Editing Optical telescope (section)

===Focal length and focal ratio===
The [[focal length]] of an [[optics|optical]] system is a measure of how strongly the system converges or diverges [[light]]. For an optical system in air, it is the distance over which initially [[collimated]] rays are brought to a focus. A system with a shorter focal length has greater [[optical power]] than one with a long focal length; that is, it bends the [[ray (optics)|ray]]s more strongly, bringing them to a focus in a shorter distance. In astronomy, the f-number is commonly referred to as the ''focal ratio'' notated as <math>N</math>. The [[f-number|focal ratio]] of a telescope is defined as the focal length <math>f</math> of an [[Objective (optics)|objective]] divided by its diameter <math>D</math> or by the diameter of an aperture stop in the system. The focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane to an [[eyepiece]], film plate, or [[Charge-coupled device|CCD]].

An example of a telescope with a focal length of 1200&nbsp;mm and aperture diameter of 254&nbsp;mm is given by:
<math>N = \frac {f}{D} = \frac {1200}{254} \approx 4.7</math>

Numerically large [[f-number|Focal ratios]] are said to be ''long'' or ''slow''. Small numbers are ''short'' or ''fast''. There are no sharp lines for determining when to use these terms, and an individual may consider their own standards of determination. Among contemporary astronomical telescopes, any telescope with a [[f-number|focal ratio]] slower (bigger number) than f/12 is generally considered slow, and any telescope with a focal ratio faster (smaller number) than f/6, is considered fast. Faster systems often have more [[optical aberrations]] away from the center of the field of view and are generally more demanding of eyepiece designs than slower ones. A fast system is often desired for practical purposes in [[astrophotography]] with the purpose of gathering more [[photons]] in a given time period than a slower system, allowing time lapsed [[photography]] to process the result faster.

Wide-field telescopes (such as [[astrograph]]s), are used to track [[satellite]]s and [[asteroid]]s, for [[cosmic ray|cosmic-ray]] research, and for [[astronomical survey]]s of the sky. It is more difficult to reduce [[optical aberrations]] in telescopes with low f-ratio than in telescopes with larger f-ratio.