An eyepiece is an optical accessory used to magnify the image seen through the telescope. The eyepiece is placed in the focuser of the telescope and is moved by the focuser until the image is sharp (focused).

There are many different types and brands of eyepieces to choose from, ranging from the standard Kellner and Plossl types up to top of the range eyepieces such as Meade's "Ultra Wide Angles" and Vixen's "LV" series.
Most new telescopes that Astro sell come with at least one eyepiece, usually a good quality Plossl. In order to expand your eyepiece range it is important first to understand the different terminology used when describing an eyepiece :

Focal Length :
Every eyepiece, like a telescope, has a focal length. This distance is expressed in millimetres.

• A long focal length eyepiece (25-55mm) gives a telescope low power and a wide field of view - perfect for scanning the sky and viewing nebulae & star clusters.
• Short focal length eyepieces give the telescope a high magnification and small field of view, good for viewing the planets and moon.

The focal length of an eyepiece is nearly always noted on the actual eyepiece. To determine the magnification that an eyepiece/telescope combination gives simply divide the telescope focal length by the eyepiece focal length.
e.g : A Meade LX90 (Focal Length = 2030mm) using a 20mm focal length eyepiece gives a magnification of about 100x.

Apparent Field of View :
How much of the sky you can see through a telescope is determined by the eyepiece's apparent field of view and by the telescope's magnification using that eyepiece.
You can see an eyepiece's apparent field of view by holding it at arms length and looking through it. The diameter of the image you see, measured in degrees, is the apparent field.
Luckily this number is noted down by the manufacturers to allow easy calculation of True Fields of View.
Most Plossl eyepieces on the market have an apparent field of view of 45-50 degrees. Wide Angle eyepieces have apparent fields of view from 60 up to 85 degrees.

In order to translate this apparent field of view into a true field of view, the magnification of the telescope/eyepiece combination must be known.
Using the example above, the Meade LX90 is giving 100x with a 20mm eyepiece. If this eyepiece has an apparent field of view of 50 degrees then the true field of view is 0.5 degrees. If the eyepiece has an apparent field of 80 degrees then the true field of view is 0.8 degrees.
As can be seen, the true field of view is found by dividing the apparent field by the magnification.

Eye Relief :
Eye relief is the maxmium distance in millimetres that the eye can be from the eyepiece and still be able to see the whole of the field of view. Longer eye relief is usually found on longer focal length eyepieces and is better for eyeglass wearers. Shorter focal lengths give a small eye relief.
There are some eyepieces such as the Vixen LV series which give a long eye relief no matter what the focal length is. These eyepieces are especially good for viewing the planets and the moon.

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