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A Guide to EyepiecesEyepiece Sizes
There is one specification that is very important when you are considering an alternative eyepiece for your telescope — its physical size. The vast majority available today have chromed barrels with a diameter of 1¼ inches (31.7 millimeters), that slide into the push-fit focusers of most telescopes. You will also encounter designs intended to show you wide views with barrels 2 inches (50.8 mm) in diameter. Occasionally you will come across scopes (typically older ones) that accommodate 0.965 inch (24.5 mm) barrels.
Field of View
Aside from the focal length, each eyepiece has an apparent field of view, measured in degrees (°). This tells you the apparent width of sky, in angular terms, that is presented to your eye — eyepieces with larger apparent fields take in greater gulps of sky than smaller ones. Simpler eyepiece designs tend to have apparent fields of about 45° ; widefield designs may be 60° or more.
What is sometimes more important to the user is the true field of view, which is obtained by dividing the apparent field by the magnification the eyepiece delivers. For example, consider a 10 mm-focal-length eyepiece of 50° apparent field in combination with a scope of 1,000 mm focal length. The magnification will be 100x (that is, 1,000/10) and the true field will be 50°/100x, or 0.5° — so the full Moon would just nestle within the field of view. Here's a general purpose true field calculator:
