Eyepieces, correctly called oculars, will, without any doubt, be the first accessory that you purchase after your telescope. Without oculars, you will not be able to see anything of importance or interest. Additional oculars, beyond the one or two supplied with most telescopes, make visual observing much more enjoyable and most variable-star observers have several oculars. Like camera lenses, they determine the field of view and magnification seen through your telescope. Owning a proper selection of oculars adds greatly to the versatility of any telescope. Before buying accessory oculars, I recommend that you take the time to understand some basic facts about these expensive little magnifying lenses that slip into the focuser or star diagonal of your telescope. A little understanding may save you some money too.

Huygens ocular - Designed by Christiaan Huygens (1629-95), this was the first eyepiece made available for use with a telescope. Huygens oculars normally consist of two plano-convex lenses with the flat side pointing at the eye. Today, these type of oculars are mainly used for solar and lunar observations (with the appropriate filter, of course). They have a small apparent field of view (AFOV), usually about 25-40n, and have no color correction. As a rule, these oculars generally have small eye relief.

Huygens-Mittenzwey ocular - A variant of the Huygens eyepiece. The lens that points to the object is replaced by a meniscus. It is a reasonable-to-good eyepiece for slow telescopes (/"/12 or more). The AFOV is approximately 45-50°. As with Huygens oculars, they generally have small eye relief.

Ramsden ocular - The first achromat, built with two plano-convex lenses, with the convex sides facing each other. They are still found nowadays, especially in small versions (0.965 inches). These oculars suffer from color aberrations but much less than the Huygens and Huygens-Mittenzwey oculars. Ramsden oculars reportedly suffer from internal reflections and usually have a small AFOV with short eye relief.

Kellner ocular - The three-element Kellner is named after Carl Kellner who was born on March 26, 1826 in Hirzenhain, Germany. With a good reputation as a producer of quality optical instruments, Kellner produced oculars for Argelander, among others. His eyepieces were primarily made for use in telescopes but he also produced some for use in microscopes. Eventually, Kellner and his twelve employees built complete telescopes. Until his early death in 1855, his workshop manufactured at least 130 microscopes, five big telescopes and a number of small hand-held telescopes.

Designed in 1849, the eyepiece known today as the Kellner, gives sharp, bright images at low-to-medium powers and is best used on small-to-medium size telescopes. Kellner oculars produce an AFOV close to 40° with good eye relief. As the power of magnification increases, the eye relief will shorten. They are usually considered good, low-cost oculars that are superior to simpler Ramsden and Huygenian designs. As you can see, the Kellner has been around for quite a while and you can find them readily available.

Orthoscopic ocular - The design of orthoscopic eyepieces dates back to the 1800s when Ernst Abbe first designed them to be used for accurate measurements of linear distance on microscope slides. The term orthoscopic means an eyepiece that introduces no barrel or pin-cushion distortion. As a result, an object will have the same size when observed anywhere in the field of view. The Abbe design employs a triplet field lens and a singlet eye lens.

Not too many years ago, the four-element ortho was considered the best all-around eyepiece available to the amateur astronomer. Today, because of its narrower field of view compared to new designs, the ortho has lost some of this reputation. Orthoscopic oculars produce excellent sharpness, color correction, contrast and have a longer eye relief than Kellners. They are considered great for planetary and lunar observing and are quite good for variable-star observing.

In most cases, the apparent field of view of each of these oculars is approximately 45°. In side-by-side comparisons, fields of view for orthos have been reported to appear equal to or slightly larger than Plossl oculars advertised as having 50: apparent field of view.

Plossl ocular - Considered today's most popular design by many, the four-element Plossl is named after Georg Simon Plossl and provides excellent image quality, good eye relief, and an AFOV of approximately 50°. Plossl was born on September 19, 1794 in Wieden near Vienna and he died on January 30, 1868 in Vienna following a severe injury after dropping a sheet of glass and cutting the artery near his right hand.

High-quality Plossl oculars exhibit high contrast and pin-point sharpness out to the edge and are considered ideal for all observing. Super-Plossls have a wider field-of-view than those simply called Plossl lenses. Plossl oculars are considered excellent for variable-star observing.

Erfle ocular - The Erfle lens is named after Heinrich Valentin Erfle who was born on April 11, 1884 in Duerkheim, Germany. His ocular is a five- or six-element optical arrangement and is optimized for a wide apparent field of 60-70°. At low powers, its view has been described as a "picture window." A wide viewing area provides impressive deep-sky views. At high powers, image sharpness reportedly suffers at the edges. Another excellent ocular for observing variable stars.

Wide field ocular - This ocular, with various names, is usually considered the premier ocular today. You will find super- and ultra-wide eyepieces that incorporate six to eight lens elements to produce apparent fields of view up to 85°. These oculars are best used at low-to-medium power. Wide field oculars have a remarkably wide field of view and you need to move your eye around to take in the whole view. Usually considered the ideal eyepiece for viewing galaxies, star fields and other deep-sky objects, they are superior for observing variable stars since many comparison stars can be viewed within the large field of view. Image quality is excellent, but the number of elements slightly reduces light transmission. This should not really present any concern; however, you will pay a premium price for the ultra-wide field of view!

Barlow lens - Not really an ocular but one of the more useful and cost-effective tools for amateur astronomers is a Barlow lens. The lens is named after Peter Barlow who was born at Norwich, England in October 1776. The now famous Barlow Lens is the result of a collaboration of Barlow with George Dollond. Barlow calculated a concave achromatic lens that Dollond made in 1833 and mounted to a telescope. Dawes employed it first while measuring close double stars.

A Barlow lens, inserted between the telescope and the ocular, can increase the power of the ocular by two, three and even five times. Of course, when you increase the power of your ocular by two using a Barlow lens, you decrease the field-of-view by two. The trade-off is often fair though. You can double the effective number of oculars that you have by purchasing one Barlow lens. For example, if you own a 40 mm, 25 mm, and 18 mm ocular then a 2x Barlow lens also provides you with a 20 mm, 12.5 mm and a 9 mm lens. Another advantage is eye relief. Looking through a nice big 25 mm ocular is effortless because the eye relief is large. You can keep that same comfortable eye relief and increase the magnifying power of your telescope by simply using a 2 x Barlow. You now have the same eye relief of the larger ocular but you are effectively using a higher-power ocular. Carefully select your oculars so that a Barlow lens doesn't duplicate an ocular already in your possession. In other words, if you have a 32 mm ocular and a 2x Barlow lens, don't buy a 16 mm ocular.

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