Telescope Eyepieces

Astronomical Eyepieces

Selecting an eyepiece to use with your telescope depends on what type of telescope and what you intend to look at. Wide angle eyepieces are great for looking at large objects like open clusters or large nemulae. Other eyepieces are better for looking at planets since magnification is more important than field of view. This page explains the differences and why you would select one over another for viewing different objects.

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.

Eyepiece Termanology

Design
Different eyepiece designs allow for a wide range of features, depending on the needs of the observer. Orthoscopic and Plossl designs can provide a great quality image with a standard field of view, but may be difficult to use at higher powers while wearing glasses due to the short eye-relief. Long eye-relief designs have more elements and can be more expensive, but work very well with glasses, even at higher powers. Wide angle designs provide a larger view of the sky at the same power as a standard eyepiece. More complex designs can combine all these attributes together in a single eyepiece.

Eyepiece Focal Length
The focal Length of the eypiece determines the magnification you will get with your telescope.

Barrel Size
Eyepices come in various barrel sizes.  The two prodomenant sizes are 1.25" and 2". 

Apparent Field of View
The edge-to-edge angular diameter of the light emerging from the eyepiece as seen by the eye. It is an inherent specification for a given eyepiece type or design. The apparent field of view of an eyepiece is directly related to the true field of view seen through the telescope; for a given focal length eyepiece, the greater the eyepiece's apparent field of view, the greater area of sky will be seen.

Eye Relief
The distance, measured in millimeters, between the observer's eye and the eyepiece lens in which the entire field of view remains visible. Eye relief varies with the optical design but generally increases with decreasing magnification. Long eye relief is advantageous for observers who wear glasses, as they don't have to put their eye directly on the eyepiece to see the entire field of view, nor do they have to remove their eyewear.

Coatings
Fully Multi-coated close × Lenses are coated with an anti-reflective material to ensure that as little light as possible is reflected away, thus more gets to your eye. Good lenses are at least "fully coated," with a single layer of magnesium fluoride applied to each air-to-glass lens surface. Multiple layers of coatings are even more effective; the term "multi-coated" means one or more lens surfaces have multiple coatings. "Fully multi-coated" is superior because all lens surfaces are multiple-layer coated. Mirror star diagonals are coated with a reflective material similar to telescope mirrors in order to reflect the light into the eyepiece. Standard aluminum coatings reflect between 88%-92%, and enhanced aluminum reflects 97%. Dielectric coatings use a different process to deposit a coating that reflects 99% of the light. In addition to the higher reflectivity, Dielectric coatings are much more durable than aluminum coatings, so they last longer and can be cleaned with less risk of damage.

Number of Elements
The number of elemts in an eyepiece refers to the number of lenses (pieces of glass).  Adding more elements allows manufacturers to create a greater depth of field.

Filter Threads
An eyepiece must have filter threads if you want to attach a filter.

Parfocal
Parfocal refers to an attribute of an eyepiece set where little to no focusing is required when swapping out one eyepiece for another.

Field Stop Diameter
The field stop is a mechanical ring in the eyepiece which provides the sharp edge to the field of view when looking through the eyepiece. The field stop diameter greatly affects the apparent field of view of the eyepiece; the larger the field stop diameter, the larger the apparent field of view is.

Barrel Security Recess
The barrel security recess is a shallow groove or depression cut into the 1.25" or 2" barrel of the eyepiece. In the event the thumb screw holding the eyepiece in the focuser becomes slightly loose, the eyepiece will not fall out because the tip of the thumb screw will catch on the lip of the security groove.

Blackened Lens Edges
Blackened lens edges in eyepieces and refractor lenses prevent unwanted light scatter inside the tube. This results in higher image contrast. 
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.