Mount Wilson Observatory (MWO) is located northeast of Los Angeles. The site benefited from ideal weather conditions, which led to the construction of a first observatory in 1889. However, the harsh climatic conditions quickly led to its abandonment. It was not until 1904 and the patronage of George Ellery HALE (1868-1938) that the current observatory was built.
George Ellery HALE received a 1.5-meter mirror, made in France at Saint-Gobain, which was used to assemble the telescope. Its commissioning took place December 8, 1908, it was then the largest telescope in the world. This revolutionary instrument propelled astronomy into a new era, offering important breakthroughs. The telescope is also famous for the photographs of nebulae that it allowed. It is no longer used for scientific measurements but remains accessible to the public.
HALE began a second telescope in 1908 with funding from the Carnegie Foundation and philanthropist John Daggett HOOKER (1838-1911). Impressive in size, 2.5 meters in diameter, the mirror was once again built by Saint-Gobain. The Hooker telescope, named after the main donor, was commissioned on November 1, 1917. It was equipped in 1919 with an optical interferometer, an instrument exploiting the interferences between the waves developed by Nobel Prize winner Albert Abraham MICHELSON. This addition allowed for more accurate measurements of star distance and size.
It is from the measurements and photographs made from this second telescope that Edwin HUBBLE (1889-1953) proved that the Andromeda galaxy was outside the Milky Way in 1923. Later, in 1929, he demonstrated the expansion of the Universe from the data collected by this same telescope. Among the great discoveries made by the Mount Wilson Observatory we can also mention the discovery of evidence of the existence of dark matter by Fritz SWICKY in 1933, or the discovery of two satellites of Jupiter by Seth NICKOLSON in 1938.