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It is important to note that, so long as the diameter of the eyepiece barrels is the same, any brand of eyepiece can be used in any telescope. The most common eyepiece types are described below. These are types and could be manufactured by many different manufacturers, just as SUVs or sedans can be made by many car companies.

Huyghenian: a two element design designed in the 1600's. It should be obsolete except that some of them (labeled ¡°H¡±) still come with low-priced telescopes. They have extremely short eye relief, chromatic aberration, field curvature, and a small apparent field of view.

Ramsden: Another old 2-element design (labeled R or SR), it¡¯s better than the Huyghenian, but it has a very small apparent field of view and is uncomfortable to use due to a small eye relief. There is a version of this design with a 2-element lens called an Achromatic Ramsden (AR).

Kellner: This is an older 3-element design. Think of it as the first of the color-corrected eyepieces. Kellners typically have an apparent field of about 40 to 45 degrees and comfortable eye relief. There are two other variants of this eyepiece, in one of which the lower lens is double instead of the upper lens, and in the other both lenses are double. This last type is usually called a Plossl.

Orthoscopic: A 4-element design with a moderately narrow apparent field of view (40-45 degrees), but still considered one of the best designs for planetary viewing. It¡¯s no longer as popular as it once was, due to superior quality eyepieces with larger apparent fields, but it has superb sharpness and color correction combined with longer eye relief than most eyepieces.

Plossl: A 4-element design, it provides excellent sharpness, comfortable eye relief and, typically, an apparent field of view of about 50 to 52 degrees. This is the most popular design on the market, and has only been improved with the addition of one additional lens. Excellent for lunar and planetary viewing. TeleVue is the premier maker of this type.

Erfle: A 5 or 6-element design, this was originally designed to provide a wider apparent field of view than the other eyepieces of its day. While originally designed with a 55 degree apparent field of view, later designs stretched this to 65 or 70 degrees. Unfortunately, the design suffers from a lack of tight color correction, and the wider-field versions have even more. The better widefield designs on the market have truly superseded this design.

[Insert Cross section diagram here] 5-Element Plossl (Improved Design). Taken from the design of the Zeiss Astroplanokular of the 1950¡¯s, this 5-element design is a significant improvement on the traditional 4-element Pl?ssl. It offers improvements in correction for lateral color and field curvature, as well as typically being from the world¡¯s highest quality manufacturers. This is the ¡°King of the Hill¡± of eyepieces with a narrower apparent field of view (50 to 52 degree apparent field). The Parks Gold Series is a premium example of this type.

Ultra-Wide Field eyepieces (various): These have many different designs incorporating 6 to 8 elements and have apparent fields of 65 to 90 degrees. The best ones have superb color correction, no astigmatism, excellent contrast, and offer a ¡°picture window¡± view of the universe. Their only drawbacks are weight and price. Meade and TeleVue are the 2 most popular sellers of this type.

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