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A Guide to EyepiecesMagnification
Every telescope has a stated focal length, which is effectively the distance from the primary lens or mirror to the point at which it forms an image of a very distant object. This is a figure that you'll usually see printed or engraved near the eyepiece focuser and usually lies in the range of 400- to 3000-mm, depending on the aperture and type of telescope. Eyepieces have focal lengths, too — 25- or 10-mm, for example. To calculate the magnification, simply divide the focal length of the scope by that of the eyepiece. A 2000-mm focal length scope used with a 25-mm eyepiece will therefore deliver 2000/25 = 80 power (or 80x). Note that the same eyepiece used with a different focal length scope will give different powers. Here's a simple magnification calculator:
But there's also a minimum magnification beyond which light emerging from the eyepiece will spill around the dilated pupil of the eye and be wasted. The largest-focal-length eyepiece you can use with your telescope is easy to calculate: multiply the focal ratio (the focal length of your scope divided by its aperture) by 7. For example, your Newtonian scope is f/5: the largest-focal-length eyepiece you should use is 35 mm. (Note: there is no harm in using a lower-power eyepiece — you will still be seeing the brightest possible view — but you will not be using all the light-collecting ability of the telescope's objective.)
