In the case of a telescope, it is called "resolution power" at the minimum angular distance at which two distant celestial bodies, or two geographical elements on the surface of a planet, can be distinguished from each other. The larger the diameter of the objective in a telescope, the greater its resolution power.
To calculate the resolution power of a target, the empirical formula established by astronomer W. R. Dawes is applied, consisting of a simple division of the fixed number 115 by the diameter of the target expressed in millimeters.
An example: if the Castor star of the Gemini constellation is observed with a 25 mm refractor, it will appear as a single; however, if it is observed with one of 150 mm., it will be presented double. Since the two components of the star are 3 ", 9 angular distance, it is clear that the first instrument has a lower resolution power and is not able to differentiate it as two distinct points (in the first case the resolution power is 115: 25 = 4 ", 6); but the second has a resolution power far superior to the necessary one, and distinguishes without difficulty the two stars.
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