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The apparent increase in angular size of an object when viewed through a telescope or binoculars, compared with the direct view of the same object. The magnification of a telescope is found by dividing the focal length of the objective lens or mirror by that of the eyepiece. The higher the magnification on a given telescope, the dimmer the image. There is a practical limit to the magnification of a telescope, which is approximately twice its aperture in millimetres. For example, a 100-mm objective has a practical magnification limit of 200, set by the effects of diffraction on the image. There is a lower practical limit, set by the size of the exit pupil. When this becomes larger than the pupil of the eye, light is wasted and the image does not appear any brighter as the magnification is reduced. A magnification of, say, 100 is often referred to as a power of 100, and written as ×100.

Subjects: Astronomy and Astrophysics.

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