History of Mount Wilson Observatory

Building the 60-Inch Telescope

(this article was written by Mike Simmons in 1984 for the Mount Wilson Observatory Association [MWOA])

With the dedication of the great 40-inch refracting telescope at Yerkes Observatory in southern Wisconsin in 1897, George Ellery Hale had completed the world's largest telescope. But in the basement below the 40-inch, the glass for a telescope of revolutionary size and design lay awaiting the funds that would eventually place it in its mounting in the mountains of southern California. It would be eleven more years before the 60-inch reflector of the Mount Wilson Observatory would become reality.

The 60-inch diameter disk of plate glass, 7 1/2 inches thick and weighing 1900 pounds (860 kg), was ordered by Hale's father, William Hale in 1894 as a gift to help his son's career. After its arrival from the St. Gobain glass works in France in 1896, the elder Hale gave the disk to the University of Chicago, which was then building the Yerkes Observatory, with the provision that a suitable mounting and housing be provided. William Hale promised to fund the grinding and figuring of the mirror himself, but his death left George Hale looking for funds for this new project. Despite some preliminary grinding of the glass by George Ritchey at Yerkes, the 60-inch would have to wait for a new benefactor.

The great refracting telescopes, which use a lens to form the image, had reached a practical limit with the 40-inch (having a lens 40 inches in diameter). A larger lens would sag under its own weight (unless it was very thick and therefore absorbed too much light), but the image-forming mirror of a telescope is supported under its back-side at the bottom of the telescope tube. A reflector is also more compact - the dome housing the 60-inch reflector is only two-thirds the size of that required for the 40-inch refractor. Even with very large mirrors, which gather great amount of light from faint objects, the much shorter focal length allowed the light to be concentrated into a a relatively small, bright image with a reflector, allowing short exposure times. Also, the lens of a refractor absorbs blue light while a mirror does not. These two factors made some photographs possible with the reflector which could not be made with the large refractors. After experimenting with a new reflector, Hale soon wrote that the 40-inch refractor ``is far outdone by the two-foot reflecting telescope recently constructed in the instrument shop of Yerkes Observatory'' for the photography of many objects.

In 1902, Hale applied to the recently formed Carnegie Institution of Washington to establish a new observatory devoted to solar research on Mount Wilson. Hale proposed that a 60-inch reflector for stellar astronomy also be built as part of a ``larger plan'', and he included letters of support from such luminaries as Sir William Huggins, who called large reflectors the ``telescope of discovery of the future.''

With the founding of the the Mount Wilson Solar Observatory in 1904 (the word ``Solar'' was dropped from the name with the completion of the 100-inch telescope in 1917), work on the 60-inch began in earnest. After six months of grinding a rough concave surface, the exacting and tedious job of figuring and polishing the surface of the mirror began in the autumn of 1905. Because the shape of the mirror had to be perfect to within a few millionths of an inch across its five-foot surface, special care was required to insure the mirror's accuracy. Accordingly, a special room was built where, for nearly the next two years, the opticians would slowly grind away a fraction of an inch of glass as the mirror's final figure was produced. The room was kept at a constant temperature to avoid changes in the shape of the glass's surface; even the distortion caused by the heat of a person's hand could be instantly detected by the test instruments. To prevent foreign material from getting into the grinding compounds and scratching the mirror, the windows were made double and sealed tight, while outside air was filtered on entering the room. The walls and ceiling were shellacked and, during polishing, canvas was hung over the mirror while the floor was kept wet to prevent flying dust. Before anyone could enter the room, he had to don a surgical gown and cap.

Despite these extraordinary precautions, the entire surface of the mirror was deeply scratched by an unknown substance in one of the polishing compounds one day in April, 1907, just as it was receiving its finishing touches. After 1 1/2 years of tedious labor, the mirror had to be ground back to a sphere and the figuring of the parabolic surface begun again. This time, though, the experience already gained by this mirror - almost twice the size of any other ever made - allowed them to complete the work in just four months. By September, 1907, the world's largest telescope mirror was ready for its mount, but other equally large problems would cause further delays.

The majority of the massive mounting and the steel for the dome was built by the Union Iron Works in San Francisco. The vital statistics of the mounting are truly impressive. The base is triangular, 15 feet by 9 feet in two parts, each of which weighs 3 1/2 tons. The polar axis, about which the telescope turns as it tracks the stars, is 15 feet long, and weight 4 1/2 tons despite being hollow. The cast-iron fork in which the telescope tube rides weighs 5 tons.

In order to move the telescope smoothly during long exposures and to accurately point it to almost any part of the sky, a new system had to be developed to maneuver the 22 tons of moving parts. Over the previous 30 years, astronomers had tried building telescope mounts with a trough in which mercury was used to float most of the weight of the telescope. This system had not always been successful, but Hale and Ritchey felt they could make it work on the 60-inch. A steel float, 10 feet in diameter, and weighing 4 tons, was fitted to the polar axis. With a 1/8-inch space between the trough and float filled with 650 pounds of mercury, over 21 1/2 tons of the telescope is supported, with just five percent of the weight left to be taken up by the bearings.

Once again, as the mounting was nearing completion, unexpected difficulties caused delays. On April 18, 1906, the great San Francisco earthquake caused considerable damage at the Union Iron Works, but the 60-inch ``escaped injury, though by the barest of margins.'' However, reconstruction and labor strikes caused the shipment of the mounting to be delayed for many months.

Even after the mounting was shipped, much work remained to be done by the Mount Wilson shops. The gear that would drive the telescope while it tracked the object under study would need to have the teeth cut in it. Cutting 1080 teeth in a gear 10 feet in diameter and weighing 2 tons is a big enough task, but any error would cause improper tracking of the object under study. Therefore, the cutting process had to be carried out with almost the precision of mirror-making. The 6-foot-tall clock drive mechanism, patterned after that of the Yerkes 40-inch refractor, also had to be built and installed. Many other parts, such as motors and mirror supports, were also built by the Mount Wilson staff.

Because the telescope's design was so revolutionary, Hale wanted to test the mount before moving it up the tortuous 9 1/2 mile road to Mount Wilson, where it would be out of reach of the shops. Therefore, a special building was constructed in Pasadena in which the mounting could be tested. The world's largest telescope, minus its mirror, was then built and tested in the city, out of the view of the night sky. The mounting moved as smoothly as had been hoped, and it was soon disassembled and readied for the trip to the summit. The test of its tracking accuracy had to await the mounting of the mirror and a test with the stars.

Transportation of such enormous parts to the top of a mountain was itself a major understaking. The narrow trail over which mule teams had hauled telescope parts and supplies to the mountaintop was widened to a road that could accommodate motor traffic. After nearly a year's work, the road was inaugurated by a brand new 1907 Franklin making the trip to the summit on May 28, 1907.

The Observatory then tested a new truck it had just received. In what might be considered state-of-the-art technology in 1907, this truck carried a generator which produced electricity for four electric motors, one on each wheel. The front and rear wheels could be steered independently in order to negotiate the sharp turns in the mountain road. Though designed for 5-ton loads, the truck proved inadequate and was rebuilt by the Mount Wilson shops. It could then take 3-ton loads to the summit, but four mules had to be added to get the 5-ton loads up the steepest slopes.

The truck proved to be too expensive to use regularly. One man with four mules could accomplish more than the truck and three men, despite the mules' limit of two tons per load. All of the material for the building and dome for the 60-inch telescope, 150 tons in all, was pulled to the top by mule teams. The truck was reserved for the heaviest pieces of the mounting, the most difficult of which was the telescope tube, 6 1/2 feet wide and 18 feet long, which was transported as a single piece. By July 1908, the mounting was on Mount Wilson.

Even the housing for this revolutionary telescope required innovations in its design. The dome, fifty-eight feet in diameter on the inside, was covered with a layer of canvas. Held in place by a metal framework, the canvas and two-foot air space between it and the sheet metal of the inner dome were designed to reduce the heating of the air on the inside during the day. The canvas was replaced by metal in 1912. To further reduce the effect of temperature variations on the telescope, the mounting was covered with blankets during the day and a refrigeration unit was planned, but these precautions have since been found to be unnecessary. A cork lining on the inside surface of the dome to prevent dripping from condensation has likewise been removed.

With all of the other pieces in place, the heart of the telescope, the 60-inch mirror, was placed in the telescope on December 7, 1908. The next evening the telescope was used for the first time, and the first photographs were taken on December 20. The great telescope at once lived up to everyone's expectations. It gathered more than twice the light of any previous telescope and it made better use of that light. It was the first major telescope to use a coudé focus, by which light could be sent to a very large spectrograph that was not attached to the telescope. The success of the telescope was not dimmed by discovery of a periodic error in its tracking, but extra care was necessary to keep the object centered in the telescope during an exposure. In fact, this slight tracking error has served as a test of Caltech graduate students' ability to guide the telescope properly.

The nature of the ``spiral nebulae'' was a question that had been debated for many years. Were these spiral-shaped objects clouds of gas within our own stellar system, the Milky Way Galaxy? Or were they galaxies themselves, ``island universes'' far beyond the limits of the Milky Way? Within the first year of operation, the 60-inch telescope shed new light on this question. Even though the ability to obtain useable spectra of even the brighter stars had been a very recent development, the 60-inch at once began providing useful spectra of the much fainter nebulae and star clusters. The Andromeda Nebula was found to have a spectrum similar to that of the Sun, leading Hale to speculate that it was composed of stars. Early 60-inch photographs were the first to show ``star-like condensations'' in the spiral nebulae - the first photographs of stars in other galaxies. Though the definitive answer to this perplexing mystery would await Edwin Hubble's work with the Mount Wilson 100-inch telescope in the 1920's, the 60-inch opened the field to study. The earliest work of the 60-inch also indicated the presence of interstellar material through the absorption of blue light from distant galaxies and star clusters.

In 1909, the program for the world's greatest telescope included stellar photography, parallax measurements, nebula and clusters photography and stellar spectroscopy. Exposure times for photographs ranged from about three minutes for bright planetary nebulae to 11 hours for some galaxies and clusters. Thirty-one photographs and three spectra were taken of Halley's comet during its last pass by Earth in 1910. Some new techniques, such as photographic photometry, were first attempted with the 60-inch in those early years. Hale also encouraged visiting astronomers from other observatories to travel to southern California to use the 60-inch telescope. Two of the first visitors were E.E. Barnard, who used the telescope to study Mars and Saturn, and Ejnar Hertzsprung now remembered for the famous Hertzsprung-Russell (H-R) diagram.

When Hale founded the Mount Wilson Solar Observatory, he expected the night-time work of the 60-inch telescope to add considerably to our understanding of the Sun, which he called a ``typical star.'' It has done that and more. For 85 years, the 60-inch telescope has been in use almost every clear night, becoming one of the most successful and productive telescopes in history. According to Dr. Allan Sandage, ``The Mount Wilson 60-inch telescope was the granddaddy of them all. where many of the problems of telescope design and solutions were first understood.'' Today it is dedicated to the study of surface activity in solar-type stars - an extenstion of the solar studies begun by Hale with the Snow telescope in 1904. Now with sensitive detectors and real-time processing capabilities via computer, the 60-inch telescope continues to play a major role in a pioneering area of stellar research.