How to Calculate Power: The magnification, or power, at which a telescope is operating is a function of the focal length of the telescope's main (objective) lens (or primary mirror) and the focal length of the eyepiece employed.
The focal length of the objective lens is the distance between the lens and the point at which it brings light rays to a focus; this focal length (in milimeters, or mm) is printed on a label affixed to the optical tube of every Meade telescope. The focal length of each eyepiece (which typically ranges from 4mm to about 40mm) is printed on the upper surface of the eyepiece. To calculate power, divide the focal length of the eyepiece into the focal length of the objective lens.
Example: The Meade Model 390 telescope has an objective lens focal length of 1000mm; when this telescope is used with a 25mm eyepiece, a power of 1000 / 25 = 40 power (written as "40X") results.
A Word about "Power": When buying a telescope one of the least important factors to consider is the power, or magnification, of the instrument. The key to observing fine detail, whether on the surface of the Moon or on a license plate one mile in the distance, is not power, but aperture - i.e., the diameter of the telescope's main (objective) lens or primary mirror. The power at which a telescope is operating is determined by the eyepiece employed; all Meade telescopes include one or more eyepieces as standard equipment, and optional eyepieces are available for higher or lower powers. Within reason power is useful, [but the most common mistake of the beginning observer is to "overpower" the telescope and to use magnifications which the telescope's aperture and typical atmospheric conditions can not reasonably support.] The result is an image which is fuzzy, ill-defined, and poorly resolved, through no fault of the telescope. Keep in mind that a smaller, lower-power, but brighter and well-resolved, image is far superior to a large, high-power, but dim and poorlyresolved, one.