Starguide - A Practical Guide to Astro-Photography

is the ratio between the apparent size of an object (or its size in an image) and its true size, and thus it is a dimensionless number. Optical magnification is sometimes referred to as "power" (for example "10× power"), although this can lead to confusion with optical power.

The angular magnification of an optical telescope is given by:

in which  is the focal length of the objective lens in a refractor or of the primary mirror in a reflector, and  is the focal length of the eyepiece.

Reference: Wikipedia Optical Magnification

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