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Telescope Accessories Help

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By — McGraw-Hill Professional
Updated on Sep 19, 2011

The Eyepiece

You’ll need certain accessories with your telescope. You’ll want at least two good eyepieces. You will need some sort of finder scope or sighting device. A Barlow lens can provide extra magnification for your eyepieces. And, of course, there are optical filters of all kinds, some for looking at the Sun, others for the Moon, some for the planets, and others for more sophisticated purposes.

There are many different designs for telescope eyepieces. All make use of two or more lenses to optimize the apparent field of view, to provide good focus from the center of the view field to the edge, and to make it easy to look through the device. Most eyepieces have focal lengths between 4 and 40 mm.

In general, the longer the focal length of an eyepiece taken as a whole, the lower is the telescope magnification, all other things being equal. Remember how to calculate telescope magnification: Divide the focal length of the objective by the focal length of the eyepiece in the same units. If a telescope has a focal length of 2 m (or 2,000 mm), then a 4-mm eyepiece provides 500× and a 40-mm eyepiece provides 50×. The overall focal length of a telescope eyepiece is not necessarily the same as the focal length of any of its individual lenses.

Eye relief is an important specification of any telescope eyepiece. This is the maximum distance, in millimeters, that the surface of the eye can be away from the surface of the eyepiece lens on the observer side while still letting the observer see the entire apparent field of view. In general, the longer the focal length of an eyepiece, the greater is the eye relief. Larger eye relief numbers translate into easier viewing.

Some people find it difficult and unpleasant to look through short-focal-length eyepieces (6 mm or less) because the lens diameter is more or less proportional to the focal length. A few eyepieces have observer-side lens diameters smaller than the diameter of the pupil of the eye itself. This makes it necessary to bring the eye very close to the eyepiece. If the observer wears glasses, viewing through such eyepieces is compromised. People who don’t wear glasses will flinch away from the eyepiece if its surface comes into direct contact with the eyeball.

An eyepiece’s outside barrel diameter always should match the inside diameter of the telescope’s focusing mount . This is 31.75 mm (1¼ in) in most telescopes, but some instruments have focusing mounts that are 50.80 mm (2 in) across as measured through the inside. Adapters can be found to get small-diameter eyepieces into large-diameter mounts. However, if you want to use a large-diameter eyepiece in a small-diameter mount, you’ll have to improvise.

Eyepiece Designs

Here are four different types of eyepieces you are likely to find on the amateur market. Of these, the Ramsden and the Kellner are the simplest and therefore the cheapest. The orthoscopic and the Plossl are more sophisticated and expensive.

Figure 20-6 A is a cross-sectional diagram of a Ramsden eyepiece. It consists of two planoconvex elements that have the same focal length. The larger lens is toward the telescope objective, and the shorter element is toward the observer. (This is true of virtually all telescope eyepieces.) The convex surfaces of the lenses face inward toward each other, and the flat surfaces face outward. This is an old design, dating all the way back to the 1700s. The Ramsden eyepiece is difficult to optimize because the spacing between the lenses is always a tradeoff between eye relief and the effects of lens aberration.

Figure 20-6 B shows the Kellner design. It is similar to the Ramsden, except that the observer-side lens is a compound element consisting of a convex lens glued to a planoconcave lens. The compound element, when designed properly, eliminates the chromatic aberration inherent in the Ramsden design. The lenses must be coated to minimize reflection of light inside the eyepiece. Kellner eyepieces work best at the longer focal lengths, providing low to medium telescope magnification.

The orthoscopic eyepiece (see Fig. 20-6 C ) is among the most popular designs in use today. Image distortion and chromatic aberration are eliminated by the three-element compound lens on the objective side. This type of lens is noted for its excellent contrast and its ability to maintain focus from the center of the view field to the edge. In addition, the view field appears relatively flat as compared with some eyepiece designs that give the view field a concave (bowl-shaped) appearance. Orthoscopic lenses work well at all focal lengths.

Your Home Observatory Telescope
Accessories Eyepiece Designs

Figure 20-6. Common telescope eyepiece designs: Ramsden ( A ), Kellner ( B ), orthoscopic ( C ), and Plossl ( D ). In each drawing, the observer’s eye is at left and the incoming light arrives from the right.

Figure 20-6 D is a cross-sectional diagram of a Plossl eyepiece. This design first gained widespread acceptance among amateur astronomers in the 1980s. It has all the assets of the orthoscopic eyepiece. The eye relief of a well-made Plossl is adequate even at the shortest focal lengths. The observer-side lens has a relatively wide diameter. Plossls with long focal lengths (25 to 40 mm) are physically bulky, projecting some distance out from the telescope’s eyepiece tube, but they offer the ultimate in viewing comfort. Some have rubber eye guards to keep out external light.

Other eyepiece designs you might encounter are the Erfle , the zoom , the RKE , and the Huygens . The sheer variety of eyepieces can confuse the novice amateur astronomer. You can get on the Internet, enter eyepiece designs as keyword phrases (for example, Kellner eyepiece ), and see what various folks have to say about the different designs. A salesperson at a hobby shop sometimes can help, but beware. A salesperson may be more motivated to get you to spend a lot of money than to sell you the best eyepieces for your needs.

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