Eyepiece design and use
Kellner: The three-element Kellner, together with its close relatives the Achromatic Ramsden (“AR”) and Modified Achromatic (“MA”), is the least expensive eyepiece suitable for serious astronomy. It gives sharp, bright images at low to medium powers. Best used on small to medium-size telescopes, Kellners have apparent fields around 40° and reasonable eye relief, though short at higher powers. They’re good, low-cost performers, far superior to simpler Ramsden and Huygenian designs. A 40mm Kellner is an inexpensive way to get very low power on most telescopes.
Plossl: Today’s most popular design, the 4-element Plossl provides excellent image quality, good eye relief, and an apparent field of view around 50°. High-quality Plossls exhibit high contrast and good sharpness out to the edge. Ideal for all observing targets. Twenty years ago, these were considered “luxury” eyepieces for the well-heeled; today they are normal general-purpose eyepieces. Eyeglass wearers can generally use Orthoscopics and Plossls with focal lengths of 17mm or greater.
Orthoscopic: The four-element “ortho” was once considered the best all-around eyepiece, but has lost some of its luster because of its narrow field compared to newer designs. Orthos have excellent sharpness, color correction, and contrast, and longer eye relief than Kellners. They are especially good for planetary and lunar observing.
Erfle: The 5- or 6-element Erfle is optimized for a wide apparent field of 60° to 70°. At low powers, its big “picture window” viewing area provides impressive deep-sky views. At high powers, image sharpness suffers at the edges.
Ultrawides: Various improved designs incorporating 6 to 8 lens elements boast apparent fields up to 85° — so wide you have to move your eye around to take in the whole panorama (which some people like and others don’t). Light transmission is slightly diminished because of the additional lens elements, but otherwise the image quality in these eyepieces is very high. So, too, can be their price.
eyepieces: Illuminated reticle eyepieces generally come in Kellner, Orthoscopic, or Plossl designs. Of these, Plossls have the largest-diameter eye lens and the longest eye relief, for more comfortable viewing. Typical focal lengths are 9mm and 12.5mm; higher powers can be achieved by attaching a barlow lens.
Barlow: One of the more useful and cost-effective tools in an amateur astronomer's accessory case is a barlow lens. Invented in the early 1800s by British mathematics professor Peter Barlow, it is a simple concave (negative) lens that, when placed between a telescope's objective lens or mirror and the eyepiece, amplifies the magnifying power of the instrument. A barlow works by reducing the convergence of the light cone heading toward the eyepiece. In this way it increases the focal length of the telescope. Since magnification is determined by dividing the telescope's focal length by the eyepiece's focal length, you can see that by doubling the telescope's focal length, a barlow lens doubles the magnification of the system for a given eyepiece.