Mount Wilson Observatory

The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. 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.

The observatory contains two historically important telescopes: the 100-inch (2.5 m) Hooker telescope, which was the largest aperture telescope in the world from its completion in 1917 to 1949, and the 60-inch telescope which was the largest operational telescope in the world when it was completed in 1908. It also contains the Snow solar telescope completed in 1905, the 60-foot (18 m) solar tower completed in 1908, the 150-foot (46 m) solar tower completed in 1912, and the CHARA array, built by Georgia State University, which became fully operational in 2004 and was the largest optical interferometer in the world at its completion.

Due to the inversion layer that traps warm air and smog over Los Angeles, Mount Wilson has steadier air than any other location in North America, making it ideal for astronomy and in particular for interferometry. 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.

The initial efforts to mount a telescope to Mount Wilson occurred in the 1880s by one of the founders of University of Southern California, Edward Falles Spence, but he died without finishing the funding effort. The observatory was conceived and founded by George Ellery Hale, who had previously built the 1 meter telescope at the Yerkes Observatory, then the world's largest telescope. The Mount Wilson Solar Observatory was first funded by the Carnegie Institution of Washington in 1904, leasing the land from the owners of the Mount Wilson Hotel in 1904. Among the conditions of the lease was that it allow public access.

There are three solar telescopes at Mount Wilson Observatory. Just one of these telescopes, the 60-foot (18 m) Solar Tower, is still used for solar research.

The Snow Solar Telescope was the first telescope installed at the fledgling Mount Wilson Solar Observatory. It was the world's first permanently mounted solar telescope. Solar telescopes had previously been portable so they could be taken to solar eclipses around the world. The telescope was donated to Yerkes Observatory by Helen Snow of Chicago. George Ellery Hale, then director of Yerkes, had the telescope brought to Mount Wilson to put it into service as a proper scientific instrument. Its 24-inch (61 cm) primary mirror with a 60-foot (18 m) focal length, coupled with a spectrograph, did groundbreaking work on the spectra of sunspots, doppler shift of the rotating solar disc and daily solar images in several wavelengths. Stellar research soon followed as the brightest stars could have their spectra recorded with very long exposures on glass plates. The Snow solar telescope is mostly used by undergraduate students who get hands-on training in solar physics and spectroscopy. It was also used publicly for the May 9, 2016 transit of Mercury across the face of the Sun.

The 60-foot (18 m) Solar Tower soon built on the work started at the Snow telescope. At its completion in 1908, the vertical tower design of the 60-foot focal length solar telescope allowed much higher resolution of the solar image and spectrum than the Snow telescope could achieve. The higher resolution came from situating the optics higher above the ground, thereby avoiding the distortion caused by the heating of the ground by the Sun. 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. A later discovery was of the reversed polarity in sunspots of the new solar cycle of 1912. The success of the 60-foot Tower prompted Hale to pursue yet another, taller tower telescope. In the 1960s, Robert Leighton discovered the Sun had a 5-minute oscillation and the field of helioseismology was born. The 60-foot Tower is operated by the Department of Physics and Astronomy at University of Southern California.

The 150-foot (46 m) focal length solar tower expanded on the solar tower design with its tower-in-a-tower design. (The tower is actually 176 feet (54 m) tall.) An inner tower supports the optics above, while an outer tower, which completely surrounds the inner tower, supports the dome and floors around the optics. This design allowed complete isolation of the optics from the effect of wind swaying the tower. Two mirrors feed sunlight to a 12-inch (30 cm) lens which focuses light down at the ground floor. It was first completed in 1910, but unsatisfactory optics caused a two-year delay before a suitable doublet lens was installed. Research included solar rotation, sunspot polarities, daily sunspot drawings, and many magnetic field studies. The solar telescope would be the world's largest for 50 years until the McMath–Pierce solar telescope was completed at Kitt Peak in Arizona in 1962. In 1985, UCLA took over operation of the solar tower from the Carnegie Observatories after they decided to stop funding the observatory.