Modern sidereal astronomy may be said to live on light. Large telescopic apertures are a sine qua non for its growth and activity.
       —Agnes M. Clerke, 1905

The rise of the large reflecting telescope in the early twentieth century parallels the rise of astrophysics and scientists' need to be able to collect ever more light for spectroscopic and photographic research programs. Refracting telescopes exhibited residual optical aberrations while the shorter focal lengths of reflectors meant that these telescopes could have shorter tubes and be covered by smaller domes, making them less expensive to build.

Nonetheless, at the turn of the twentieth century, astronomers still disagreed about which telescope design was best. Some argued that giant reflecting telescopes were awkward and imprecise and noted the thermal distortions caused by nightly temperature changes at the observatory. For some years, the refracting telescope was still seen by many as the premiere tool of the professional astronomer while amateur astronomers experimented with reflectors.

With the refinement of techniques to place reflective metal coatings on glass mirrors, and the increasing ability of technicians at glass companies to make very large mirror blanks, by 1920 the giant reflecting telescope supplanted the refracting telescope—except for a few specialized functions—as the astronomer's workhorse.

This advance depended not only on progress in optical techniques, but also on mechanical engineering. A large mirror would slightly change shape as the telescope slewed from pointing high in the sky to lower down. To support the mirror so it would stay focussed with extreme precision, engineers devised ingenious lever systems and rigid backings. Using these remarkable instruments, astronomers began to offer compelling new evidence that changed how people understood the history of the universe.

24-inch telescope
at Yerkes

As he did for so many areas of astronomy, George Ellery Hale was a prime force behind the acceptance of the reflecting telescope as a valid tool for modern astrophysics. Even as he was overseeing the installation of the world's largest refracting telescope—the famous 40-inch instrument at Yerkes— Hale was advocating the utility of reflecting telescopes for astrophysical research and demonstrating its value for spectroscopic research and more photographic purposes.

Aiding him in his endeavors was George W. Ritchey, a young optician whom Hale hired in 1896. Ritchey, a perfectionist by nature, had already made several modest-sized reflecting telescopes and was aware of their value for astro-photography. With his own money, Ritchey made a 24-inch silvered glass mirror which was later mounted in a tube of steel and aluminum pipes. Photographs taken with this instrument were of high quality and showed that a good reflector in the hands of a skilled user could be scientifically powerful. Its success also inspired Hale to consider building a larger version of it. But instead of the low altitude site of Yerkes, Hale's vision was to build his new telescope on a mountaintop in the western United States where it would be unfettered by cloudy weather and poor seeing conditions.

Who Was
George Willis Ritchey?

In the summer of 1903, encouraged by reports he had heard, Hale visited Mt. Wilson, a 5700 foot peak, overlooking the relatively unpopulated cities of Los Angeles and Pasadena. He was so encouraged by the possibilities of the mountaintop site that he soon relocated his family to Pasadena (he later played a prominent role in the establishment of the California Institute of Technology there) and began to make plans for what would become the Mount Wilson Observatory.

Hale's first action was to build a telescope especially designed for solar observing on Mt. Wilson. His bold moves were aided by a $300,000 grant from the Carnegie Institution in December 1904 (that would be roughly ten times as much in modern dollars). Mt. Wilson was unlike Yerkes or Lick Observatory in that the staff and their families did not live permanently on the mountain. Instead, scientists would visit the observatory, conduct their research, and stay temporarily in accommodations that became known as "the Monastery." The rest of time, they were based at the observatory's headquarters in Pasadena.

Horses hauled the heavy instruments up Mount Wilson.

Hale soon persuaded the Carnegie Institution to donate additional money for the construction of what would be the world's largest reflecting telescope. The design featured a 60-inch mirror that would be designed and build by Ritchey. Hale's actions set the stage for what would become a motif of his scientific career—the pursuit of ever-larger telescopes, funded by philanthropic institutions, often conceived while other smaller projects he had initiated were still being completed.

How Shapley
used the 60-inch

60-inch at
Mt. Wilson
Observatory

Who Was Hale?

The 60-inch saw its first light in December 1908 and by 1909 it was such a success that there was little doubt in the minds of astronomers that large reflecting telescopes were the scientific instruments of the future. Even before the 60-inch was done, Hale was pursuing bigger goals. In 1906, he persuaded J.D. Hooker, a wealthy Los Angeles merchant, to donate $45,000 as seed money to cast and grind a 100-inch mirror.

Getting a high quality glass disk to serve as the mirror blank for the world's biggest telescope was not easy. The first efforts of the St. Gobain glassworks in France failed, and Hale and Ritchey began to despair of getting suitable raw materials. A 100-inch glass disk sent from France to Pasadena in late 1908 was full of small bubbles but, despite Ritchey's objections, Hale persuaded the stubborn optician to start grinding and polishing it. In 1910 Hale and Hooker prevailed upon the industrialist and philanthropist Andrew Carnegie to contribute money to the project. With Hale estimating the cost of the 100-inch at half a million dollars, Carnegie persuaded the foundation that bore his name to provide funding.

Polishing a mirror as large as 100 inches was a major challenge for Ritchey. Dust in the optical shop had to be removed through air filters and the mirror could not be tested during the winter months due to temperature variations in the room. The mirror blank was not completed until the end of 1916. Delays caused by the U.S. entry into World War I hindered the telescope's completion, and the telescope mounting on Mt. Wilson did not receive the giant mirror until July 1917. Finished over budget and behind schedule, the 100-inch telescope was in regular use by 1919. Its completion had another cost—the delays and stress of building the telescope took a great toll on Hale's health. And conflicts between him and George Ritchey resulted, after the 100-inch was completed, in the firing of the dour yet painstakingly careful optician.

Ritchey, however, did not stop developing designs for bigger telescopes. In 1924, he moved to France and began a series of experiments to build giant mirrors. Ritchey was inspired by Progressive-era ideals and envisioned a series of observatories around the world whose telescopes— he called them "massive guns of Peace"—would have mirrors as large as 400 inches in diameter. While his ultimate dream was not realized, Ritchey continued to conceive many important ideas for telescope design that later became standard in telescope design decades later.

[We] shall look back and see how inefficient, how primitive it was to work with thick, solid mirrors, obsolete mirror curves, equatorial telescope-mountings of antiquated types requiring enormous domes and buildings, and similar anomalies in a progressive age.
       —G.W. Ritchey, 1928

Andrew Carnegie and George Ellery Hale beside the telescope onMt. Wilson.

Meanwhile, back at Mount Wilson, the 100-inch could collect almost three times as much light as the 60-inch, enabling astronomers like Edwin Hubble and Walter Baade to bring about revolutionary changes in cosmology in later decades.

Hale was constantly imagining even bigger telescopes. In 1928, he authored an influential article in Harper's Monthly Magazine entitled, "The Possibilities of Large Telescopes," which became the opening salvo in his campaign to see a telescope with a mirror as big as 300 inches realized. After recounting the successes he had witnessed with the 60-inch and 100-inch telescopes at Mount Wilson, Hale spelled out the scientific possibilities of telescopes with bigger mirrors.

Starlight is falling on every square mile of the earth's
surface, and the best we can do at present is to gather and concentrate the rays that strike an area 100 inches in diameter... I believe that a 200-inch or even a 300-inch could now be built and used to the great advantage of astronomy.
       —George Ellery Hale, 1928

Hale's article provided the basis for conversations with Wickliffe Rose, the president of philanthropic organizations associated with the Rockefeller Foundation, and other potential donors. This proselytizing paid off in autumn of 1928 when the International Education Board, an organization associated with the Rockefeller Foundation, gave $6 million to Caltech to build a 200-inch telescope. At the time, this was the largest contribution ever made for a scientific enterprise. Even this tremendous amount of money was not enough, however. The Rockefeller money was only to build the telescope; money to operate the new telescope still needed to be raised, and Hale worked hard to ensure that Caltech raised additional money for an endowment.

The design and construction of the 200-inch telescope was a massive undertaking and an excellent example of what historians call "Big Science." A series of committees was established to oversee the construction of the telescope's various parts and to provide the engineering and management skills necessary to see the telescope come to fruition. The advice and input of industrial firms were enlisted by Hale and the other three members of the Observatory Committee. Teamwork and the involvement of industry, in fact, became characteristics that would mark the building of large instruments for astronomy henceforth.

Engineers and scientists building the 200-inch telescope had to overcome many challenges. The most serious of these was securing a glass mirror blank of sufficient size. After failed attempts by General Electric to make a mirror from fused quartz—a process that itself took three years—Hale terminated the experiment and the project turned instead to getting a glass disk made of cast pyrex from Corning Glassworks in New York. Even this was a process fraught with danger and uncertainty. Corning was unable to produce a perfect 200-inch disk until December 1934. The casting aroused enormous public interest, which continued when the massive piece of glass was packed and shipped by train to Pasadena the following year to be polished.

Astronomers and dignitaries assembled on Palomar for the dedication of the 200-inch Hale telescope, 1948.

Other innovations came about as the 200-inch was being built. Engineers learned how to coat big mirrors with thin layers of metal that were deposited in chambers under a high vacuum. Aluminum was used, resulting in a coating that was more durable than easily-tarnished silver. In 1934, after extensive study, the site of Palomar Mountain in southern California was chosen as the location for the 200-inch. To support the massive mirror and its mounting, a giant horseshoe-shaped bearing was built that could support up to 500 tons on a thin film of pressurized oil. An elaborate truss and support system had to be designed that could keep the telescope aligned as it pointed and tracked objects in the night sky.

Hale himself died in 1938—his last words about his telescopic legacy were, "It is a beautiful day. The sun is shining and they are working on Palomar." The 200-inch's completion was delayed by World War Two and the technical difficulties associated with finishing its massive mirror. The polished mirror was not transported to the top of Palomar until November 1947. Less than a year later, hundreds of dignitaries assembled in the dome of the 200-inch to attend its dedication. Raymond B. Fosdick, president of the Rockefeller Foundation, spoke of the telescope as a tool to heal an ailing world, calling it the "lengthened shadow of man at his best." The telescope was given Hale's name to honor him.

Finally, several hundred tons of telescope and dome slowly and quietly began to move. A radio announcer gave a real-time description of the events. Many of the dedication's attendees stayed to watch a nighttime demonstration in which the moons of Saturn could be viewed with light collected in the massive 200-inch mirror.

In many ways, the Hale Telescope became the archetype for the modern research telescope. With its giant primary mirror, an observing cage where the astronomer could sit and collect data, a sophisticated truss system to keep the primary and secondary mirrors aligned, the 200-inch remained the largest telescope in the US for over forty years. Extremely innovative for its time and still a very usable and important instrument today, the Hale Telescope represented the basic "technological paradigm" for large telescope design and construction well into the 1970s.

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