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Physics - Formulas - Telescope Magnification

A common question when purchasing a telescope is what "power" it is. The power of the telescope is the ability to magnify an object.

While magnification is really not as important as field of view of aperture, to determine the power of a telescope, simply divide the eyepiece diameter to the telescope focal length:

Eyepiece diameter is also known as the eyepiece focal length. Do not confuse this with the exit pupil which is a different concept: based on the telescope aperture and magnification or telescope focal and telescope focal ratio. The exit pupil is the term that defines the size of the light cone that enters your eye.

Additionally, the resolution of a telescope must be considered. For ideal "seeing" conditions, the following formula applies:

Example: a 254mm telescope (a 10")

The size of an image depends on the focal length of your telescope. For example, the longer the focal length, the larger the object:

How faint an object can your telescope see:

Where m is the limiting magnitude. Example, our 10" telescope:

The faintest object a 10" telescope can see is with a visual magnitude of 14.7 (Pluto has a magnitude of around 13.8).


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