Eyepiece Wisdom

September 9, 2008

Think it’s tough to choose a telescope? Wait until you try to choose a good eyepiece: there’s a bewildering variety on the market right now. Here are a few words of wisdom to help you choose an eyepiece that coaxes the most out of your telescope.


The Basics

Here are arguably the most important numbers you need to know about an eyepiece:

• Focal length. Eyepieces have focal lengths of 2 mm to 56 mm. Longer focal lengths give lower magnification and larger exit pupils. You learned how to calculate these numbers earlier. Whatever magnification you choose, just make sure the exit pupil lies between 0.7 mm and 7 mm.

• Barrel diameter. Most eyepieces have a barrel diameter of 1.25”. Some low-power eyepieces use a 2” barrel rather than 1.25” to accept a wider cone of light and allow a wider field of view. You may need an adapter to accommodate these larger eyepieces in your scope.

• Apparent Field of View (AFOV). This is the angular diameter of the circle you see when you hold an eyepiece up to your eye. A wider field shows you more sky, so it’s handy if you don’t have a motor drive on your telescope since you have to move the telescope less frequently. A Plossl-type eyepiece has a 50-degree AFOV. A more expensive Nagler-type has an 82-degree field of view… this means you see 160% more sky than a Plossl at the same magnification.

• Eye Relief. This is the distance you hold your eye from the outer lens to see the full exit pupil. Short eye relief means you have to jam your eye up close to the lens. If your eyes suffer from astigmatism, you’ll need to wear glasses when using your telescope and you’ll need eyepieces with longer eye relief… at least 17-20 mm. Without glasses, 10-20 mm of eye relief is fairly comfortable.

Tele Vue Nagler 9.0mm Type 6 (1.25

A 9 mm Nagler eyepiece: “spacewalk views”

• Size and Weight. Some complex eyepieces weigh in at nearly 2 pounds and are the size of a hand grenade! That’s too much for many small telescope mounts to support.

A Deeper Look

• Your telescope likely came with 2 or 3 eyepieces of moderate quality. Don’t bother “upgrading” with a full set of 7-8 similar eyepieces: you don’t need that many. Instead, buy 3 or 4 premium eyepieces that cover low, medium, and high magnification. But buy the highest-quality eyepieces you can afford.

• As a rule of thumb, if you can afford only one premium eyepiece, get one with a focal length twice the focal ratio of your telescope. This gives you an exit pupil of 2 mm. For example, if you have an f/10 Schmidt-Cassegrain, many celestial objects look sharpest with a 20 mm eyepiece. If you have an f/5 Dobsonian, get a good eyepiece with a 10 mm focal length.

Good To Know

If you have a decent telescope and your budget allows, don’t be shy about spending $300-$400 on a premium eyepiece. It will hold its value over time, just like a fine watch. And the views are well worth it.

Personal View

For many years, I used a couple of decent-quality Plossl eyepieces with my 8-inch SCT. They worked fine. But when I had my first look through a top-notch eyepiece, all I could say was, “Holy @!#*.” Not very eloquent, I know. But truly… the contrast, image, and amazingly wide field of view of a premium eyepiece were absolutely stunning.

Posted in Telescopes and Observing



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