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How Telescopes Work

Schematic diagrams of various eyepieces
Schematic diagrams of various eyepieces

Next to the telescope itself and the mount, the eyepieces will be your most important purchase. Most telescopes come with one eyepiece (low power), some telescopes come with none. Therefore, you may have to purchase eyepieces so that you can vary the magnification of your telescope.

Eyepieces come in many designs:

  • Huygens
  • Ramsden
  • Orthoscopic
  • Kellner and RKE
  • Erfle
  • Plossl
  • Nagler
  • Barlow (used in combination with another eyepiece to increase magnification 2 to 3 times)

The designs vary in terms of the number and types of lenses, or elements, they use. Eyepieces should be evaluated for the following:

  • optical quality
  • field size
  • brightness
  • sharpness
  • lack of aberrations (chromatic aberrations, ghost images)
  • eye relief (distance from focal point, your eye, to the lens -- especially important for eyeglass wearers)
  • barrel size - 0.965 inches, 1.25 inches, 2 inches
  • price

Huygens and Ramsden eyepieces are the oldest designs. They suffer from chromatic aberrations and are often included with "cheap, department store" 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 (RKE is Edmund Scientific's patented modification of Kellner) are three-element designs that produce images in a 40-degree field of view, and they have some chromatic aberration. They have good eye relief. Kellners work best in long focal length telescopes. They are a good balance between performance and economy, varying from $30 to $50 each.

Set of RKE eyepieces
Set of RKE eyepieces

Erfle eyepieces were invented during World War II. They have a five-element design and a wide field of view (60 degrees). They suffer from ghost images and astigmatism, which makes them unsuitable for planetary viewing. Improvements on the Erfle design are called wide-field eyepieces. They can range from $50 to $300 each.

Plossl eyepieces have a four- 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 and range from $50 to $150 each.

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, are heavy (up to 2 pounds / 1 kg) and expensive ($150 to $400 each).

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. A Barlow lens can range from $30 to $70.

One final category of eyepieces are those with illuminated reticles. They 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. They can range from $100 to $200 dollars each.