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Eyepieces
 
Choosing an eyepiece depends on what type of telescope and what you intend to look at.
 
There are many types of eyepieces. They have been evolving over the past 300 years. This page lists and describes them in a somewhat historical order. Some relatively old designs are still popular today.
Huyghens
  Developed more than 300 years ago by Christian Huyghens this eyepiece was used in many early telescopes.
Kellner
  About 150 years later in 1849 Carl Kellner created the first achromatic eyepiece using special glasses. This was a vast improve reducing aberrations and also had good color correction. The only problem was this eyepiece had internal reflections. Today modern coatings are applied to eliminate reflections making this a fair inexpensive eyepiece.
Orthoscopic
(A planetary eyepiece)
 

The next innovation came about 30 years later in 1880 by Ernst Abbe. With good color correction, minimal spherical aberration, good contrast, and a flat field of view these have been one of the most popular eyepieces for more than 100 years. With newer wider field eyepieces developed in recent years this eyepiece is not quite as popular. Since contrast and color are more important than field of view when viewing planets, this is still a popular planetary eyepiece. This is one of my favorite eyepieces for viewing planets.

Plossl
(A versatile eyepiece)
  Developed in 1860 by an Austrian optician named G.S. Plossl, it was nearly 100 years before this eyepiece became popular. These are expensive to make and therefore cost more than an ortho. These eyepieces have great eye relief, field of view, and contrast making them a ideal for all observing targets.
Erfle
  These eyepieces use five or six elements to give a very wide field of view. This made the eyepiece popular from 1917 when it was invented until about recent years when advanced wide field eyepieces were developed.
Super Wide These modern eyepieces have a field of view from 65-70 degrees. With the creation of the Televue Panoptic by Al Nagler a revolution in eyepiece design was spawned. The competition is fierce which is great for eyepiece consumers. These eyepieces are great for viewing large nebulae and star clusters.
Ultra-Wide With field of view of 80 degrees and greater, these eyepieces have become very popular with astronomers will to lay out the big sums of money. While these are great for many objects, a simpler design may give a better view of small bright objects like planets.
Barlow Lenses While not actually an eyepiece, a barlow is attached to an eyepiece that magnifies the image. Barlows are purchased by magnification power (1.5x, 2x, 4x...) Barlows are mainly used for planetary or binary star viewing since they can reduce the amount of light passing through. They are also a way to get high magnification and good eye relief at the same time.
Zooms Zoom eyepieces are very handy since as their name implies the magnification factor can be adjusted. While this makes them useful, a good quality zoom is very expensive and still not as good as having several eyepieces of varying magnification.
Bino Viewers

Binoviewers are devices that allow you to use both of your eyes to view the image in a telescope. The results are truly dramatic. Images have an an almost 3 dimensional look. When you use both eyes you are also using both sides of you brain.

The thing to keep in mind about binoviewers is that you need two of each eyepiece. This can become very expensive.

   
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