Most people have about a 7 mm pupil diameter when dark-adapted, and that
reduces some with age. If you are using an exit pupil (= ocular focal
length divided by f/ratio of the scope) larger than your dark-adapted pupil,
you are literally waisting light and not using the full aperture of the
telescope.
Kevin
On Wed, Aug 27, 2008 at 1:36 PM, Peter Santangeli <peter@No-Spam>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 my it might be 8).
>
> Pete
>
> Jamie Dillon 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. I just
>> remind myself to avoid the kidney-beaning. Yes this has to do with exit
>> pupil, just a simple way to work the equation. Dickinson and Dyer bring
>> the
>> x7 bit up in the new version of The Backyard Astronomer's Guide.
>> Invaluable
>> book.
>>
>> Once I got a free 40mm Plossl from Discovery scopes. In my 11" f/4.5 the
>> kidney-beaning was impressive.
>>
>>
>>
>>
>
>
> ---
>
> Aug 25, 2008: TAC Web Page Updated
> http://observers.org/TAC.cgi/Announcements/
> CalStar - Sept 25-27, BYO Party! http://www.sjaa.net/calstar/
> TAC mailing list - join or leave here:
> http://seds.org/mailman/listinfo/sf-bay-tac
>
---
Aug 25, 2008: TAC Web Page Updated http://observers.org/TAC.cgi/Announcements/
CalStar - Sept 25-27, BYO Party! http://www.sjaa.net/calstar/
TAC mailing list - join or leave here: http://seds.org/mailman/listinfo/sf-bay-tac
Received on Wed Aug 27 15:01:54 2008
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