* Astronomy

The eyepiece that you use in your telescope determines what magnification or power that you can see.  Eyepieces are usually described by their "focal lengths". A 4mm eyepiece is an example of a very short focal length, and will deliver high power. A 40mm eyepiece is a long focal length, and will render low power . These two examples are the approximate extremes of both ends of the typical scale. The typical eyepiece(s) that come with a new telescope are likely to be 25mm or 10mm, and yield 40x to 100x magnifications.  It should be noted that parfocal eyepieces can be interchanged without the need for refocusing (so it is worth sticking to one brand of eyepiece).

Eyepieces also come in different barrel diameters, 0.965", 1.25" (31.7mm), and 2".

Eyepieces are available in different basic designs. Some are suited for specific situations. Here are the most common types:

Eyepiece Constructions

The number on an eyepiece is its focal length in millimetres. The most common letters and their meanings are:
Er Erfle.	An extremely good relatively wide-angle eyepiece with a flat field. Invented in the 1940's, the 5 or 6-element Erfle delivers a wide 60- 70 degree apparent field of view. At low power they offer a "picture window" deep-sky experience. Erfle's are mostly available in long focal length or in 2-inch configurations, as they aren't suitable for high power, where they tend to exhibit ghost images. Also used in good quality wide-angle binoculars. Use at f/6 and greater.
H Huyghenian.	Most commonly provided with budget telescopes. A simple two-element eyepiece which gives a small field of view (about 30 degrees) but is clear of optical defects. Use at f/10 and greater.
HM Huygen Mittenzwey	Huygens-Mittenzwey is a variant of the Huygens eyepiece with the field lens replaced by a meniscus. The eyepiece will produce a 45-50° field of view but with low eyerelief.    Use at f/12 and greater
AH Achromatic Huygen
Ramsden	A simple two-element eyepiece of flat-convex lenses. It gives a small field of view (30°) with distinct optical defects. They are achromatic when the lenses are spaced apart at exactly the length of their focal lengths. However, such a system is extremely sensitive to dirt.
SR Special Ramsden	Usually a 3-element eyepiece similar to a reversed keller configuration, But designs vary.
AR Achromatic Ramsden
K Kellner.	A 3-element eyepiece usually provided with budget telescopes.They yield a sharp and bright image at low and medium power, with small or medium apertures. They have good eye relief, and about 40 degrees of apparent field. They are inexpensive, and far superior to Ramsden and Huygenian designs. It is often used in binoculars. Use at f/7 and greater.
MA Modified Achromat.	A variation on the Kellner.
RKE.	An RKE eyepiece has an achromatic field lens and double convex eye lens, a reversed adaptation of the Kellner eyepiece. It was designed by Dr. David Rank for the Edmund Scientific Corporation, who marketed it throughout the late 1960s and early 1970s. This design provides slightly wider field of view than classic Kellner design and makes it design similar to a widely spaced version of the König. There is some ambiguity about what RKE stands for. According to Edmund Scientific Corporation, RKE stands for Rank Kellner Eyepiece
Or Orthoscopic.	Orthoscopic eyepieces (Abbe eyepiece) are an excellent eyepiece, but with a relatively small field of view by today's standards. They have a slightly smaller field of view than Kellner eyepieces, but the image is fully achromatic, giving a higher quality image. Four-element "orthos" were once prized as the best all-around eyepiece for planetary, lunar, and double star observing. Use at f/6 and greater.
Monocentric	A Monocentric is an achromatic triplet lens with two pieces of crown glass cemented on both sides of a flint glass element. The elements are thick, strongly curved, and their surfaces have a common center giving it the name "monocentric"
Pl Plössl.	A good mid-range eyepiece. Currently probably the most common eyepiece. The 4-element Plössl is an achromatic configuration,  and yields an excellent image, good eye relief, and about a 50 degree apparent field of view. Use at f/5 and greater.
Super Plössl.	The Super Plössl Eyepiece is a modified Plössl, which thanks to the introduction of an additional lens increases the field of view to about 55°. The eyepiece consists of five lenses. This is the achromatic configuration, which gives a higher quality picture than standard Plössls..
Lanthanum	Lanthanum Eyepieces use lenses made of lanthanum doped glass which perfectly reducing the formation of a secondary spectrum and chromatic aberration. They are characterised by high eye relief. The picture quality is near perfection.
Panoptics & Naglers	Expensive high-end eyepieces for stunning images and performance. Panoptics and Naglers manufactured by Al Nagler (TeleVue). Panoptics are a 6-element design, and exhibit a 68 degree field of view. Naglers are a 6 or 7-element design, and have an 82 degree field. They have superb bright wide field views. (bit watch out for the weight) TeleVue also make "Radian", 6 or 7-element eyepieces, with 60 degree fields of view. All have 20mm eye-relief. Long eye-relief is important for eyeglass-wearers.
Ultrawides	They have apparent fields up to 85 degrees. The 6 to 8 element Superwide designs offer wide panoramic views. Although brightness is slightly diminished, image quality is very high.
Barlow	A"2X" Barlow will double the power of any eyepiece . A "3x" Barlow will triple the power. A 2x Barlow functions by dividing the focal length of any eyepiece by two. An eyepiece, for example, with a 20mm focal length becomes a 10mm eyepiece. Remember to fit the Barlow into the diagonal... Generally it is best not to use the barlow if you can avoid it. Though it must be said that the eye relief for those who wear eyeglasses does balance out the degraded image quality. (Solution - invest in a good quality one); An Achromatic (a double lens) should be the minimal quality, those with single lens should be avoided.  Apochromatic, Apo (triple lens) or ED, using special glass, (double lens) Barlow Lens would be the preferred choice.
Star diagonal	A commonly skimped on bit of equipment is the 'star diagonal'. Refractor telescopes have mirrored "diagonals" that angle the image to a more convenient eye position. The diagonals are either mirror or prism-diagonals. Mirror-diagonals yield better images than prisms. The standard diagonal on most telescopes are "ok" , but they will absorb some light.

Don't make the mistake of assuming that all things are better viewed with high powers. Most general viewing of comets or deep sky objects are done with modest 25x to 80x power.
However, to view Planets like Jupiter and Saturn high power is recommended. The moon, and many double stars can require the maximum that your telescope's optics can deliver. Upwards of 200x power is usual.