This is the third of a series of articles about astronomy equipment to help you decide what you might like. If you have questions about a specific piece of equipment, and how it compares to alternatives, send a note to me at and I'll see what I can find out.

Now that we have talked about eyepiece basics, it's time to talk about eyepiece designs. We will discuss the pros and cons as well as some of the characteristics of various designs available. Specifically we will cover Ramsden, Kellner, Orthoscopic, Plossl, 5 and 6 Element Erfle, Panoptic, and Nagler.

Ramsden eyepieces consist of 2 optical elements and are very lightweight. They often exhibit moderate chromatic aberration and field curvature, and have a small apparent Field of View. (20 to 30 degree apparent field of view)

Kellner eyepieces consist of 3 optical elements at a very low cost and fairly good eye relief. They do often have some chromatic aberration and field curvature. (40 to 45 degree apparent field of view)

Orthoscopic eyepieces consist of 4 optical elements. They deliver excellent contrast, and minimal chromatic aberration and field curvature. They also have some internal reflections. (45 to 50 degree apparent field of view)

Plossl eyepieces consist of 4 or 5 optical elements at relatively low cost. Eye relief is 8/11 of the focal length of the eyepiece and there are minimal optical aberrations. They are prone to internal reflections. (50 to 55 degree apparent field of view)

5-Element Erfle and 6-Element Erfle eyepieces are low cost with minimal chromatic aberrations and field curvature. They sometimes exhibit astigmatism at the field edges. (60 to 70 degree apparent field of view)

Panoptic eyepieces consist of 6 optical elements. Eye relief is 17/25 of the focal length of the eyepiece and they have a large true field of view. They are rather pricey and the edge of the field often has a pincushion distortion. (68 degree apparent field of view)

Nagler eyepieces consist of 6 to 8 optical elements. They provide a huge field of view, have no optical aberrations, and provide a very nice contrast. The only con for the Nagler is their price.

Our ideal telescope probably deserves Nagler eyepieces, or at least one of them.