Editing LIGO (section)

==Observatories==<!-- This section is linked from [[LIGO Livingston Observatory]] -->
LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory ({{Coord|30|33|46.42|N|90|46|27.27|W}}) in [[Livingston, Louisiana]], and the LIGO Hanford Observatory, on the [[Hanford Site|DOE Hanford Site]] ({{Coord|46|27|18.52|N|119|24|27.56|W|}}), located near [[Richland, Washington]]. These sites are separated by 3,002 kilometers (1,865 miles) straight line distance through the earth, but 3,030 kilometers (1,883 miles) over the surface. Since gravitational waves are expected to travel at the speed of light, this distance corresponds to a difference in gravitational wave arrival times of up to ten milliseconds. Through the use of [[trilateration]], the difference in arrival times helps to determine the source of the wave, especially when a third similar instrument like [[Virgo interferometer|Virgo]], located at an even greater distance in Europe, is added.<ref>{{cite web|title=Location of the Source|url=http://www.sr.bham.ac.uk/gwastro/what-are-we-looking-for/4|website=Gravitational Wave Astrophysics|publisher=University of Birmingham|access-date=28 November 2015|url-status=dead|archive-url=https://web.archive.org/web/20151208133633/http://www.sr.bham.ac.uk/gwastro/what-are-we-looking-for/4|archive-date=8 December 2015}}</ref>

Each observatory supports an L-shaped [[ultra high vacuum]] system, measuring four kilometers (2.5 miles) on each side. Up to five [[interferometer]]s can be set up in each vacuum system.

The LIGO Livingston Observatory houses one laser [[interferometer]] in the primary configuration. This interferometer was successfully upgraded in 2004 with an active vibration isolation system based on hydraulic actuators providing a factor of 10 isolation in the 0.1–5&nbsp;Hz band. Seismic vibration in this band is chiefly due to [[microseismic]] waves and anthropogenic sources (traffic, logging, etc.).

The LIGO Hanford Observatory houses one interferometer, almost identical to the one at the Livingston Observatory. During the Initial and Enhanced LIGO phases, a half-length interferometer operated in parallel with the main interferometer. For this 2&nbsp;km interferometer, the [[Fabry–Pérot]] arm cavities had the same optical finesse, and, thus, half the storage time as the 4&nbsp;km interferometers. With half the storage time, the theoretical strain sensitivity was as good as the full length interferometers above 200&nbsp;Hz but only half as good at low frequencies. During the same era, Hanford retained its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.