Editing Mount Wilson Observatory (section)

== Interferometry ==
{{further|Astronomical interferometer|List of astronomical interferometers at visible and infrared wavelengths}}

[[Astronomical interferometry]] has a rich history at Mount Wilson. No fewer than seven interferometers have been located here. The reason for this is the extremely steady air over Mount Wilson is well suited to interferometry, the use of multiple viewing points to increase resolution enough to allow for the direct measurement of details such as star diameters.

=== 20-foot Stellar Interferometer ===

The first of these interferometers was the 20-foot Stellar Interferometer. In 1919 the 100-inch Hooker telescope was equipped with a special attachment, a 20-foot optical astronomical interferometer developed by Albert A. Michelson and Francis G. Pease. It was attached to the end of the 100-inch telescope and used the telescope as a guiding platform to maintain alignment with the stars being studied. By December 1920, Michelson and Pease were able to use the equipment to determine the precise diameter of a star, the red giant Betelgeuse, the first time the angular size of a star had ever been measured. In the next year, Michelson and Pease measured the diameters of six more red giants before reaching the resolution limit of the 20-foot beam interferometer.<ref>{{Cite web |url=http://www.mtwilson.edu/vir/100/20fti/ |title=Mount Wilson Observatory |access-date=2015-08-20 |archive-url=https://web.archive.org/web/20150712085353/http://www.mtwilson.edu/vir/100/20fti/ |archive-date=2015-07-12 |url-status=dead }}</ref>

=== 50-foot Stellar Interferometer ===

To expand on the work of the 20-foot interferometer, Pease, Michelson and George E. Hale designed a 50-foot interferometer which was installed at Mount Wilson Observatory in 1929. It successfully measured the diameter of Betelgeuse, but, other than beta Andromedae, could not measure any stars not already measured by the 20-foot interferometer.<ref>{{cite journal |last=Vaughan |first=Arthur H. Jr. |title=Interferometer Measurement of Star Diameters |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1967ASPL...10...57V&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES |journal=[[Astronomical Society of the Pacific]] |date=August 1967 |volume=10 |issue=458 |page=57 |bibcode=1967ASPL...10...57V |access-date=21 September 2020}}</ref>

Optical interferometry reached the limit of the available technology and it took about thirty years for faster computing, electronic detectors and lasers to make larger interferometers possible again.

=== Infrared Spatial Interferometer ===
{{main|Infrared Spatial Interferometer}}

The [[Infrared Spatial Interferometer]] (ISI), run by an arm of the [[University of California, Berkeley]], is an array of three 1.65 meter telescopes operating in the mid-infrared. The telescopes are fully mobile and their current site on Mount Wilson allows for placements as far as 70 meters apart, giving the resolution of a telescope of that diameter. The signals are converted to radio frequencies through [[heterodyne]] circuits and then combined electronically using techniques copied from [[radio astronomy]].<ref>{{cite web |url=http://isi.ssl.berkeley.edu/system_overview.htm |title=Infrared Spatial Interferometer Array – System Overview |work=University of California at Berkeley |access-date=31 August 2015}}</ref> The longest, 70-meter baseline provides a resolution of 0.003 arcsec at a wavelength of 11 micrometers. On July 9, 2003, ISI recorded the first [[closure phase]] [[aperture synthesis]] measurements in the mid infrared.<ref>{{cite web |url=http://isi.ssl.berkeley.edu/papers/Chandler2007ApJ657.pdf |title=The Radiative Pattern and Asymmetry of IRC +10216 at 11 μm Measured with Interferometry and Closure Phase|work=Space Sciences Laboratory and Department of Physics, University of California, Berkeley, CA |author1=A. A. Chandler |author2=K. Tatebe |author3=D. D. S. Hale |author4=C. H. Townes |date=10 March 2007}}</ref>

[[File:CHARA 1497.jpg|thumb|upright|One of six telescopes of the [[CHARA array]]]]

=== CHARA array ===
{{main|CHARA array}}

The [[CHARA array|Center for High Angular Resolution Astronomy]] (CHARA), built and operated by [[Georgia State University]], is an interferometer formed from six 1 meter telescopes arranged along three axes with a maximum separation of 330 m. The light beams travel through vacuum pipes and are delayed and combined optically, requiring a building 100 meters long with movable mirrors on carts to keep the light in phase as the Earth rotates. CHARA began scientific use in 2002 and "routine operations" in early 2004. In the infrared, the integrated image can resolve down to 0.0005 arcseconds. Six telescopes are in regular use for scientific observations and as of late 2005 imaging results are routinely acquired. The array captured the first image of the surface of a [[main sequence star]] other than the Sun published in early 2007.<ref>[http://www.ns.umich.edu/htdocs/releases/story.php?id=5877 U-M astronomers capture the first image of surface features on a sun-like star], [[University of Michigan]], 2007-05-31</ref>