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Mount Wilson Observatory | Building the 100-inch Telescope

Building the 100-inch Telescope

By Mike Simmons

(Scroll down to watch video slideshows of the construction from the archives of the Huntington Library and Carnegie Observatories. Ray Blumhorst created the slideshows for the centennial. The photos really capture the effort that went into building this famous telescope.)


With the completion of the 60-inch telescope in 1908, Mount Wilson became the home of the largest astronomical instrument in the world. But in typical fashion, George Ellery Hale was already making plans for a much larger instrument, one of unheard-of dimensions. Before the revolutionary 60-inch could even be tested, the 4 1/2-ton disk of the 100-inch telescope was already cast. No one, not even Hale himself, was certain that such an instrument could achieve the hoped-for improvement over the 60-inch, and the 60-inch telescope had not yet been proven! It would be a long wait before Hale and his colleagues would know if their half-million dollar gamble would pay off. More than eleven years would pass from the day the order for the 100-inch glass was placed before it would focus the light of the distant stars — eleven years of problems and serious doubt. Even on the night the telescope finally saw “first light,” those doubts persisted.

With the construction of the 60-inch telescope imminent, John D. Hooker, a prominent local businessman and friend of Hale, offered to pay for the production of the mirror for an even larger instrument, an 84-inch. Hale jumped at the opportunity, and his enthusiasm prompted Hooker to raise the ante. Hale would have his mirror, but it must be 100 inches in diameter. $45,000 was pledged for the glass disk, the building for the shop, and the equipment necessary to shape the glass into a mirror. Hale wrote to Hooker that the new telescope, with a light-gathering ability almost three times that of the 60-inch and six times that of the largest telescope currently existing, would “enormously surpass all existing instruments in the photography of stars and nebulae…”

Hale expected the optical work to take four years to complete, and the mounting and dome one year. That would give him time to raise the funds for the rest of the telescope and building, over $500,000, while Hooker’s mirror was being shaped.

There were other problems to overcome, though. It was not at all certain that a glass disk of optical quality that size could be cast. Giving the correct shape to the glass, to within a few millionths of an inch across its eight-foot diameter, would at least be a challenge. The mounting must point the mirror to any part of the sky with great accuracy and move smoothly and precisely to follow objects across the sky. Its design and construction would be aided by experience with the 60-inch, but could still be a huge problem. Keeping the hard-won shape of the mirror constant despite possible contraction and expansion of the glass was thought to be a necessity. Hale speculated that the entire dome might be refrigerated to the nighttime temperature to keep the mirror from heating up during the day. And finally, there was a problem over which Hale could exert no control. The atmosphere that blankets the Earth is constantly in motion, bending the incoming starlight and blurring the images. Tests indicated that the air would be steady often enough to allow the huge telescope to make use of its great resolving power, but the only real test would have to wait for “first light.”

Hale wasted no time in starting the new project, determined to overcome all of the difficulties encountered along the way. The massive glass disk was ordered from the French Plate Glass Companies in St. Gobain, France, on September 19, 1906. The “Hooker building” was built at the Observatory headquarters in Pasadena with rooms for grinding and figuring the mirror and a 100-foot-long testing hall. Keeping in mind the San Francisco earthquake that had occurred five months earlier (while the mounting for the 60-inch telescope was still at the manufacturer’s plant there), the mirror room was made fire- and earthquake-proof. Hale traveled to St. Gobain the next year and reported that the first casting attempt would be made in July, although several castings were expected to be made before the techniques (and the glass) were perfected. The process took a year to complete, and in August, 1908, on the day that the 60-inch was on its way up to Mount Wilson, Hale received word that the 100-inch disk had been cast.

Everything was prepared for the arrival of the 9,000 pound slab of wine bottle glass. George Ritchey, Hale’s chief optician, began preliminary design work on the telescope for the estimation of costs, waiting until the 60-inch was tested before going into detail. The world’s largest grinding machine was built. The glass company had reported that bubbles in the glass might be a problem, but the astronomers insisted on seeing for themselves. But when the disk arrived at the observatory shops in Pasadena in December of 1908, on the same day that the 60- inch mirror was placed in its mounting on Mount Wilson, they could see immediately that it was useless. Two sheets of bubbles had formed between the layers of glass when each layer was poured into the mold. The company agreed to keep trying, but in Hale and the others, the doubts grew stronger.

This edge-on photo was taken during the recent re-aluminizing of the 100-inch mirror. The three layers are clearly visible, as are the multitude of bubbles and swirls. Photo: Mike Simmons

Ritchey went to Paris to discuss the recasting with officials of the glass company. A new furnace and annealing oven (for cooling the glass at a very slow rate to prevent strain) were built. Hale followed a few months later. A satisfactory disk was cast in the spring of 1910, more than a year after the delivery of the rejected disk, but it was broken by internal strains while cooling. Another broke almost a year later. A thinner disk was successfully poured, but it might not hold its shape while in use in the telescope.

Finally, Walter Adams, the Assistant Director, did some tests on the rejected disk in Pasadena and found that it might be suitable for use after all. A glass expert was brought in, and he suggested that the bubbles might even be beneficial, helping to relieve the strains within the glass as the temperature changed and thereby helping to retain the mirror’s accurate figure. The glass was put on the huge machine for the first time and ground to a spherical shape for testing, but in some quarters the doubts increased; Ritchey, whose responsibility it was to fashion the giant disk into a mirror, and Hooker, its donator, were opposed to putting effort into this disk. Adams and Hale performed the delicate tests on the mirror themselves, not trusting their chief optician to be objective enough.

George Ritchey, master optician, designed this machine to figure the parabolic surface of the 100-inch mirror. Here the mirror can be seen in its horizontal position with the large grinding tool above and to the right. Photo: Carnegie/ Huntington Library

Problems did appear in some of the early tests, causing Ritchey to proclaim the glass useless, but the cause was soon traced to the mirror’s support system. Ritchey, however, continued to insist that the mirror was flawed. The work continued despite Ritchey’s criticism. The spherical shape seemed to hold with large changes in temperature, so the job of transforming the spherical shape into that of a parabola, the mirror’s final form, was begun. This took almost a year, working only during warmer weather when artificial heat was not needed in the building; the warm air would form layers that would make testing the mirror’s progress impossible.

With the mirror under way and the testing of the 60-inch telescope completed, the observatory staff could turn their attention to the problem of designing the telescope mounting. The delays caused by the problems with the mirror had one beneficial effect: Hale reported that this allowed more time to improve the mounting design for the 100-inch. Ritchey again became a dissident minority. He wanted to fashion a special wide angle instrument, but Hale rejected the idea because he felt much of the work to which the new instrument would be suited required a different design. Hale also pointed out that the telescope could be reconfigured to Ritchey’s design later if that seemed desirable. The new mounting incorporated many of the features of the successful 60-inch telescope, including the mercury flotation bearings to support the 100 tons of moving telescope that had to track the stars so precisely and smoothly. A new addition would be coils of water pipes in the support cell of the mirror to keep it at a constant temperature. The order for the mounting went out to the Fore River Shipyards in Quincy, Massachusetts, where the construction of battleships had created the technology needed for building the massive mounting. Unfortunately, an excess of other orders at the shipyards caused more delays.

Even with the telescope under construction, the funding was not at all certain. As he had done several times in the past, Hale had begun a monumental venture without sufficient funds, feeling that he would find them when necessary. Hooker’s gift would cover only one tenth of the total cost, and the Carnegie Institution of Washington, which owned and operated the observatory, needed its entire endowment to continue its several research departments. But again Hale found a sympathetic benefactor, Andrew Carnegie himself. Carnegie had been very enthusiastic about the success of the 60-inch telescope when he visited Mount Wilson in 1910. The next year, Carnegie donated an additional ten million dollars to the Carnegie Institution with the following suggestion: “I hope the work at Mount Wilson will be vigorously pushed, because I am so anxious to hear the expected results from it. I should like to be satisfied before I depart, that we are going to repay to the old land some part of the debt we owe them by revealing more clearly than ever to them the new heavens.” Once again, Hale’s optimism and drive had found a way.

Even the building to house the telescope did not escape some controversy. Ritchey wanted just a cylindrical windscreen placed around the telescope with a roof on wheels that could be rolled away for observing. Hale settled on a conventional dome, but again with some possible modifications because of the unique nature of the project. To keep the air inside the dome from heating up during the day, Hale planned to line the walls with cork, place a sunscreen over the building and refrigerate it when necessary. Only the sunscreen was incorporated into the final plans, with a second layer of sheet metal placed a few feet over the dome, a system first tested on the 60-inch dome.

The dome was built by the Morava Construction Company in Chicago and first tested there in a way that must have startled more than a few Chicagoans on their way to work. The 100-foot steel hemisphere was completely assembled in an empty lot for testing, then disassembled and shipped. The smooth motion of the dome had to be assured, so a motor-driven grinding machine, mounted on the end of a 50-foot arm radiating from the center of the building, was used to grind the rails on which the dome would roll. By the fall of 1915, the housing for the telescope was ready.

The dome was first assembled in Chicago to make sure everything fit properly before transporting it to Mount Wilson. Note the smokestack in the background. Photo: Carnegie/ Huntington Library

In order to get all of the material safely to the mountaintop, transportation had to be improved. When Hooker made his gift for the mirror in 1906, the mountain was at the end of a steep and dusty 9-mile footpath, with no way to get there except by foot or on the back of a horse or mule. The Snow solar telescope had been transported to the top in 60 trips by mule trains. The 60-inch was conveyed by both mules and a primitive truck. The staff could make use of a mule train only every other day and even that did not run in winter. By 1915, when the material for the dome was transported to Mount Wilson, trucks were traveling to the top daily on the toll road in just over two hours year-round. There’ were still dangers, however. A brand new Mack truck went over the edge near a place called Buzzard’s Roost, carrying its driver down 300 feet into a steep canyon. Walter Adams and the assistant driver jumped before the truck took the plunge. The three escaped injury, but the truck was a total loss. The road was soon widened to allow for larger trucks carrying the 11-foot diameter telescope tube and other large parts. During the construction season in 1915, 650 tons of material for the dome were transported to the top, with pieces up to 24 feet long and weighing 41/2 tons.

The telescope residing in that dome could not just sit on the ground. It would need a roost of appropriately gargantuan proportions. The pier on which the telescope would rest had to be isolated from the rest of the building in order to prevent vibrations of the instrument when the dome was turned or the wind blew against the walls. It would also provide a solid foundation while holding the telescope at the observing level of the building, well above the ground. Built two years before the dome, the pier stands 33 feet high. At the bottom it is a rectangle 20 by 45 feet across, while the top mushrooms out to the circular observing floor, 54 feet in diameter and held in place by massive concrete support brackets. Within the hollow structure of the pier are several floors. Besides storage space at ground level, there was equipment for cooling the mirror in rooms at the 16-foot level (more recently used for darkrooms and rest rooms), while the mirror silvering equipment resided just under the telescope at the 25-foot level.

The first steel supports for the dome begin to go up around the massive concrete pier which will support the mount for the telescope on the platform above. Photo: Carnegie/ Huntington Library

One final accessory remained to be built, the clock drive that would regulate the speed at which the telescope would sweep across the sky, tracing the arcs followed by the stars during the night. The telescope is like a giant grandfather clock, with the tube moved by the force of a falling weight. The clock drive mechanism keeps that rate constant and allows for minor adjustments. However, this clock had to be considerably more massive than any timepiece. A 2- ton falling weight drives the machine, while over 1,000 pounds of bronze parts were cast for the mechanism, and almost 3,000 pounds of iron. The force is then transmitted to the drive gear on the polar axis of the telescope, a precision gear like that of a fine Swiss watch, but 17 feet across!

Visible before its cover was installed, the 17-foot gear slowly turns the telescope on its equatorial axis to compensate for Earth’s rotation. Sometimes called a “de-rotator,” it keeps the telescope locked on the target object being photographed. Photo: Carnegie/ Huntington Library

The clock drive that turns the worm gear under the seventeen-foot right-ascension gear. Powered by a heavy weight on a long cable, the clock turns the telescope to compensate for Earth’s rotation. Photo: Carnegie/ Huntington Library

By 1916, after more than five years of grinding and polishing, the huge mirror was ready. It had been over seven years since the disk was initially rejected by the observatory staff, and now all their hopes lay in the smooth curve on the front surface of that glass. Its specifications were impressive. Almost 101 inches across and 12 inches thick at the edge, the mirror weighs almost 9,000 pounds. It is still the largest solid glass mirror ever made (later mirrors of that size were honeycombed in back to reduce the weight). The curve of the mirror is 1¼ inches deep and required 35 gallons of solution to fill the concavity when a new silver reflective layer needed to be deposited (a procedure superseded in 1935 by the deposition of a thin aluminum layer in a vacuum chamber). Most importantly, the shape of the mirror was just what it was supposed to be. Focusing the light reflected from its surface on a photographic plate over 42 feet away, all of the light focused within six thousandths of an inch of the plate’s surface, a maximum error of one part in 92,000. And all of this was achieved, for the first time, almost entirely through the use of a carefully monitored grinding machine. Less than 20 hours of the final figuring had been done by hand.

On July 1, 1917, the mirror made the long awaited trip to Mount Wilson. The top of its container rose 14 feet above the dirt road while on the truck. Nearly 200 men accompanied the truck as it made the 8-hour trip at an average speed of about one mile per hour. Many of them held ropes attached to the truck as it negotiated the many switchbacks. Others were there to protect the precious cargo from another potential danger: a caller the day before had threatened to blow up the mirror while it was en route to the mountain.

The secondary mirrors, which reflect the collected light of the 100-inch mirror to the plates and instruments at the telescope’s different stations, also required a great deal of precision work. The largest of them, the Newtonian secondary, required over seven months work to produce a flat figure on its two- by three-foot front surface.

The mounting was now assembled and waiting to carry the telescope’s optics. Some parts of the mounting had been assembled and tested at the shipyards, but complete testing there was not possible. The structure was shipped to Mount Wilson; it would not be fully tested until assembled on the mountain. Four sections of the tube were too large to travel by train, so they made the trip by ship through the Panama Canal. Once in place, the mounting and dome had 35 motors attached for the myriad of motions required.

With the closed-yoke mount of the telescope in place, workers begin to install the telescope tube, with the heavy lower section first. The two round, mercury floats that will support 95% or the telescope’s weight are clearly visible. Photo: Carnegie/ Huntington Library

The imminent completion of the 100-inch telescope changed the observatory in almost every way. A two-story wing was added to the astronomers’ dormitory, the “Monastery.” For the first time, power lines were strung from the valley below; before that all electricity had been generated by the observatory. Even the name of the observatory was changed. When founded with a solar telescope, the facility was named the Mount Wilson Solar Observatory. With the completion of the 100-inch, the word “Solar” was dropped from the name.

Delays still plagued the project, this time due to the World War. Most of the optical and instrument shops’ time was devoted to supporting the war effort, and Hale was in Washington, D.C. most of the time. But not even the Great War could keep the astronomers away now. The 100-inch was finally ready for testing.

A small crowd gathered in the new dome at sunset on November 1, 1917, as the 100-inch telescope was readied for its first look at the sky. Ritchey still insisted that internal weaknesses in the mirror doomed the telescope to failure. Hale and Adams were fairly sure that he was wrong, but uncertainties remained. Hale had brought the visiting English poet Alfred Noyes to the mountain for the occasion. Noyes later chronicled the unfolding drama in his epic poem “Watchers of the Sky”:

“… The explorers of the sky, the pioneers
Of science, now made ready to attack
That darkness once again,
and win new worlds.”

Shortly after dark the giant tube was turned toward Jupiter. What they saw was not what had been hoped for. In Walter Adams words, “The sight appalled us, for instead of a single image we had six or seven partially overlapping images irregularly spaced and filling much of the eyepiece.” Could Ritchey have been right after all? Some investigation revealed that the dome had been left open during the day while workmen were preparing the instrument for its test, allowing the air in the dome, and thus the mirror, to heat up. The sun may even have shone directly on the mirror, temporarily ruining the figure as the surface expanded. There was nothing to do but wait for the glass to cool and see if the image improved.

They stayed in the dome for two or three hours, occasionally peeking in the eyepiece. They thought they saw some improvement, but more time was needed. Hale and Adams agreed to meet in the dome at 3:00 A.M., and they retired to the Monastery. Neither could sleep though, and they both arrived back at the 100-inch early. With Jupiter no longer in position, the telescope was swung to a star. Hale’s first look told the story; in the center of the eyepiece the star’s light shone as a brilliant, precisely focused pinpoint of light. Hale’s biggest gamble had paid off; the 100-inch was a success.

The telescope was not without some problems, but they were all readily solved. The removable sections of the telescope tube that support the secondary mirrors were not rigid enough, so compression rings were added around them. The large, movable observing platform had to be partially reconstructed. The clock drive had a periodic error that was traced to the springing of a shaft in the drive train. An additional bearing solved this problem. The optics and the spectrograph, the telescope’s primary research tool, performed up to the highest expectations. In comparisons of plates taken with the 60- and 100-inch telescopes, some of them exposed simultaneously with identical equipment on the telescopes, the full theoretical gain of the larger instrument was attained. The 100-inch not only worked, but it would be as powerful a research instrument as anyone could have hoped.

Hale could now turn his attention to a research program for the 100-inch, but that program needed to be flexible. Whatever discoveries were made with the new telescope would change our understanding of the universe and point the way to new areas of investigation. However, some principles would always guide the work of the observatory. Hale was a solar astronomer, but he always recognized that the sun is a typical star, stating “…in the long run, every advance in our knowledge of the sun is likely to find application in the study of other stars. The principle of initiating many stellar researches from suggestions afforded by solar investigations, and of preparing an observing program which intimately unites both of these classes of work with laboratory studies, is undoubtedly sound, and should continue to form the basis of our procedure.” This solar-stellar connection is the basis of research still underway at Mount Wilson today.

Hale also felt that the 100-inch would be useful in solving the mystery of the spiral nebulae. A debate was raging as to whether these graceful objects were “island universes” or “lesser systems tributary to the Galaxy, which dominates a single universe.” The great light gathering power of the huge mirror would allow astronomers to learn more about these faint objects. Into the fray stepped Edwin Hubble, who joined the Mount Wilson staff in 1919.

Hubble was a man of many talents. He studied mathematics and astronomy at the University of Chicago, but he was also an accomplished boxer. After graduation, Hubble turned down an offer to train for the world heavyweight championship and accepted a Rhodes scholarship to study jurisprudence at Oxford. He returned to the United States to practice law, but soon returned to astronomy, taking graduate work at the University of Chicago and working at Yerkes Observatory. Once at Mount Wilson, Hubble used the 100-inch to attack the spiral nebulae problem. Believing them to be “island universes” (now called galaxies), Hubble looked for variable stars (which vary in brightness) in the Andromeda nebula. He found several, but one proved to be very important. It was a Cepheid variable, a type that had been shown to be useful as an indicator of distance. The search for Cepheids was expanded to other spiral nebulae, and dozens were found, all of which indicated that the distances to the nebulae were far too great for them to be a part of our galaxy. When the results were announced to a meeting of the American Astronomical Society on New Year’s Day, 1925, the debate was over. According to Mount Wilson and Las Campanas Observatories staff member Allan Sandage in the “Hubble Atlas of Galaxies”, Hubble’s discovery with the 100-inch had “proved beyond question that nebulae were external galaxies comparable to our own. It opened the last frontier of astronomy, and gave, for the first time, the correct conceptual value of the universe. Galaxies are the units of matter that define the granular structure of the universe.”

Ironically, some strong evidence against the existence of other galaxies continued to come from Adriaan Van Maanen who was also at Mount Wilson. This led to some personal problems, but like the spiral nebulae question itself, these were eventually resolved.

Hubble used the 100-inch for decades to push back the limits of the universe. He measured galactic distances and speeds to prove that the universe is expanding, changing our perception of the nature of the universe and leading to the “Big Bang” theory of its origin. In recognition of his achievements, the Space Telescope, a 94-inch telescope that will orbit the earth, has been named the “Hubble Space Telescope.”

Another important user of the 100-inch telescope was already close at hand long before it was built, but at that time he did not seem destined to become one of the great figures in astronomy. Milton Humason quit school at the age of 14 and went to work on Mount Wilson in 1905. He worked at the Mount Wilson Hotel and as a mule driver. In 1911 he married the daughter of the observatory’s chief electrician. Humason moved to the valley floor to be foreman on a relative’s ranch, but went back to the mountain six years later to join the observatory staff. The annual report of the Director for 1918 announced the completion of the 100-inch telescope, but in the back of the same report the name of Milton Humason appeared for the first time — as janitor. Within a few years Humason had become a night assistant on the telescopes, and even began taking some photographic plates himself. By 1924 he had become Hubble’s chief assistant, using the 100-inch telescope in the search for variable stars in spiral nebulae. He became a member of the research staff and worked with Hubble to chart the expansion of the universe. As one of astronomy’s most skilled observers, he used the 100-inch telescope to great advantage, reaching further into space than anyone had thought possible. He was eventually recognized with an honorary doctorate from the University of Lund in Sweden. Hubble the former boxer and lawyer, and Humason the one-time mule skinner, formed perhaps the most unusual team in the history of astronomy, and one of the most important.

Another famous early user of the 100-inch telescope was the Nobel Prize winning physicist Albert Michelson of the University of Chicago. Michelson came to Mount Wilson in 1919 and returned frequently as a guest investigator through much of the next decade. In historic experiments he made very accurate measurements of the speed of light at Mount Wilson. He also developed the stellar interferometer, a device for use on the telescope to allow the measurement of very small distances in the sky. With extra mirrors mounted on a 20-foot beam attached to the top end of the 100-inch telescope tube, Michelson and Mount Wilson staff member Francis Pease made the first direct measurements of the sizes of stars other than the sun. On December 13, 1920, they found the diameter of Betelguese in the constellation of Orion to be 215,000,000 miles. If the sun were to grow to that size it would swallow the Earth, yet at the distance of Betelguese they were dealing with an object that appeared no larger than a one-foot ruler would at a distance of 800 miles. Other stellar diameters were measured, and they were able to prove the validity of Henry Norris Russell’s theory of red supergiant stars.

Important work done with the 100-inch telescope also included finding a method of determining the distance to stars, as well as their composition, by inspection of their spectra. The refinement of these methods required the work of many scientists over a period of decades. As Hale had expected, new discoveries, new equipment and new ideas continued to lead to new research questions for the 100-inch telescope. For forty years it was the largest in the world, and many believe the quality of its spectrograph has never been surpassed. It has probed the skies under the direction of many of the great astronomers of this century. The great discoveries made with it and the mass of important data collected with it for more than six decades have made the 100-inch Hooker telescope one of the most important scientific instruments of all time.