Eyepiece designs

After a century of stagnation, eyepiece technology began to advance rapidly around 1980, and today you can buy finer eyepieces than previous generations would have dreamed of. You can also spend more money on them. There have been three main advances: sharper images, more eye relief, and wider fields.

Figure 6.3 shows a range of eyepiece designs. At the bottom is the 300-year-old Huygens (HOY-khens) or Huygenian (H) design, a cheap two-element eyepiece still supplied with some low-cost telescopes. It works well only at high f-ratios and I do not recommend it. The improvement when switching to a better eyepiece, even a humble Kellner, is often dramatic.

The Kellner, Achromatic Ramsden, and Modified Achromatic (K, AR, and MA respectively) are three variations on a 150-year-old three-element design. This was the usual general-purpose eyepiece a generation ago; today, most people go for Plossls and Orthoscopics, which are sharper and give more eye relief.

The Abbe Orthoscopic has a following among planet observers and until the 1970s was generally considered the best eyepiece available. The images are sharp and eye relief is relatively abundant.

Figure 6.3. Some popular eyepiece designs, with the newest at the top.

In 1979, Al Nagler of Tele Vue Optics invented a minor improvement to the century-old Plossl (PLUS-l) design, and Tele Vue Plossls quickly became the standard to which other eyepieces were compared. Celestron, Meade, and other manufacturers responded by marketing their own Plossls. Today, the Plossl is the workhorse of eyepieces, the usual choice when other concerns are not overriding. No other type gives a sharper image at the center of the field. To get a slightly wider field of view, several manufacturers add a fifth lens element.

Eyepieces of the Erfle type are popular with observers who enjoy wide fields.1 Many observers first learned about the Erfle via surplus eyepieces from World War II gunsights.

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