Editing Mount Wilson Observatory (section)

=== Construction ===
[[File:MtWilsonGlass-1917.jpg|thumb|left|The mirror of the Hooker telescope on its way up the [[Mount Wilson Toll Road]] on a [[Mack Truck]] in 1917]]

Once the sixty-inch telescope project was well underway, Hale immediately set about creating a larger telescope. [[John D. Hooker]] provided crucial funding of $45,000<ref name="NYT-1907">{{citation |author=Serviss, Garrett P. |date=January 27, 1907 |title=The Greatest Telescope in the World; Monster Instrument Ordered by Carnegie Institution Will Far Exceed in Power All Other Watchers of the Skies |work=New York Times |url=https://timesmachine.nytimes.com/timesmachine/1907/01/27/106737435.pdf}}</ref> for the purchase and grinding of the mirror, while [[Andrew Carnegie]]<ref>[http://www.mtwilson.edu/index.html History of Mount Wilson Observatory - Building the 2.5 meter Telescope] {{Webarchive|url=https://web.archive.org/web/20150905060014/http://www.mtwilson.edu/index.html |date=2015-09-05 }}. Article written by Mike Simmons in 1984 for the ''Mount Wilson Observatory Association'' (MWOA).</ref> provided funds to complete the telescope and dome. The Saint-Gobain factory was again chosen to cast a blank in 1906, which it completed in 1908. After considerable trouble over the blank (and potential replacements), the Hooker telescope was completed and saw "first light" on November 2, 1917. As with the sixty-inch telescope, the bearings are assisted by the use of mercury floats to support the 100 ton weight of the telescope.

In 1919 the Hooker telescope was equipped with a special attachment, a 6-meter optical [[astronomical interferometer]] developed by [[Albert A. Michelson]], much larger than the one he had used to measure Jupiter's satellites. Michelson was able to use the equipment to determine the precise diameter of stars, such as [[Betelgeuse]], the first time the size of a star had ever been measured. [[Henry Norris Russell]] developed his star classification system based on observations using the Hooker.

In 1935 the silver coating used since 1917 on the Hooker mirror was replaced with a more modern and longer lasting aluminum coating that reflected 50% more light than the older silver coating. The newer method of coating for the telescope mirrors was first tested on the older 1.5 meter mirror.<ref>{{cite magazine|magazine=Popular Science|author=Bonnier Corporation|title=Aluminum-Coated Mirrors Boost Power of Giant Telescope|url=https://books.google.com/books?id=ySUDAAAAMBAJ&pg=PA17|date=July 1935|publisher=Bonnier Corporation|page=17}}</ref>

[[Image:Assembling hooker polar axis.jpg|thumb|250px|Workmen assembling the polar axis of the Hooker telescope]]

Edwin Hubble performed many critical calculations from work on the Hooker telescope. In 1923, Hubble discovered the first [[Cepheid variable]] in the spiral nebula of Andromeda using the 2.5-meter telescope. This discovery allowed him to calculate the distance to the spiral nebula of Andromeda and show that it was actually a [[galaxy]] outside the [[Milky Way]]. Hubble, assisted by [[Milton L. Humason]], observed the magnitude of the [[redshift]] in many galaxies and published a paper in 1929 that showed the universe is expanding.

The Hooker's reign of three decades as the largest telescope came to an end when the [[Caltech]]-Carnegie consortium completed its {{convert|200|in|m|adj=on}} [[Hale Telescope]] at [[Palomar Observatory]], 144&nbsp;km south, in [[San Diego County, California]]. The Hale Telescope saw first light in January 1949.<ref>{{Cite web|url=http://calteches.library.caltech.edu/997/1/Telescope.pdf|title=Engineering and Science monthly May 1949}}</ref>

By the 1980s, the focus of astronomy research had turned to deep space observation, which required darker skies than what could be found in the Los Angeles area, due to the ever-increasing problem of [[light pollution]]. In 1989, the [[Carnegie Institution]], which ran the observatory, handed it over to the non-profit Mount Wilson Institute. At that time, the 2.5-meter telescope was deactivated, but it was restarted in 1992 and in 1995 it was outfitted with a visible light adaptive optics system and later in 1997, it hosted the UnISIS, laser guide star adaptive optics system.<ref>{{Cite web |url=http://www.mtwilson.edu/cent.html |title=Mount Wilson Observatory |access-date=2015-08-11 |archive-url=https://web.archive.org/web/20150529195621/http://www.mtwilson.edu/cent.html |archive-date=2015-05-29 |url-status=dead }}</ref><ref>{{cite web |last=Thompson |first=Laird A. |author-link=Laird A. Thompson |title=Laird A. Thompson: Professor of Astronomy |url=http://www.lairdthompson.net/ |work=jc-t.com |date=2 September 2013 |access-date=21 September 2020 |archive-date=September 5, 2015 |archive-url=https://web.archive.org/web/20150905090019/http://www.lairdthompson.net/ |url-status=dead }}</ref>

As the use of the telescope for scientific work diminished again, a decision was made to convert it to use for visual observing. Because of the high position of the Cassegrain focus above the observing floor, a system of mirrors and lenses was developed to allow viewing from a position at the bottom of the telescope tube. With the conversion completed in 2014, the 2.5 meter telescope began its new life as the world's largest telescope dedicated to public use. Regularly scheduled observing began with the 2015 observing season.<ref>{{Cite web |url=http://www.mtwilson.edu/100in.html |title=Mount Wilson Observatory |access-date=2015-08-11 |archive-url=https://web.archive.org/web/20150905060025/http://www.mtwilson.edu/100in.html |archive-date=2015-09-05 |url-status=dead }}</ref>

The telescope has a resolving power of 0.05 [[Minute of arc|arcsecond]].