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The clear aperture of a telescope is the diameter of the objective lens or primary mirror specified either in inches or millimeters. The larger the aperture the more light it collects and the brighter (and better) the image will be. Greater detail and image clarity are observed as aperture increases.
This is the distance (usually in millimeters) of an optical system from the lens (or primary mirror) to the point where the telescope is in focus (focal point). The longer the focal length of the telescope, generally the more magnifying power it has, the larger the image and the smaller the field of view.
This is the ratio of the focal length of the telescope to its aperture in the same units of measurements. For example, the f/ratio of a telescope with a 200mm aperture and a focal length of 1000mm is: 1000 / 200 = 5, or f/5. Telescopes of f/4 to f/6 are called "fast" systems. They offer lower magnifying power and wider fields of view than slow f/8 to f/15 systems.
The most common misunderstanding of telescopes is that they are rated by their magnifying power. The fact is, telescopes are rated by their aperture or light gathering capability. The aperture of a telescope is far more important than the magnifying power, because it determines the telescope's ability to resolve small or distant objects. See Usable Magnifications.
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