Choosing Eyepieces

While the telescopes' main lens or mirror collects the light and focuses it into an image, it is the eyepiece that magnifies this image. To change the magnification you need to change the eyepiece or invest in a zoom eyepiece.Eyepieces are not rated by magnification, instead they are classified by their focal length, always given in millimetres (i.e. 10mm, 14mm, 25mm etc. on the barrel).

To calculate the magnification power that you will achieve with a given eyepiece, you need to divide the focal length of your telescope in millimetres by the focal length of the eyepiece:

Focal length of Telescope in MM
--------------------------------------------------- = Magnification
Focal length of Eyepiece in MM

For example: if you use a 10mm eyepiece in a telescope with a focal length of 1000mm, this will give you 100x magnification. The same eyepiece in a telescope with a focal length of 500mm will give you 50x magnification.

Idealy you should have about 3 eyepieces, a low power one (35x to 50x mag.), a medium power one (80x to 120x mag) and a high power eyepiece (150x+, mag). If you buy an eyepiece that will magnify greater than the effective maximum magnification of your telescope (see buying a telescope) the image will become fuzzy and less detailed.

Eyepieces come in a three main barrel sizes, 0.965 inch (cheap starter scopes), 1.25 inch and 2 inch. With 1.25 inch being the most common and 2 inch being suitable for more advanced scopes.

There are a multitude of eyepieces available under such brand names as Meade, Celestron, Antares, Orion and Baader, with the Plossl design being the most economical to start off with, while yet yielding good contrast views.

At the higher end, are generic eyepieces known as Erfles, and brand names such as Wide Fields, Ultra Wide Angles, Naglers and Panoptics. These are available in focal lengths ranging from 35mm to 4mm, however, they give a much wider field of view than the Plossl, allowing you to take in more of the sky.

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