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The resolution, or resolving power, of a telescope is defined as being the minimum angular separation between two different objects which can be detected. The angular resolution limit, q, of a telescope operating under ideal conditions is given by the simple formula:

where λ is the wavelength of radiation being detected and D is the limiting aperture of the telescope, usually the diameter of the objective, or primary optic. Unfortunately, we are not able to increase the resolution of a telescope simply by increasing the size of the light gathering aperture to as large a size as we need. Disturbances and nonuniformities in the atmosphere limit the resolution of telescopes to somewhere in the range 0.5-2 arc seconds, depending on the location of the telescope. Telescope sights on top of mountains are popular since the light reaching the instrument has to travel through less air, and consequently the image has a higher resolution. However, a limit of 0.5 arc seconds corresponds to an aperture of only 12 in (30 cm) for visible light: larger telescopes do not provide increased resolution but only gather more light.

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Science EncyclopediaScience & Philosophy: Swim bladder (air bladder) to ThalliumTelescope - Resolution, Overcoming Resolution Limitations, Space Telescopes, Adaptive Optics, Recording Telescope Data, Modern Optical Telescopes - Operation of a telescope, Types of telescope, Alternative wavelengths