Editing Optical telescope (section)

===Light-gathering power===
{{further|Etendue}}
[[File:KeckObservatory20071013.jpg|thumb|The [[W. M. Keck Observatory|Keck II telescope]] gathers light by using 36 segmented hexagonal mirrors to create a 10 m (33 ft) aperture primary mirror]]
The light-gathering power of an optical telescope, also referred to as light grasp or aperture gain, is the ability of a telescope to collect a lot more light than the human eye. Its light-gathering power is probably its most important feature. The telescope acts as a ''light bucket'', collecting all of the photons that come down on it from a far away object, where a larger bucket catches more [[photons]] resulting in more received light in a given time period, effectively brightening the image. This is why the pupils of your eyes enlarge at night so that more light reaches the retinas. The gathering power <math>P</math> compared against a human eye is the squared result of the division of the aperture <math>D</math> over the observer's pupil diameter <math>D_{p}</math>,<ref name="SaharaSkyObservatory"/><ref name="RyukyuAstronomyClub"/> with an average adult having a [[pupil]] diameter of 7&nbsp;mm. Younger persons host larger diameters, typically said to be 9&nbsp;mm, as the diameter of the pupil decreases with age.

An example gathering power of an aperture with 254&nbsp;mm compared to an adult pupil diameter being 7&nbsp;mm is given by: <math>P = \left(\frac {D}{D_{p}}\right)^2 = \left(\frac {254}{7}\right)^2 \approx 1316.7</math>

Light-gathering power can be compared between telescopes by comparing the [[area]]s <math>A</math> of the two different apertures.

As an example, the light-gathering power of a 10-meter telescope is 25x that of a 2-meter telescope: <math>p = \frac {A_{1}}{A_{2}} = \frac {\pi5^2}{\pi1^2} = 25</math>

For a survey of a given area, the field of view is just as important as raw light gathering power. Survey telescopes such as the [[Large Synoptic Survey Telescope]] try to maximize the product of mirror area and field of view (or [[etendue]]) rather than raw light gathering ability alone.