The MWO is located on Mount Wilson, a 5,710-foot (1,740-meter) peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles.

[1] The increasing light pollution due to the growth of greater Los Angeles has limited the ability of the observatory to engage in deep space astronomy, but it remains a productive center, with the CHARA array continuing important stellar research.

George Ellery Hale, then director of Yerkes, had the telescope brought to Mount Wilson to put it into service as a proper scientific instrument.

On June 25, 1908, Hale would record Zeeman splitting in the spectrum of a sunspot, showing for the first time that magnetic fields existed somewhere besides the Earth.

Although slightly smaller than the Leviathan, the 60-inch had many advantages including a far better site, a glass mirror instead of speculum metal, and a precision mount which could accurately track any direction in the sky, so the 60-inch was a major advance.

ACE was developed by DARPA for the Strategic Defense Initiative system, and the National Science Foundation funded the civilian conversion.

Custom made 10 cm eyepieces are fitted to its focus using the bent cassegrain configuration to provide views of the Moon, planetary, and deep-sky objects.

It was used by Edwin Hubble to make observations with which he produced two fundamental results which changed the scientific view of the Universe.

Using observations he made in 1922–1923, Hubble was able to prove that the Universe extends beyond the Milky Way galaxy, and that several nebulae were millions of light-years away.

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.

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.

[15] 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.

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.

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.

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.

[20] 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.

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.

[22] The 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.

[23] A 61 cm telescope fitted with an infrared detector purchased from a military contractor was used by Eric Becklin in 1966 to determine the center of the Milky Way for the first time.

[24] In 1968, the first large-area near-IR (2.2 μm) survey of the sky was conducted by Gerry Neugebauer and Robert B. Leighton using a 157 cm reflecting dish they had built in the early 1960s.

[25] Known as the Caltech Infrared Telescope, it operated in an unguided drift scanning mode using a lead(II) sulfide (PbS) photomultiplier read out on paper charts.

[35] The idea to use the dome as a venue for live music originated in 2017 from a conversation between Dan Kohne, a board member of the Mt.

[36] Tsan agreed that the acoustics in the dome were "extraordinary", comparable to such world-renowned venues as the Palais Garnier (Opéra de Paris) and the Coolidge Auditorium at the Library of Congress.

[38] The observatory was the primary setting of "Nothing Behind the Door," the first episode of the radio series Quiet, Please which originally aired June 8, 1947.