F number is the ratio of focal length of the optical system to aperture of the telescope. f5 means you have a focal length five times longer than the lens or mirror diameter. f15 means a focal length 15 times longer than the lens or mirror diameter.
f numbers are used in photographic lens systems to indicate the "speed" of the system - a low f number means a fast system that supplies light quickly to the camera. A high f number is a slow system, taking much longer to supply the same amount of light to the camera. For taking pics of very faint objects, a low f number is much more desirable.
For visual work, a low f number produces less magnification with the same eyepiece than a high f number. Low f number optical systems are tougher to correct, optically, than long focal length systems becuase low f number lenses and mirrors have more curvature. That's why very low f number APO, highly corrected refractors are so expensive per inch of apaerture.
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