Ken Hough's Website
Astronomy: Telescope Eyepieces
For normal viewing, telescopes must be fitted with an eyepiece. As with telescopes and mounts, there are many types of eyepiece available. Not all are suited to modern wide aperture telescopes. To begin with, just stick with standard Plössl or Super Plössl types and you won't go far wrong. These eyepieces are popular and so are also relatively cheap. Most are of good quality and typically with all optical surfaces having been coated to minimise internal reflections and light scatter.
The main points concerning choice of eyepieces are as follows :
Size: Eyepiece size is a measure of the diameter of the barrel that fits into the focuser/eyepiece holder of a telescope. Nowadays, only two sizes of eyepiece need be considered. ie 1 1/4" (32mm) and 2" (50.8mm). Smaller telescopes are fitted with focusers that can only take 1 1/4" sized eyepieces. Medium to large telescopes might accomodate 2" eyepieces. The larger focusers can be used with adaptors that allow the use of the smaller eyepieces, but the converse is not possible.
2" eyepieces can be nice to use, but they are expensive and heavy.
Note: Previously, eyepieces of 0.965" in diameter were produced, but this size is no longer supported on modern telescopes. Other sizes that might be used on very large telescopes need not concern us here.
Focal length: Just as telescopes are described as having certain focal lengths, so are eyepieces. It is the combination of these two parameters that defines the overall image magnification that will be seen via the eyepiece. Refer to the section on Magnification.
Eyepieces of between 2mm and 40mm focal length are available, but depending on the focal length of the telescope being used, the most commonly used are between 7mm and 30mm. Eyepieces of very short focal length tend to provide only limited eye relief (see below). The optics of some telescopes cannot fully accomodate eyepieces of very long focal length.
Telescopes are often supplied with two Plössl or Super Plössl type eyepieces, chosen to give low magnification (approx 30x) and medium/high magnification (approx 100x).
Field of view: The angular field of view provided by modern Plössl / Super Plössl eyepieces is approximately 50 to 60 degrees which is adequate for most purposes. Wide angle and ultra wide angle eyepieces with fields of view of 80 and even 100 degrees or so are possible, but these don't always work well with wide aperture telescopes and likely to produce lower contrast images than is the case for 'normal' eyepieces.
I have an f=30mm / 2" ultra wide angle eyepiece (weight approx 0.5kg!). This works well on a long focal length refractor telescope (f9) and on a Maksutov (F11), but on a wide aperture (f4.8) Newtonian telescope, the outer parts of the field are very poorly defined. The very wide views that are seen through this eyepiece can be stunning, but for viewing of single objects, the higher contrast and sharpness provided by conventional eyepieces (eg Plössl) is preferable.
Eye relief is defined as the optimum distance bewtween the viewers eye and the front of the eyepiece. As a rule, long focal length eyepieces provide the longest eye relief, whereas eyepieces of short focal length can in some cases have uncomfortably short eye relief. Depending on focal length, eye relief can be between 2mm and 20mm.
Viewing from outside the eye relief distance results in a decreased field of view. A minimum eye relief of around 5mm is suggested so as to allow for the viewers eye lashes and comfortable viewing. Those who must wear glasses will need longer eye relief of up to 20mm, which can be a problem with short focal length Plössl type eyepieces. There are other types of eyepiece which provide extra long eye relief, but you should do your own research on this. Choice is likely to depend on the type of telescope being used and on available budget.
A discussion about eyepieces wouldn't be complete without mentioning 'Barlow lenses'. These devices can be inserted between the eyepiece and the focuser of a telescope so as to increase the effective focal length of the telescope and thus the magnification of the system. Typically a 'kit' telescope will be supplied with two eyepieces and a Barlow, thus providing four possible magnifications. Most Barlows are marked as X2 which means that the effective focal length of a telescope will be doubled. In this way a 25mm and a 10mm can also be made to behave as 12.5mm and 5mm eyepieces. An advantage in doing this is that eye relief distance of the basic eyepieces is retained in the presence of a Barlow.
I have deliberately avoided specific mention of other types of eyepiece mainly because Plössl/Super Plössl eyepieces will provide a good and sensibly economical starting point. There are many other types of eyepiece, each with their own advantages, and in some types, serious disadvantages. Some can be expensive! I suggest that you refer to wikipedia for descriptions of the various types and then to the catalogues of suppliers. You should also try to talk with experienced astronomers and try out their equipment.