|
exit pupil considerations
The points made about apparent fields of view and astigmatism are relevant.
When selecting eyepieces, exit pupil is another consideration. While a healthy young persons eyes may dilate to 7mm, older observers may be limited to 5-6mm. If the scope exit pupil exceeds your pupil dilation you are not utilizing the full aperture, light from the outer part does not enter your pupil reducing the effective diameter of the scope.
EXIT PUPIL(mm) = EYEPIECE FL(mm) ÷ telescope ƒ ratio
another way to look at it is for a 7mm exit pupil :
Longest useable eyepiece focal length = ƒ ratio x 7
So for a 7mm exit pupil : -
f ratio ……….. Longest
eyepiece
(mm) 3 ……….. 21 3.3 ……….. 23 3.6 ……….. 25 4 ……….. 28 4.3 ……….. 30 4.7 ……….. 33 5 ……….. 35 5.3 ……….. 37 5.6 ……….. 39 6 ……….. 42 6.5 ……….. 46 7 ……….. 49 7.5 ……….. 53 8 ……….. 56 8.5 ……….. 60 9 ……….. 63 9.5 ……….. 67 10 ……….. 70 10.5 ……….. 74 11 ……….. 77
So for an f4.7 scope, 33-34mm is the about the longest eyepiece you should purchase. If you want wider fields, the 30mm or 31mm x 82 deg eyepieces are a better way than going to longer focal lengths.
Some people go longer just because they want the low power and accept the loss of light. Just also be aware that the central obstruction from the secondary starts to become visible as a dark central patch at large exit pupils. This can be quite annoying.
Joe
|