Exit pupil is another important concept to understand

Exit pupil is another important concept to understand, though it is not a property of an eyepiece by itself; instead, it is a characteristic of one specific eyepiece when used with a particular telescope. When the eyepiece is in perfect focus, all of the light from it will be contained in a tiny circle at the distance above the eye lens where the image is focused. The exit pupil is the diameter of this small circle. The exit pupil of an eyepiece on a particular telescope equals the diameter of the telescope’s objective divided by the magnification. Thus, if your telescope’s objective has a diameter of 125 mm and your eyepiece is giving you a magnification of 25 times, the exit pupil of the eyepiece will be 5 mm.

So, why is exit pupil so important? If the diameter of your eye’s pupil is the same diameter or a greater diameter than the exit pupil of the ocular you are using, your eye will receive all of the light from the eyepiece. In that case, the image will appear the brightest it can possibly be. However, should it be that the pupil of your eye can not dilate to a size that is at least equal to the exit pupil, some of the outer edge of the small circle of light will not be able to enter your eye and the image you see will be dimmed.

Given the explanation in the immediately preceding paragraph, it becomes apparent that it would be very useful to know what is the maximum size exit pupil that you as an individual can use. This size varies from one individual to another, though it is possible to make an educated guess as to what your maximum allowable exit pupil will be. The pupils of teenagers and young adults can typically dilate to at least 7 mm. As a person gets older this maximum dilation diameter becomes gradually smaller until it reaches a lower limit. For most people this lower limit is about 5 mm. There you have it, shy away from eyepieces which will yield an exit pupil greater than 5 mm if you are age 30 or older. Everyone should avoid oculars yielding exit pupils greater than 7mm.

Now on to a discussion of ocular types.

The two simplest eyepiece types to be had are of the Ramsden and Huygens varieties. These contain two simple lenses (read that non-achromatic lenses); hence, they will add some false color to your view. The lens closest to the eye is called the eye lens. The other lens is called the field lens because it is this lens that widens the field that you see. The field lens keeps you from experiencing the effect of looking through a narrow tunnel that you would usually get with a five dollar toy telescope. Of the two, the Huygens eyepiece has slightly better color correction, while the Ramsden has far less spherical aberration. Ramsden or Huygens oculars are usually supplied with low cost 2.4 inch telescopes.