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  Telescope Type: Astronomy: Telescope Type - SCOPE SPEED
Posted on Saturday, December 11 @ 01:08:36 CST by astronomy
  Telescope Type


A specification usually marked on the telescope's tube or given in its instruction manual is the focal length, almost always given in millimeters. This is the length of the light path from the lens or mirror to the focus point. With refractors and reflectors, the focal length is roughly equal to the physical length of the tube.

But in Maksutovs and Schmidt-Cassegrains, the light path is folded back and forth several times, allowing long focal-length optics to reside in a shorter, more compact tube.

Dividing the focal length of a telescope by its aperture in millimeters gives its ratio. A telescope with a focal length of 2,000 mm and an aperture of 200 mm (or 8 inches) is an f/10 telescope. If the focal length were 1,000 mm, it would be an f/5 scope.

The smaller the f-ratio, the faster the telescope. Fast telescopes are an advantage for deep-sky photography— an f/5 telescope will record a nebula in a quarter of the exposure time required by an f/10 telescope. Hence, the term "fast" telescope. For visual use, however, there is little advantage of one f-ratio over another. Slower telescopes are sometimes sharper, making them better suited to high-power planetary viewing, but they often have longer, less portable tubes. Fast telescopes give lower powers and wider fields with any given eyepiece, making them better suited to deep-sky viewing, but they also tend to exaggerate flaws in the optics.

There is no single ideal telescope, but by taking into account your skywatching interests, you can certainly make an informed choice.




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