The eyepiece is half of your telescope. It is the part of the telescope that we become most familiar with. The eyepiece determines the magnification, the field of view, and, to a large degree, our level of comfort while observing. Let’s take a closer look.
Your eyepiece has three specifications. The focal length of the eyepiece determines your magnification. Divide the focal length of your telescope by the focal length of your eyepiece. For a 2000 mm telescope, a typical 25 mm eyepiece will yield 80X. Want to get closer? Try a 10 mm eyepiece, which yields 200X. Want to back off? A 32 mm eyepiece yields 62X.
The field of view determines how much sky you will see. Eyepieces with smaller fields of view (around 40 degrees) look “narrow”, but tend to be more accurate throughout their fields. Those with larger fields of view are more immersive and involving, but tend to be more expensive.
The eye relief measures how far away you need to be from the eyepiece to see the view. If you wear eyeglasses, you will need eye relief figures of 18mm to 20mm.
There are three sizes of eyepieces, measured by their barrel diameters: .965 inches, 1.25 inches, and 2 inches. The smallest .965 inches eyepieces tend to be found in department store toys and are almost always worthless. Stay away from them. The 1.25-inch units are the standard of the industry. Around 80 percent to 90 percent of all eyepieces are in this category. The large 2-inch eyepieces are becoming more popular due to their huge fields of view and generous eye relief, the latter being important for aging observers who wear glasses. However, 2-inch units tend to be large and expensive.
There are many different eyepiece designs. Most of them are found among generic manufacturers, although a selection have known brand names attached. Among the eyepieces:
- The Huygenian and Ramsden designs are simple, outdated designs, found packaged with cheap .965-inch department store telescopes. Avoid them.
- The Kellner is a simple, inexpensive three-element design found in less expensive telescopes. These are acceptable for general use where cost is an issue.
- The four-element Orthoscopic has been a favorite among lunar and planetary observers due to its high accuracy. However, it tends to have a narrow field of view and tight eye relief.
- The four-element Plossl has become the standard general-purpose eyepiece. You can find the Plossl with manufacturers such as Orion, Celestron, Meade and Explore Scientific. It has a good field of view, and better eye relief than the simpler designs.
- The five-element Erfle was the first attempt at a wide-field eyepiece. Its wide field was compromised by less-than-sharp views, especially as you get near the edges.
Eyepieces stayed within these same designs for hundreds of years. Then, in the early 1980s, everything changed. An optical engineer named Al Nagler developed a large, heavy, and (for its time) very expensive eyepiece he coined a Nagler, which had a whopping 82-degree field of view. He coined the view through these eyepieces a “spacewalk.”
I remember seeing ads for these eyepieces as a kid, thinking this must be some kind of joke. At a time when eyepieces were selling for $20 to $50, the Naglers were close to $200. Then one day I looked through one. The human eye cannot take in an 82 degree field of view all at once, so it felt like looking out the porthole of a spaceship. Unlike the Erfle, stars were sharp all the way out to the edge.
Al Nagler’s company, Tele Vue, the firm produces several lines of premium eyepieces that are the standard for serious observers. They include the Panoptic, DeLite, and the new (and to the author, amazing) 100-degree field-of-view Ethos series. The latter can cost up to $1,000 each, making the author actually wish for the days of $200 Naglers!
We live in a market full of excellent eyepiece choices today. Most reputable manufacturers have excellent quality choices at reasonable prices, in both the traditional and Nagler-type “clone” designs. As always, try before you buy if possible.