Astronomy and Space Science: The Telescope

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Astronomy and Space Science: The Telescope

Introduction

The telescope, invented in the first decade of the seventeenth century, transformed the ways in which astronomers and natural philosophers both viewed and thought about the universe. Telescopes quickly became indispensable to astronomical research and over time grew enormously in variety, size, power and expense. By the late twentieth century, astronomers even escaped the obscuring layers of Earth's atmosphere by

launching telescopes into space. Many thousands of people labored to build, launch, operate, and maintain the Hubble Space Telescope (HST), which to date has cost over $10 billion, making it a leading example of very large-scale science, as well as the most famous telescope of all.

Historical Background and Scientific Foundations

The details of the invention of the telescope are unclear. What is certain is that in September 1608, Hans Lippershey, a spectacle maker from Middleburg, the Netherlands, journeyed to the Hague to seek a patent for what later would be termed a refracting telescope. His device consisted of two lenses, one convex and one concave, placed at opposite ends of a tube; this magnified far-off images a few times. Lippershey was not granted a patent because, while his contrivance was deemed useful, it was simple to copy. Skilled spectacle makers in fact had little trouble fabricating their own examples even without sight of a working instrument.

In May 1609 Galileo Galilei (1564–1642) heard rumors of the remarkable new device that enabled distant objects to be viewed as if they were close. When news arrived confirming the rumor, Galileo threw himself into constructing a telescope. Before long he was making telescopes that magnified around twenty times. He soon applied them to astronomy and rapidly made a string of astonishing discoveries. Armed with these finds, Galileo then began to attack the dominant Aristotelian notions of the cosmos, challenging the very foundations of European thought.

When Galileo saw the moon pitted with craters and studded with mountains, he concluded it was quite Earth-like—definitely not a perfect Aristotelian sphere. When he turned to Jupiter, he spotted four companions to the planet. These were moons of Jupiter, known today as Io, Europa, Ganymede, and Callisto. His telescopes also revealed many more stars than are visible to the naked eye. Galileo published his remarkable findings shortly thereafter in the Sidereus Nuncius (Starry messenger), where he announced that a set of phenomena had been added to science that had been revealed by an instrument, an unprecedented event in the history of science.

Refractors Grow Longer

Galileo also found that Saturn's appearance varied over time. When he observed the planet in July 1610, it seemed to consist of three bodies arranged in a line, a central larger one with two smaller ones at its side. After two years the two smaller bodies had disappeared. Later, to his surprise, they reappeared.

In 1659, nearly 50 years later, the Dutch mathematician, astronomer, and philosopher Christiaan Huygens (1629–1695) solved the puzzle in his book Systema Saturnium (The Saturnian system), in which he found that these “bodies” were the result of a thin, disk-like ring that surrounded the planet. Huygens also improved on telescope design, grinding superior lenses and building long refractors. Galileo's longest telescope, for example, was about 4 feet (1.2 m) long. Huygens's were generally longer, and sometimes very much longer. In 1655, for example, Huygens discovered Titan, Saturn's largest moon, with a telescope that had an aperture of a little over 2 inches (5.1 cm) but a focal length of 12 feet (3.7 m). Later he built one with a 123-foot (37.5 m) focal length. What drove these increases in telescope length?

In 1611 Johannes Kepler (1571–1630) designed a new kind of refracting telescope in which the eyepiece is composed of a convex, not a concave, lens. Although this type of refractor would displace the Galilean variety in time, at this point in history the resolution (or image detail) for both types was weak. One reason was “spherical aberration,” a limitation of seventeenth-century lens makers, who were only able to create spherical lenses. Because these do not focus the light striking them on a common point, they produce a somewhat blurred image. In addition, astronomers knew that observing, say, a star through a telescope produced a colored appearance that also signified a loss of sharpness, a problem known as “chromatic aberration.” By the middle of the seventeenth century astronomers had discovered the benefits of long focal lengths (and long telescopes) in combating both spherical and chromatic aberration.

IN CONTEXT: LONG REFRACTORS

In time astronomers recognized that all telescopes exhibit various kinds of imperfections and that devising a perfect telescope is impossible. By the middle of the seventeenth century astronomers coped as best they could with spherical and chromatic aberration by increasing the focal length of their telescopes. These longer telescopes used main lenses that were only slightly curved, which reduced the effects of spherical aberration. A longer focal length also diminished chromatic aberration. Telescopes began to grow; by the middle of the seventeenth century good ones were 30 ft (9 m) or longer.

One avid astronomer and telescope maker in Danzig (now Gdansk, Poland) was the wealthy brewer Johannes Hevelius (1611–1687). When Hevelius read Huygens's accounts of telescopes much longer than those he had built, Hevelius was inspired. His most famous instrument had a focal length of no less than 150 feet (45 m) and had to be suspended from a mast 90 feet (27 m) high. Observing with this gigantic instrument posed all sorts of practical difficulties. Edmond Halley (1656–1742, of Halley's comet fame) pronounced this enormous telescope effectively useless. This would not be the last time a telescope maker pressed available technology too far.

Newton and the Reflecting Telescope

Hevelius died in 1687, but by the end of his life telescope development had diverged in two directions: traditional refractors and reflectors, telescopes that employed mirrors. In 1668 British mathematician and physicist Isaac Newton (1642–1727), then a fellow of Trinity College, Cambridge, made the first successful reflecting telescope. Newton's novel device emerged from his research into the nature of light.

Against the general opinion of the day, Newton argued that white light is made up of different colors, and that each color is refracted by a slightly different amount when it passes through a lens. Newton therefore believed refractors would inevitably suffer from chromatic aberration. In Newton's design, a mirror of speculum metal (an alloy of copper and tin with sometimes small amounts of other materials added) sat at the bottom of a tube. Inserted into the tube near its top was a flat mirror, tilted to direct light to an eyepiece on the side of the tube. This type of optical configuration became known as a Newtonian to distinguish it from other sorts of reflectors. As it relied only on mirrors, which would not split the incoming light into colors, Newton avoided the problem of chromatic aberration.

Newton need not have been so pessimistic about the prospects of contriving a refractor free from chromatic aberration, however. In 1729 Chester Moor Hall (1703–1771), a London barrister who pursued optical experiments and devices as a hobby, invented an achromatic lens by combining lenses of two types of glass with different refractive properties: a convex lens of crown glass and a concave lens of flint glass. This combination significantly reduced light dispersion and therefore the blurring caused by chromatic aberration. John Dollond (1706–1761), a leading eighteenth-century British instrument maker, secured a patent on this idea in 1759, and also placed achromatic lenses into general production. Although Dollond died in 1761, his son Peter vigorously defended his patent in lawsuits; as a result Dollond-built achromatics became very popular among astronomers.

William Herschel and His Reflectors

In the late eighteenth century, state-of-the-art telescopes were extremely expensive. For an amateur astronomer of modest means who wanted a sizable telescope, the only realistic option was to build one. One such highly ambitious amateur was William Herschel (1738–1822).

William Herschel's Reflecting Telescope

Early in his astronomical career, Herschel set his sights on building big telescopes. He was particularly intrigued by nebulae (what appeared to be misty clouds of light) and star clusters, and he knew that to detect and examine them he needed telescopes with large mirrors that could collect lots of light. Herschel's finest instrument had a 20-foot (6.1 m) focal length and contained a primary mirror 18 inches (45 cm) in diameter. He devised a mount for the telescope that kept it very stable, but also allowed it to be easily and promptly directed to any part of the sky. In 1833, his son John took the 20-foot reflector to the Cape of Good Hope so that he could explore the southern skies as his father had in the north.

William Herschel's efforts transformed the reflecting telescope into a powerful tool for exploring the heavens. His reach exceeded his grasp, however, when he built a reflector with a primary mirror of 4 feet (1.2 m) and a focal length of 40 feet (12.2 m). Completed in 1789, it was by far the largest reflector attempted to this time, but was very cumbersome to operate. Built with thousands of pounds of King George III's money, its speculum metal mirrors tarnished rapidly and required frequent polishing. Herschel was unwilling to admit it publicly, but the 40-foot telescope was a grand failure.

Precision and Reliability

Other astronomers did not follow Herschel in building reflectors. For them, astronomy's goals were to determine the positions of the celestial bodies as accurately as possible and then to interpret these positions in terms of Newton's law of universal gravitation. To this end, they needed telescopes that were precise and reliable—light grasp was not so important. Refractors set on massive mounts for maximum stability were the choice of these professionals.

WILLIAM HERSCHEL (1738–1822)

William Herschel (1738–1822) was born in Hanover, in what is now northwest Germany. He left school in 1753 to join his father's military band. The very close links at the time between Britain and Hanover led William to visit England in 1756, but he soon moved there permanently to pursue a musical career; in 1774, he became the organist at the Octagon Chapel in Bath.

Herschel's life was transformed by the events of March 13, 1781. That night, employing a fine Reflector of 7-foot (2.1-m) focal length, he came upon an object that was, in his words, “a curious either Nebulous Star or perhaps a Comet.” It proved to be a planet, now called Uranus. Although other astronomers had observed this same object earlier, they had not been able to identify it. Such was the quality of Herschel's telescope, however, that he immediately realized it was not a star.

This was the first new planet in recorded history and its discovery caused a sensation. Herschel became famous overnight. King George III granted him a royal pension, and Herschel abandoned his musical career to devote himself to astronomical endeavors, including the construction of telescopes. Herschel's devoted sister Caroline, a successful astronomer in her own right, with the discovery of eight comets to her name, also played a very important role as her brother's indefatigable assistant.

In the early 1800s, Germany overtook England as the primary manufacturer of refractors. The leading firm was that of the engineer Joseph von Utzschneider (1763–1840) and physicist Joseph von Fraunhofer (1787–1826) who ran an optical shop in Munich, Bavaria. Advances in glassmaking led to better quality glass and, when allied to Fraunhofer's deep knowledge of optical techniques, larger and better telescope objectives. Perhaps Fraunhofer's finest achievement was his 9.6-inch (24.4-cm) refractor for the Dorpat Observatory, finished in 1824. For some years it was the biggest refractor in the world.

The Leviathan of Parsonstown

While precision, stability, and reliability were the keywords for refractors, some astronomers, like Herschel, wanted more light, the better to find and examine faint objects. The Irish atronomer William Parsons, third Earl of Rosse (1800–1867), was one such astronomer. By 1839 Rosse had constructed a 36-inch (91-cm) reflector at his family seat at Parsonstown (now Birr), Ireland. Rosse, however, harbored still bolder plans for a reflector with a primary mirror measuring an unprecedented

6 feet (1.8 m) in diameter, and great efforts were undertaken at Parsonstown to this end. Visitors were awed by the scale of the work: telescope tubes though which people could walk upright, mirrors weighing tons, steam-powered machines for polishing those mirrors, and masonry supports for the telescope that one likened to a Norman keep. This monster telescope, the “Leviathan of Parsonstown,” was completed in 1845; it became one of the scientific wonders of its age.

The Leviathan, however, was handicapped by the Irish climate and its location next to a bog. The number of clear nights on which it could be used effectively were few. It was also so massive that it could be pointed to only a limited part of the sky. Large reflectors such as these were also idiosyncratic, needing the coaxing of experienced observers to bring out their best. In the middle of the nineteenth century they were still not the instruments of choice for professional astronomers.

Refractors versus Reflectors

Refractors grew bigger and more powerful in the last decades of the nineteenth century as an international race pitted builders (and their patrons) against each other for the prestige of creating the largest telescope. The culmination of this competition was the great Yerkes Observatory refractor at the University of Chicago, completed in 1897. This enormous scope, with its 40-inch (100-cm) objective, was funded to the tune of over $500,000 (well over $7 million in modern currency) by Charles T. Yerkes, a disreputable but wealthy figure who had secured his fortune through the manipulation of streetcar and railroad franchises in various cities.

The Yerkes was the last of the great refractors; a larger one has never been built. Reflectors were better suited to the newer astrophysical questions astronomers were tackling by the first decade of the twentieth century: the physics and chemistry of celestial bodies, not their positions. In addition, rather than observe these bodies through an eyepiece, early twentieth-century astronomers attached cameras or spectroscopes to their telescopes to photograph them or filter their light through a prism or diffraction grating. By the end of the century, in fact, the photographic plates would themselves be replaced by electronic light detectors.

Telescope mirror manufacturers also abandoned the problematic speculum metal mirrors by the late-nineteenth century. Through advances in chemistry, glass mirrors had become easier to grind and polish to the correct shape than speculum metal. A very thin layer of silver was then deposited on the front surface of glass to reflect the incoming light (replaced in the 1930s by aluminum).

Hale's Ambitions

One of the key figures in securing Yerkes's funding for the 40-inch (100-cm) refractor was American astronomer George Ellery Hale (1868–1938). One of the leading promoters and fund-raisers in the history of astronomy, he was key to the establishment of three observatories, all of which would at one time possess the world's most powerful telescope. After the Yerkes refractor, however, his other giant telescopes would be reflectors.

Hale left Yerkes in 1902 to establish the Mount Wilson Observatory in California. By this time astronomers were seeking optimal sites for their telescopes rather than simply operating them wherever an observatory happened to be located. Hale recognized the big advantages to be gained by building telescopes thousands of feet above sea level (and much atmospheric disturbance) at sites where the skies were favorable for observing. Mount Wilson fit the bill, and Hale was intent on building a large reflector, among other instruments. Supported by the private philanthropy of the Carnegie Institution, Hale and his staff, most notably the master optician George Ritchey, completed a 60-inch (152-cm) Reflector in 1908. In 1919 the observatory gained an even more powerful reflector, the primary mirror of which was 100 inches (254 cm) in diameter. It would be the biggest in the world for 30 years.

Hale was not done: Despite incapacitating bouts of ill health, he obtained funds for what would become for many years the largest telescope in the world, the 200-inch (508-cm) reflector on Palomar Mountain in California. Sadly, Hale did not live to see his finest creation come into operation in 1949.

An even bigger 236-inch (600-cm) Russian telescope was built in 1975 at Mount Pastukhov in the Caucasus Mountains. Although it suffered from major problems at first, its performance later improved. Since then a surge of telescope building has put instruments with even bigger mirrors on mountaintops around the world. In 1991, for example, atop Hawaii's Mauna Kea, the 32.8-foot (10-m) Keck telescope went into operation, followed shortly thereafter at the same site by the 27-foot (8.3-m) Japanese Subaru telescope.

Beyond Visible Light

World War II (1939–1945) both spawned new technologies and forced the rapid evolution of existing ones. One of the most important was radar. At war's end, scientists joined forces with astronomers to detect radio waves from astronomical objects. Radio astronomy reshaped astronomers' concepts of the universe as evidence from radio telescopes painted a more violent and tumultuous picture of the universe than had been seen from observations with optical telescopes.

New sources of support were vital, too, as science became a key battleground in the Cold War between the United States and the Soviet Union. By the 1950s funds for science flowed from national governments as never before. Some of this money was used to build radio telescopes, including a number with enormous dishes to collect and focus radio waves. The British Mark I radio telescope at Jodrell Bank, University of Manchester, had a dish diameter of 250 feet (76 m).

The Cold War contest for technological supremacy was one driver for x-ray astronomy. Because Earth's atmosphere prevents celestial radiation from reaching Earth, astronomers must launch their instruments into space to detect x-ray radiation. While early instruments contained relatively crude means of detecting x rays, in the early 1960s Italian-born American physicist Riccardo Giacconi (1931–), decided to try a new approach.

Giacconi knew using a conventional mirror to focus the x rays was fruitless since they would penetrate the surface rather than be reflected. To overcome this, Giacconi and another Italian-born American experimental physicist, Bruno Rossi (1905–1993), devised a way to form x-ray images by nesting several mirrors in concentric cylinders. This meant that incoming x rays hit the mirrors at very small angles and reflected from, rather than penetrated, the surface. The Einstein Observatory, launched into orbit in 1978, became the first large x-ray telescope observatory.

Modern Cultural Connections

Until the 1930s almost all the information astronomers learned about the universe came from observations of visible light. But visible light forms only one very small part of the electromagnetic spectrum, which extends from radio waves at long wavelengths to gamma rays at very short ones. Wartime advances, however, greatly advanced the progress of radio techniques, and spurred the study of the universe in other regions of the spectrum.

IN CONTEXT: LYMAN SPITZER ON SPACE TELESCOPES

In 1946 the American astrophysicist Lyman Spitze Jr. (1914–1997), wrote a report called “The Astronomical Advantages of an Extra-Terrestrial Observatory.” More than a decade before Sputnik 1 was launched, and at a time when most of his colleagues were highly skeptical of pursuing astronomy from space, Spitzer strongly advocated a very large space telescope.

While a large Reflecting satellite telescope (possibly 200 to 600 inches [5 to 15.2 m] in diameter) is some years in the future, it is of interest to explore the possibilities of such an instrument…. Most astronomical problems could be investigated more rapidly and effectively with such a hypothetical instrument than with present equipment. However, there are many problems which could be investigated only with such a large telescope of very high resolving power…. It should be emphasized, however, that the chief contribution of such a radically new and more powerful instrument would be, not to supplement our present ideas of the universe we live in, but rather to uncover new phenomena not yet imagined, and perhaps to modify profoundly our basic concepts of space and time.”

Because Earth's atmosphere blocks much of the radiation that would otherwise reach it, scientists needed a way to analyze this energy. Advances in rocket technology allowed telescopes to be launched into space, expanding the sections of the electromagnetic spectrum they could explore. Infrared, ultraviolet, x-ray, and gamma ray astronomy have all been made possible (or have benefited enormously from) the technologies of the space age.

The world's most famous telescope is the Hubble Space Telescope (HST), launched into orbit in 1990 by the space shuttle Discovery. HST is a completely automated observatory and works principally in the visible and ultraviolet regions of the spectrum, although an infrared instrument was added in 1997. At its heart is a primary mirror 7.9 feet (2.4 m) in diameter that collects light for the telescope's battery of scientific instruments. Although this mirror is relatively small by the standards of the biggest telescopes on ground, the HST's position above Earth's atmosphere more than compensates for its size.

A joint project of NASA and the European Space Agency, HST is designed to be updated over time. Space shuttle astronauts have visited the telescope regularly to replace, for example, old instruments with new ones. Thousands of people have worked on and used the HST, which cost around $2 billion to build; its time in orbit has added billions more to this figure. It is probably the single most costly scientific instrument devised, as well as the most productive telescope ever. At the time of writing it is uncertain whether NASA will fly one more shuttle mission to update and repair HST. If not, its productive scientific life is likely to be over by around 2010.

The design and use of telescopes have been driven largely by technological and scientific changes, but also by the people and societies that build them. Herschel's career as a telescope builder, for instance, underscores the critical role of individuals in late-eighteenth and early nineteenth century science, while the multibillion dollar Hubble Space Telescope is a prime example of very large-scale science and interlocking teams of engineers and scientists, as well as of the extremely high premium placed on scientific research by modern Western societies. Engraving of the “Leviathan of Corkstown,” completed by William Parsons, 3rd Earl of Rosse (1800–1867) on his home estate in Ireland 1845. At the time it was the world's largest telescope.

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Panek, Richard. Seeing and Believing: How the Telescope Opened our Eyes and Minds to the Heavens. New York: Penguin Books, 1998.

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Rice University. “The Galileo Project.” http://galileo.rice.edu (accessed April 14, 2008).

Robert W. Smith

Scientific Thought: In Context