Famous Telescopes

Name:	Hooker 100" Reflector
Observatory:	Mount Wilson Observatory
Original Site:	Mount Wilson, California, USA
Location:	Mount Wilson, California, USA
Date:	1917
Type:	Mixed Reflector
Designer:	George Willis Ritchey, Saint-Gobain, Daniel H. Burnham.
Aperture:	100" Silvered Glass Mirror


	Sponsor	While the 60-inch reflector was under construction, George Ellery Hale began to plan an even larger telescope. Hale had learned that men with lots of money often had an amateur interest in astronomy. Los Angeles hardware king John D. Hooker was no exception. He was fascinated when Hale spoke about the stars. Hale became a regular visitor to the Hooker mansion where he expounded on his plans for an 84-inch reflector on Mount Wilson. Hooker said "make it 100 inches and I shall pay for it." Hale was elated. In 1906 obtained a $45,000 grant from Hooker.

	Construction:	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 travelled 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 mould. The company agreed to keep trying, but in Hale and the others, the doubts grew stronger. 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 night-time 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." 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. 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. When Ritchey finished, the 100-inch mirror weighed about 9,000 pounds. 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 floatation 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. In 1910 Hale and Hooker persuaded the Carnegie Institution to grant $500,000 to complete the construction of the telescope. 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. The preliminary design of the dome was drawn up by members of the observatory staff and turned over to the firm of Daniel H. Burnham, in Chicago, for completion. The English yoke mounting for the telescope was designed by Ritchey and then modified by Hale and another member of the Mount Wilson staff, Francis Pease. The use of the English yoke mounting to provide much stiffer support for the telescope meant that the instrument could never see the stars near the north celestial pole. In 1915 the Fore River Shipbuilding Company of Quincy, Massachusetts, began sending the various components of the mounting to Pasadena. Since the parts were too large and heavy for the old road to the top of the mountain, a new road, widened to 12 feet, had to be built. 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 travelling 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. To support the telescope, a concrete pier 33 feet high was built into the bedrock of the mountain. At the bottom the pier is a rectangle 20 x 45 feet across, while the top mushrooms out to a circular observing floor, 54 feet in diameter, supported by two massive concrete support brackets. Within the hollow structure of the pier are several floors used for storage and darkroom space. Equipment to re-silver the huge mirror is found just under the telescope. 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 centre 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. 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 aluminium 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. 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 centre 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 final cost of the telescope to Carnegie was $600,000, not including the labour of the Mount Wilson staff, or the original contribution by Hooker. In 1981 the 100-inch Hooker reflector was designated an International Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers. In 1985, due to the lack of operating funds, the Carnegie Institution mothballed the Hooker reflector.

	Specification:	Optically, the Hooker telescope is very similar to the 60-inch telescope. It can be used as an f/5 Newtonian, f/16 modified Cassegrain, or an f/30 Coude. In the latter case the beam is brought through the hollow polar axis into a constant temperature room in the south pier for high-dispersion spectrography. The polar axis of the telescope, on which the 87-ton instrument must turn smoothly to counteract the rotation of the Earth, is defined by self-aligning journal bearings, while the bulk of the load is carried by two steel drums that float in mercury. The resulting friction is so low that the telescope, with 200,000 pounds of moving parts, can be rotated by the force of one hand at the end of the tube. To rotate the telescope, a large worm gear 18 feet in diameter is mounted on the south end of the polar axle. The gear, of cast iron with hollow spokes, is made in two halves bolted together. It meshes with a tool-steel worm that is rotated by a mechanical driving clock at one turn per minute. Because of the accuracy required in tracking the stars as they move across the night sky, great precision was required in cutting the teeth of the worm gear. This operation was done by the Italian instrument maker, C. Jacomini, with the gear in place on the telescope. Using a microscope and a diamond scriber, Jacomini, divided the gear into 1440 equal segments, one for each tooth, scribing the marks on an inserted brass ring near the edge of the gear. Later he gashed the teeth individually. The gear and worm were "run in" with rouge and oil. The rotating 100-foot, 500-ton double skin dome is similar to that found in the 60-inch telescope. The dome is supported by railroad trucks on a precision-ground double circular railroad track and is controlled by electric motors.

	Fame:	With the completion of the 60-inch telescope in 1908, Mount Wilson became the home of the largest operational astronomical instrument in the world. No one, not even Hale himself, was certain that the 100-inch 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. After the completion of the mirror, Hale turned his attention to defining 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 Adrian 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 Betelgeuse 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 Betelgeuse 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.



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