Hooker Telescope Is Installed on Mount Wilson
The Hooker Telescope, installed on Mount Wilson in California, represents a significant advancement in astronomical technology due to its large 100-inch (254-centimeter) mirror, which was the largest in the world at the time of its completion in 1917. Developed under the guidance of astronomer George Ellery Hale and funded by businessman John Daggett Hooker, the telescope was constructed amidst various challenges, including difficulties in casting the mirror and logistical hurdles in transporting it to the summit. After initial setbacks, including distorted images during its first trials, the telescope proved its capabilities by producing clear and detailed observations of celestial objects such as the Moon and distant nebulas.
One of the telescope's most notable contributions was to the debate over the nature of spiral nebulas, culminating in Edwin Hubble's groundbreaking work that identified them as separate galaxies rather than parts of the Milky Way. This discovery significantly altered the understanding of the universe's structure and its expansion. The Hooker Telescope's ability to gather light from faint stars also facilitated critical measurements, such as the diameter of Betelgeuse, a giant star in Orion. The innovations and observations made with the Hooker Telescope laid the groundwork for future astronomical endeavors and ultimately inspired the construction of even larger telescopes, like the 200-inch Hale telescope at Palomar Observatory. Its legacy continues to influence modern astronomy.
Hooker Telescope Is Installed on Mount Wilson
Date November, 1917
George Ellery Hale oversaw the Mount Wilson Observatory’s installation of the Hooker telescope, the world’s largest telescope until 1948.
Locale Mount Wilson, California
Key Figures
George Ellery Hale (1868-1938), American astronomerJohn Daggett Hooker (1838-1911), American businessmanGeorge Willis Ritchey (1864-1945), American opticianWalter Sydney Adams (1876-1956), American astronomer
Summary of Event
Since Galileo first used a telescope to view celestial objects in 1609, astronomers have worked to make bigger and better telescopes. Galileo’s first telescope was a refracting telescope; that is, it worked by using lenses to bend, or refract, light. In the seventeenth century, Sir Isaac Newton and N. Cassegrain worked to perfect reflecting telescopes, which worked by using mirrors to reflect starlight.
There is a limit to how big a refracting telescope can be, because a lens supported only around its edges will distort under its own weight past a certain point. The largest refracting telescope is the 40-inch (102-centimeter) telescope at Yerkes Observatory in Wisconsin. A reflecting telescope can be made much larger than this, however, because the mirror is supported across its whole lower surface. Astronomers want to have big telescopes because the bigger a telescope’s aperture (the width of the primary mirror or lens receiving the starlight), the more light the telescope gathers. Given that stars are extremely small and faint sources of light as seen from Earth, the more light-gathering capacity a telescope has, the better.
In 1906, astronomerGeorge Ellery Hale of the Mount Wilson Observatory in California was overseeing the construction of a 60-inch (153-centimeter) reflecting telescope. With characteristic enthusiasm and energy, Hale described to John Daggett Hooker how a larger telescope would perform even greater wonders than the 60-inch telescope could. Hooker, a wealthy businessman who had contributed money for the initial expedition to set up a telescope on Mount Wilson, was excited to hear this and immediately offered the observatory $45,000 to have an 84-inch (213-centimeter) mirror cast. He decided later to fund a 100-inch (254-centimeter) mirror instead. He wanted the telescope to be the biggest in the world and thought that a 100-inch mirror would not be easy for anyone else to surpass in size.
The Saint-Gobain Company in France was the only company willing to cast such a large mirror. The glass for the mirror had to be melted in three separate batches and then all poured into the mold simultaneously. The meeting of the three streams of molten glass formed large clouds of air bubbles in the glass, and the glass also crystallized partially during its cooling. Nevertheless, the Saint-Gobain Company shipped the disk to Hoboken, New Jersey; from there it was shipped to New Orleans and then sent on the final leg of its journey to Pasadena, California, where it arrived on December 7, 1908. Both Hale and Hooker were disappointed with the quality of the mirror blank, and George Willis Ritchey—who was to be responsible for the mirror grinding—believed that the glass would never hold the necessary shape. The Saint-Gobain Company accepted financial responsibility for the failure and proceeded to cast another disk. The second disk broke during cooling, and further tries at casting a new disk were also unsuccessful. The advent of World War I halted the attempts.
Ritchey and Hooker were both reluctant to use the original disk, which they thought was worthless. In fact, Hooker refused to give any more financial support to the enterprise, and the remainder of the expenses entailed in building the telescope were paid by a gift from Andrew Carnegie. Astronomer Walter Sydney Adams carried out tests on the original disk and decided that his results showed that it could be ground to the proper shape. Despite Ritchey’s disagreement, in 1910 Hale told Adams that the polishing of the mirror was to begin. The blank contained roughly 7,905 square inches (51,000 square centimeters) of surface, all of which had to be polished and shaped accurately to within millionths of an inch. Ritchey began this painstaking job, and it was completed by W. L. Kinney.
Meanwhile, the construction of the mounting and controls for the telescope also was being carried out. The mounting design was Ritchey’s, with major revisions from Hale, Francis Gladheim Pease, and F. L. Drew. The mounting, a huge rectangular steel cradle, was built in Quincy, Massachusetts; it was shipped to San Pedro Harbor in California by way of the Panama Canal. The finer parts of the instrumentation, such as the driving mechanisms that would enable the telescope to track an object across the sky, were crafted by the machinists in the observatory’s workshops. The dome that would house the telescope was designed by architectDaniel Hudson Burnham of Chicago. The motions of the telescope and dome were controlled by more than thirty electric motors, which worked to control the motion of the massive mirror and mounting smoothly and accurately, so that the telescope could track and photograph an object with no blurring of the image.
The telescope was designed to have several “foci,” or places where the light is focused and observed. Newton had developed a telescope in which the light was reflected from the main or primary mirror up to a much smaller secondary mirror, which reflected it out the side of the telescope, where it was brought to focus and observed. Cassegrain had developed a telescope in which the light, after leaving the primary, was reflected from a secondary mirror at the opposite end of the telescope and then back down to the vicinity of the primary. The Hooker telescope utilized both of these types of focus: The Newtonian focus was used for photography and the recording of stellar spectra (the light of stars broken down into its component wavelengths), and various instruments were used at the Cassegrain foci. Each focus has different characteristics that make it suitable for a particular type of work.
By early 1916, the grinding and testing of the mirror were completed, the great piers to hold the mounting and telescope were ready, and the trip up Mount Wilson could begin. Getting the huge mirror and its massive mounting and machinery to the observatory site was a harrowing task, requiring the building of a special road and involving some physical danger to the participants. On July 1, 1917, the mirror was safely carried up the mountain, and in November, 1917, the telescope was assembled. The astronomers and technicians were ready for first light, the first trial of the telescope.
On the evening of November 1, 1917, Hale pointed the telescope at Jupiter. Despite the fact that the telescope weighed 198,416 pounds (90,000 kilograms), it moved smoothly. Unfortunately, the image of Jupiter was distorted disastrously. Rather than one clear image, there were six or seven overlapping images. The horrified astronomers attempted to guess what could have gone wrong with their new telescope. The dome had been open during the day and the sun had warmed the mirror; perhaps this warming and the subsequent cooling it was undergoing were distorting the mirror’s shape and thus the image. The only way to test this idea was to wait and let the mirror cool further. This process took several hours. The astronomers waited, even attempting to go home and get some sleep. Hale could not sleep, however, and returned to the observatory at around 2:30 a.m., where he was joined by Adams. Jupiter had set, and Hale moved the telescope to view the bright star Vega. One can only imagine how delighted Hale must have been to see at last a clear, sharp image of the star in the telescope. This was the beginning of the long and useful career of the Hooker telescope.
Significance
The Hooker telescope began to prove its worth immediately. Photographs of the Moon and of nebulas revealed previously unseen fine details. The great light-gathering capacity of the huge mirror allowed astronomers to study the spectra of very faint stars. Over the years of its use, the telescope contributed a vast amount of information to human knowledge of the stars and their properties.
Rather than make many unrelated observations with the new telescope, Hale organized the first observing program to focus on one of the great controversies of the time: whether the spiral nebulas visible in the sky were relatively nearby parts of the Milky Way or whether they were huge, distant, independent systems. The Hooker telescope was used to make photographs of the Andromeda nebula. Edwin Powell Hubble studied these photographs; the detail they showed enabled him to identify a star of a type known as Cepheid variables. These stars vary in light output, and the period of variation is related to the absolute brightness of the star; the absolute brightness can be compared to the brightness as seen from Earth and this comparison used to calculate the star’s distance. Hubble calculated the distance of this star, and thus of the entire Andromeda nebula; the distance was very great, and this proved to be decisive evidence in the controversy over the spiral nebulas. They are, indeed, remote separate galaxies that are not part of the Milky Way. The Hooker telescope was crucial in the resolution of this basic question, which changed astronomers’ view of the universe and Earth’s place in it.
After this 1924 discovery, Hubble and Milton L. Humason worked with the Hooker telescope and gathered the data that would enable them to conclude that all the galaxies are receding from one another, and the speed of their recession can be used to measure the age of the universe.
The Hooker telescope was also used to measure the diameter of a star. Stars are so tiny as seen from Earth that the measurement of their diameter is exceedingly difficult. Albert A. Michelson had developed a method of measuring star diameters, and in 1920, Francis Pease and John August Anderson carried out the measurements for the bright-red star Betelgeuse in the constellation Orion. Ejnar Hertzsprung had used indirect means to deduce that Betelgeuse was a giant star; this measurement confirmed his deduction, giving the result that the diameter of Betelgeuse was about 350 times that of the Sun.
This and other work done with the Hooker telescope led to results that astronomers could not investigate further without a larger telescope. The success of this instrument thus led to Hale’s plans for the 200-inch (508-centimeter) Hale telescope, which was eventually completed at the Palomar Observatory in California in 1948.
Bibliography
Asimov, Isaac. Eyes on the Universe: A History of the Telescope. Boston: Houghton Mifflin, 1975. Written in Asimov’s customary engaging style, this book focuses on the development of the telescope. The chapter “Reflectors Take Over” covers the period during which the Hooker telescope was built and briefly discusses its construction and some of the astronomical advances to which the telescope contributed. Includes photographs.
Brunier, Serge, and Anne-Marie Lagrange. Great Observatories of the World. Richmond Hill, Ont.: Firefly Books, 2005. Oversize volume presents profiles of thirty-six of the world’s leading observatories (including Mount Wilson), ten space-based telescopes, and eleven “observatories of the future.” Focuses on telescope technology. The many illustrations include photographs of the observatories themselves as well as of the celestial objects seen through their telescopes.
King, Henry C. The History of the Telescope. 1955. Reprint. Mineola, N.Y.: Dover, 2003. Includes a chapter that discusses the various telescopes of Mount Wilson, including the Hooker telescope. Provides drawings, diagrams, and technical details on the mounting and optics of the telescopes. Recommended for readers interested in the technical aspects of telescopes. Includes photographs.
Struve, Otto, and Velta Zebergs. Astronomy of the Twentieth Century. New York: Macmillan, 1962. Contains some discussion of the building of the Hooker telescope and of the work of Hubble and Harlow Shapley in which this telescope played a part. Discussion of Hale and Mount Wilson, however, emphasizes solar work rather than the Hooker telescope. Includes photographs, drawings, bibliography, and glossary.
Watson, Fred. Stargazer: The Life and Times of the Telescope. New York: Da Capo Press, 2005. History of the telescope’s development includes discussion of the impacts on society of the discoveries the instrument has made possible. Presents the stories of the astronomers and other scientists responsible for advances in telescope technology, including Hale.
Wright, Helen. Explorer of the Universe: A Biography of George Ellery Hale. 1966. Reprint. Melville, N.Y.: American Institute of Physics Press, 1994. Excellent source for general readers on Hale’s work on Mount Wilson, including the building and use of the Hooker telescope. Personal correspondence and other original documents convey the thoughts and hopes of the astronomers of the time. Includes photographs, drawings, and bibliography.
Wright, Helen, Joan N. Warnow, and Charles Weiner, eds. The Legacy of George Ellery Hale: Evolution of Astronomy and Scientific Institutions, in Pictures and Documents. Cambridge, Mass.: MIT Press, 1972. Includes some discussion of the Hooker telescope, although emphasis is on the larger Hale telescope and on solar telescopes. Interesting and useful for the many photographs and references to original letters, newspapers, and other documents.