Every so often, you might hear astronomers mention the exit pupil of certain eyepiece/telescope combinations. While exit pupil is more commonly discussed when considering binoculars, it is also quite important to telescope users. So what does it all really mean? Also, how can this exit pupil affect my views through a telescope?
The exit pupil is the image of the objective element as projected by the eyepiece. Thus, all light collected by the telescope passes through the exit pupil. Ideally, the exit pupil of the telescope should be approximately the size of the pupil in your eye. Any larger and light might be wasted by not entering the eye's pupil.
Calculating Exit Pupil
Exit pupil is defined as the objective diameter divided by the magnification. So to find your telescope and eyepiece combo's exit pupil, take the focal length of your telescope and divide it by the focal length of your eyepiece. This is the magnification of your telescope and eyepiece combo. Now, divide the objective diameter by this magnification. Make sure you do this all in the same units, i.e. completely in millimeters. The result is your telescope and eyepiece combo's exit pupil. So,
For example, my telescope has an objective diameter of 254 mm. If I use a magnification of 40x, I would have an exit pupil of 6.35 mm, near the 7mm "maximum" available to young eyes.
Find Your Eye's Pupil Size
In order to fully understand the effects of exit pupil, you should know the maximum dilated pupil size of your own eyes. This sounds like it might be difficult to measure, but is not as hard as it sounds. Go into a dark room and let your pupils adjust for a minute or two. Take a piece of dark paper, and cut two slits in it, spaced a certain distance apart. Start with 7 mm and work down. You will be able to see two images through these slits. When the two images just barely touch, you have reached your pupil size. Pupil size typically decreases with age. Young people may have pupil sizes over 7 mm, but will sharply decrease between the ages of 30 and 60 to 6 mm and below.
Also, check out Mang's page on how to find your pupil size using a camera!
The importance of this is that if you have a pupil size of 6 mm, any exit pupil over 6 mm is a waste of light! Thus you are not using the full aperture of your telescope. Using an exit pupil that is the same size of your eye's pupil will allow you to see the widest field of view and see the "richest field" your telescope can provide. Keep this in mind when pricing your next eyepiece purchase!