An eyepiece is the second lens in a refractor, or the only lens in a reflector. Eyepieces come in many optical designs, and consist of one or more lenses in combination -- they are almost like mini-telescopes themselves.
The purposes of the eyepiece are to:
- produce and allow you to change the telescope's magnification
- produce a sharp image
- provide comfortable eye relief (the distance between your eye and the eyepiece when the image is in focus)
- determine the telescope's field of view: apparent - how much of the sky, in degrees, is seen edge-to-edge through the eyepiece alone (specified on the eyepiece; true or real - how much of the sky can be seen when that eyepiece is placed in the telescope (true field = apparent field/magnification)
There are many types of eyepiece designs:
- Kellner and RKE
- Barlow (used in combination with another eyepiece to increase magnification 2 to 3 times)
Huygens and Ramsden eyepieces are the oldest designs. They suffer from chromatic aberrations and are often included with the least expensive and least effective telescopes.
Orthoscopic eyepieces were invented by Ernst Abbe in 1880. They have four elements and a 45-degree apparent field of view, which is somewhat narrow. The optical design gives a crisp view, has a good eye relief, and is considered excellent for planetary viewing. Orthoscopic eyepieces can range from $50 to $100 each.
Kellner and RKE (Edmund Scientific's patented modification of Kellner) are a three-element design that produce images in a 40-degree field of view, with some chromatic aberration. They have good eye relief. Kellner eyepieces work best in long focal length telescopes. They are a good balance between performance and economy. They vary from $30 to $50 each.
Erfle eyepieces were invented during World War II. They have a five-element design and a wide, 60-degree field of view. They suffer from ghost images and astigmatism, which makes them unsuitable for planetary viewing. Improvements on the Erfle design are called wide-field eyepieces.
Plossl eyepieces have a four-element or five-element design, with a 50-degree field of view. They have good eye relief (except for 10 mm and shorter lenses). They work best in the 15- to 30-mm size. The quality is good, especially for planetary viewing. They have some astigmatism, especially at the edge of the field. They are popular eyepieces.
Nagler eyepieces were introduced in 1982, advertised as "like taking a spacewalk." They have a seven-element design with an incredible 82-degree field of view. They come in 2-inch barrel size only, and are heavy -- up to 2 pounds (1 kg) -- and expensive.
Barlow lenses can be an economical way to increase magnification and/or provide better eye relief with an existing eyepiece. The eyepiece fits into the Barlow lens, which then fits into the eyepiece holder.
One final category of eyepiece is the eyepiece with illuminated reticles. These eyepieces come in many designs, and are used exclusively for astrophotography. They aid in guiding the telescope to track an object during a film exposure, which can take anywhere from 10 minutes to an hour.