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The aperture of a telescope is several times larger than the aperture of human eye so that the objects that cannot be normally seen by unaided eye can be seen. Light-gathering power of a telescope is proportional to the area of its aperture and hence depends on the square of the radius of the mirror. Therefore, a 20 cm diameter telescope collects four times more photons than a 10cm diameter telescope. A telescope can be equipped to record light over a long period of time, by using photographic film or electronic detectors such as photometers or CCD detectors while the eye has no capability to store light. Along-exposure photograph taken through a telescope reveals objects too faint to be seen with the eye, even by looking through the same telescope. A third major advantage of large telescopes is that they have superior resolution, the ability to discern fine detail. Small resolution is good. The resolution is directly proportional to the wavelength being observed and inversely proportional to the diameter of the telescope.