The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles.
The observatory contains two historically important telescopes: the 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 solar tower completed in 1908, the 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.Harris Newmark, Sixty Years in Southern California (1916). 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.
Solar telescopes
There are three solar telescopes at Mount Wilson Observatory. Just one of these telescopes, the Solar Tower, is still used for solar research.
Snow Solar Telescope
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 primary mirror with a 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.
60-foot Solar Tower
The 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.
150-foot Solar Tower
The focal length solar tower expanded on the solar tower design with its tower-in-a-tower design. (The tower is actually 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 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.
60-inch telescope
For the 60-inch telescope, George Ellery Hale received the mirror blank, cast by Saint-Gobain in France, in 1896 as a gift from his father, William Hale. It was a glass disk thick and weighing . However it was not until 1904 that Hale received funding from the Carnegie Institution to build an observatory. Grinding began in 1905 and took two years. The mounting and structure for the telescope were built in San Francisco and barely survived the 1906 earthquake. Transporting the pieces to the top of Mount Wilson was an enormous task. First light was December 13, 1908. It was, at the time, the largest operational telescope in the world. Lord Rosse's Leviathan of Parsonstown, a 72-inch (1.8-meter) telescope built in 1845, was, by the 1890s, out of commission.
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.
The 60-inch telescope is a reflector telescope built for Newtonian, Cassegrain and coudé configurations. It is currently used in the bent Cassegrain configuration. It became one of the most productive and successful telescopes in astronomical history. Its design and light-gathering power allowed the pioneering of spectroscopic analysis, parallax measurements, nebula photography, and photometric photography. Though surpassed in size by the nine years later, the 60-inch telescope remained one of the largest in use for decades.
In 1992, the 60-inch telescope was fitted with an early adaptive optics system, the Atmospheric Compensation Experiment (ACE). The 69-channel system improved the potential resolving power of the telescope from 0.5 to 1.0 arc sec to 0.07 arc sec. ACE was developed by DARPA for the Strategic Defense Initiative system, and the National Science Foundation funded the civilian conversion.
The telescope is used for public outreach as the second largest telescope in the world devoted to the general public. 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. Groups may book the telescope for an evening of observing.
100-inch Hooker telescope
The Hooker telescope was completed in 1917, and was the world's largest telescope until 1949. It is one of the most famous telescopes in observational astronomy of the 20th century. 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. He then showed that the universe was expanding.
+ Discoveries made with the Hooker 100-inch telescope: Year Description 1923 Edwin Hubble conclusively proves the Andromeda nebula to be external to the Milky Way galaxy 1929 Hubble and Milton Humason confirm that the Universe is expanding, measure its expansion rate, and measure the size of the known Universe 1930s Fritz Zwicky finds evidence for dark matter 1938 Seth Nickolson finds two satellites of Jupiter, referred to as #10 and #11.Chicago Tribune archives- Retrieved 2017-05-16 1940s Walter Baade's observations lead to the distinction of stellar populations and to the discovery of two different types of Cepheid variable stars, which double the size of the known universe previously calculated by Hubble 1963 Water vapor on Mars.
Construction
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 for the purchase and grinding of the mirror, while Andrew CarnegieHistory of Mount Wilson Observatory - Building the 2.5 meter Telescope . Article written by Mike Simmons in 1984 for the Mount Wilson Observatory Association (MWOA). 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.
The dome housing the telescope was designed by D. H. Burnham & Company of Chicago and fabricated by the Morava Construction Company, also of Chicago, founded by Bohemian-born engineer Wensel Morava. The steel hemisphere was fully pre-assembled in a vacant Chicago lot for testing, then disassembled and shipped to California for reassembly on the mountain.
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. The newer method of coating for the telescope mirrors was first tested on the older 1.5 meter mirror.
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 Hale Telescope at Palomar Observatory, 144 km south, in San Diego County, California. The Hale Telescope saw first light in January 1949.