhaps also with fatigue.
The correct number is the size in mm of your dark-adapted pupil.
For young people (under 30 or so) it is typically close to 7 mm. For
elderly ones, it may be 3 mm. I know someone whose dark-adapted pupil
was 10 millimeters, but that's rare.
(Focal ratio) times (eye's pupil diameter) gives the longest
eyepiece that will not "waste" light. Depending on conditions, it may
be useful to use a longer eyepiece, for instance to get a wider true
field. But be aware that the scope is no longer functioning at its
full aperture.
I've used a 56mm eyepiece in my 4.25 inch f/4 Newtonian, to get 6
degrees of true field at 7.7 power. But the beam coming from the
eyepiece (the telescope's exit pupil) was 14mm! Since my eye's pupil
may be about 4mm, the scope was effectively stopped to 1.2 inches.
Clear Skies!
--
Jim Van Nuland, San Jose (California) Astronomical Association
<http://www.sjaa.net/school/indexs.htm> School star party site
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Received on Wed Aug 27 14:57:44 2008
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