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Originally Posted by yagon
Can anyone comment on the effect on the exit pupil?
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The exit pupil is the diameter of the light cone from a specified eye piece. What that means is if the exit pupil is too large a lot of the telescopes light is wasted. For example a healthy 20 year old's pupil diameter is approx 7mm. If the exit pupil of a particular eyepiece/ telescope combination is more than 7mm some of the light is wasted and the image is not as bright. To determine the exit pupil you divide the telescopes aperture by the eyepieces magnification
aperture/ magnification= exit pupil
80mm/x25 = 5mm exit pupil
or
aperture/ (telescope focal length/ ep focal length)=exit pupil
80mm/ (400mm/16mm) = 5mm
A 5mm exit pupil is good for low power eyepieces. A 7mm exit pupil is good under a very dark sky but any higher than that you will get dimminishing returns on your telescope as you start wasting light.
using the above equation with a 72mm eyepiece
80mm/ (400mm/72mm)= 14mm a twice the exit pupil of a 20 year old under dark skies the telescope effectively has an aperture of 40mm with the 72mm eyepiece.
I hope I haven't confused you
