MAGNIFICATION

The magnification, also called power and symbolized ×, is the extent to which a telescope makes objects look closer. (Actually, telescopes increase the observed sizes of distant objects, but they do not look closer in terms of perspective.) The magnification is a measure of the factor by which the apparent angular diameter of an object is increased. A 20× telescope makes the Moon, whose disk subtends about 0.5° of arc as observed with the unaided eye, appear 10° of arc in diameter. A 180× telescope makes a crater on the Moon with an angular diameter of only 1 minute of arc (1/60 of a degree) appear 3° across.
Magnification is calculated in terms of the focal lengths of the objective and the eyepiece. If f_o is the effective focal length of the of the objective and f_e is the focal length of the eyepiece (in the same units as f_o), then the magnification factor m is given by this formula:
m = f_o /f_e

For a given eyepiece, as the effective focal length of the objective increases, the magnification of the telescope also increases. For a given objective, as the effective focal length of the eyepiece increases, the magnification of the telescope decreases.