Editing F-number (section)

== {{anchor|Focal ratio}}Focal ratio in telescopes ==
[[Image:Focal ratio.svg|right|thumb|250px|Diagram of the [[focal ratio]] of a simple optical system where <math>f</math> is the [[focal length]] and <math>D</math> is the diameter of the [[Objective (optics)|objective]]]]

In astronomy, the f-number is commonly referred to as the ''focal ratio'' (or ''f-ratio'') notated as <math>N</math>. It is still 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:

<math display=block>N = \frac fD \quad \xrightarrow {\times D} \quad f = ND</math>

Even though the principles of focal ratio are always the same, the application to which the principle is put can differ. In [[photography]] the focal ratio varies the focal-plane illuminance (or optical power per unit area in the image) and is used to control variables such as [[depth of field]]. When using an [[optical telescope]] in astronomy, there is no depth of field issue, and the brightness of stellar point sources in terms of total optical power (not divided by area) is a function of absolute aperture area only, independent of focal length. The focal length controls the [[Field of view#Astronomy|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]].

For example, the [[Southern Astrophysical Research Telescope|SOAR]] four-meter telescope has a small field of view (about {{f/|16}}) which is useful for stellar studies. The [[Large Synoptic Survey Telescope|LSST]] 8.4&nbsp;m telescope, which will cover the entire sky every three days, has a very large field of view. Its short 10.3&nbsp;m focal length ({{f/|1.2}}) is made possible by an error correction system which includes secondary and tertiary mirrors, a three element refractive system and active mounting and optics.<ref name=RefDesign>{{Cite journal |title=LSST Reference Design |publisher=LSST Corporation |author1=Charles F. Claver |date=19 March 2007 |url=http://lsst.org/files/docs/LSST-RefDesign.pdf |pages=45–50 |access-date=10 January 2011 |display-authors=etal |archive-url=https://web.archive.org/web/20090306173830/http://lsst.org/files/docs/LSST-RefDesign.pdf |archive-date=6 March 2009 |url-status=dead }}</ref>