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Book Review - Fred Watson, Stargazer |
Posted by Michael Cohen, Aug 24, 2006
Fred Watson, Stargazer: The Life and Times of the Telescope (Da Capo, 2004). 342pp. $24.95
We need a new history of the telescope every ten years or so. Fred Watson brings us up to date with a book that finds a middle ground between the very detailed early history to be found in Henry C. King’s 1955 The History of the Telescope (now available again in a 2003 Dover reprint) and the emphasis on scientific revolution that characterizes Richard Panek’s Seeing and Believing: How the Telescope Opened Our Eyes and Minds to the Heavens (Viking Penguin, 1998) Watson combines the credentials of a professional astronomer with the ability to write clear prose that an amateur can understand.
Watson begins by describing a gathering of astronomers and instrument-makers in 2000. He reminds us that aperture fever in professional astronomers makes the malady in amateurs look small—literally. In the twentieth century, telescopes which contemporaries considered big went from 60 inches at the turn of the century to 200 inches at mid-century (the Hale telescope installed on Mount Palomar in 1948). During the 70s and 80s, four-meter telescopes, slightly smaller than the Hale, became common, but in the last decade of the century, ten telescopes were completed with mirrors from eight to ten meters (320 to 400 inches) in diameter. Of course, the land-based telescopes would probably not have increased so much in diameter if engineers had not come up with “adaptive optics,” the system of bending parts of the mirror to compensate for distortions caused by turbulence in the column of air above the telescope.
Watson includes colorful details like the duel that destroyed the bridge of Tycho Brahe’s nose. Brahe is only included here (since he died in 1601, before the telescope was invented) because he designed an equatorial mount for one of his instruments, an armillary sphere, and such mounts were later adapted for telescopes. Watson also thinks Brahe’s organizing skills (“he was the first modern-style director of a scientific institution”) make him a forerunner of the contemporary organization man necessary to develop the funding and coordinate the construction of a big telescope.
Watson explores and then dismisses the possibility that the telescope was known before the early seventeenth century—to the ancient Romans, for instance, or to those making lenses for spectacles since the invention of corrective eyeglasses in the thirteenth century. As he implies, it is rather odd that no one in several hundred years put two lenses together, either two convex ones or one convex and one concave, thereby discovering that they would magnify distant scenes. He does explain the difficulty of making good glass lenses; telescope making is dependent on other technologies, which clearly had not developed much before the moment when Hans Lipperhey, a German-Dutch spectacle-maker, applied to the governing council of the Netherlands for a patent for his telescope in September, 1608. The council paid him for making several instruments, but he never got his patent, partly because the simple principle of Lipperhey’s refracting telescope, which used a concave glass lens (an eyepiece) to magnify an image formed by a convex glass lens (an objective), quickly spread across Europe.
Galileo Galilei built one as soon as he heard about it, and within six months published his historic observations of four of Jupiter’s moons. Meanwhile others (Thomas Harriot in England, Simon Marius in Germany) were using telescopes to make observations. Johannes Kepler improved the telescope’s design by inventing the astronomical telescope, whose convex rather than concave eyepiece keeps the magnified image inverted but allows for a larger field than the Galilean design.
Over the next few decades other combinations of lenses were tried, William Gascoigne invented a reticle to insert between the lenses that allowed for measuring the angular diameter of sky objects or the angular distance between close pairs of stars, and refracting telescopes began to grow in length. Longer telescopes reduce the natural distortions of glass lenses: chromatic aberration (the prismatic effect of glass breaks light into its component colors, causing images with colored “ghosts”) and spherical aberration (the more a lens is curved the more it blurs an image). Fifteen-foot telescopes were common, and some designs were for instruments over a hundred feet long.
Modern refracting telescopes use a combination of precision-ground lenses to correct both spherical and chromatic aberration, but this solution was not discovered until the next century. But before the seventeenth century was over, another answer was found: the reflecting telescope. The medieval Arab scientist Alhazen knew that curved mirrors have properties similar to lenses. Contemporaries of Galileo such as René Descartes and Marin Mersenne had theorized about a reflecting telescope, and James Gregory, a mathematician at Scotland’s St. Andrews University, had even tried to have one built. But the technology took time. It is harder to make an accurate mirror than an accurate lens, because, in Isaac Newton’s elegant statement of the problem, “the Errors of reflected Rays, caused by any Inequality of the Glass, are about six times greater than the errors of refracted Rays caused by the like Inequalities.” Newton was the first to build a working reflecting telescope, approaching the problem not only as a theoretical one, but as a practical matter of lens construction, polishing, and design. Newton knew that theoretically, the mirror of a reflecting telescope should have the shape of a parabola, but he also knew that he couldn’t make such a mirror, but that a spherical shape would do for a short focal length telescope. He made his own alloy for the mirror, which like all mirrors before nineteenth-century glass-silvering techniques, was of metal. He chose the simplest design, still called the Newtonian, in which light rays enter a tube, bounce off a curved primary mirror at the bottom, and then are diverted to an eyepiece outside the tube by a flat mirror within the light path near the top of the tube.
Mersenne’s contribution was to conceive of a primary mirror with a hole in the center through which the observer looked toward the secondary mirror. Gregory’s design also had a pierced primary, and shortly after Newton built a working model of his very different design, Laurent Cassegrain designed a telescope with a perforated paraboloid mirror and a convex secondary that needed the shape of a hyperbola. These mirrors could not be made in 1672, but the design became the basis for almost all modern research telescopes.
About 1720, John Hadley, who also invented the precursor of the nautical sextant called the octant, developed a technique to make paraboloid mirrors needed for the Newtonian telescope by first producing a spherical surface of the desired focal length and then “figuring” the mirror to a parabola. A little later James Short developed his own technique, one he never disclosed, for producing both the paraboloid and the ellipsoid mirrors needed for the Gregorian design, and he produced telescopes for four decades, one with a primary mirror of 18 inches.
But the most impressive developments in astronomy and telescope –making in the eighteenth century have to do with an expatriate German composer and organist at the Octagon Chapel in Bath, England. Wilhelm Friedrich (or as he later Englished his name) William Frederick Herschel cast and ground metal mirrors 12 inches in diameter for a twenty-foot telescope he used for most of his observations, and he eventually got King George III to underwrite the building of a 40-footer with a 48-inch mirror. Herschel started a journal of his observations in 1774, and in 1781 he discovered the planet Uranus. Among his innovations was doing away with the secondary mirror; he just pointed his eyepiece down from the rim of the tube toward the primary. Since only one metal mirror was thus involved, the image was brightened considerably. He used eyepieces of many different focal lengths, was the first to use a “monk’s hood” to eliminate extraneous light, and used an accurate timepiece and patient, careful notes about the size, brightness, and appearance of objects. Herschel had added thousands of objects to our catalogue of the sky by the time of his death in 1822.
The nineteenth century was the heyday of the refracting telescope. At the beginning of the century, a Swiss cabinet-maker, Pierre Louis Guinand, had produced flint glass lens blanks up to 5 inches. Later he discovered how to make larger ones by pressing softened glass into a mold. Joseph von Fraunhofer produced a refractor of almost 10 inches for the Russian observatory at Dorpat, where Wilhelm Struve used it to measure more than 3000 double stars. For this instrument, Fraunhofer invented what is now known as the German Equatorial Mount. Wilhelm Bissel used another of Fraunhofer’s instruments to measure the parallax of 61 Cygni in 1838, the first time the distance between our star and another had been measured.
Back in the 1730s, Chester David Hall had guessed that two lenses, one concave and one convex, made of two different kinds of glass, might cancel out the color errors that plagued refracting telescopes. He was right, but because he did not patent the idea, John Dolland the optical instrument maker picked it up, got a royal patent in 1758, and prospered. His son Peter designed the apochromat lens, which uses three lenses to eliminate color errors; the two-lens model is called an achromat.
In the 1880s, University of Jena physicist Ernst Abbe realized that if glass of desired refractive power could be manufactured, all sorts of wonderful lenses were possible. He joined with chemist Otto Schott and the Zeiss father and son team to build, at the end of the nineteenth and the beginning of the twentieth centuries, lenses and prisms of unprecedented quality and astronomical refractors with apertures up to 30 inches.
During the first half of the nineteenth century, Fraunhofer, Gustav Kirchoff, Robert Bunsen and others had come to realize that the dark lines in the sun’s spectrum were absorption lines corresponding to bright lines that were emitted in those places when particular elements burned. In 1862, William Huggins, an amateur astronomer, fitted his 8-inch Alvan Clark refractor with a two-prism spectroscope built by the chemist William Miller, and the two men studied and described the spectra of fifty stars. Huggins then pointed his spectroscope at a planetary nebula, got a single emission line, and realized the nebula was made of a luminous gas.
In the last quarter of the nineteenth century, the largest refracting telescopes were built, almost all with lenses made by Alvan Clark: a 26-inch at the U. S. Naval Observatory (1872), with which Asaph Hall discovered Phobos and Deimos, the moons of Mars (1877); a 36-inch at the Lick Observatory (1888) on Mount Hamilton near San Jose; and the largest, the 40-inch Yerkes Observatory telescope at the University of Chicago (1897).
Meanwhile the reflecting telescope was also being vastly improved. The Dublin engineer Thomas Grubb designed a mirror cell in 1835 to support a reflecting mirror without compressing or distorting it. This meant that there was no longer any practical limit to the size of a reflecting mirror in a telescope. Refracting lenses are limited because glass flows slightly, and with a very large lens on edge the effect becomes detrimental in an objective size beyond about forty inches, the size of the largest such lens, in the Yerkes refractor.
William Parsons, third Earl of Rosse, began making telescopes in 1827. Eventually he built an instrument with a 6-foot mirror, mounted in a wooden tube suspended between masonry walls (with some play so that it could follow objects for about an hour outside of their meridians) and nicknamed “the Leviathan of Parsonstown.” When Rosse began using it in 1845, he was able to see spiral structure in M51 and in sixty other “nebulae,” a term that denoted any deep sky object with unresolved stars.
The amateur William Lassell, an English brewer, discoverer of Neptune’s moon Triton (1846), was the first to make an open-framework tube with a counterbalance and a fork equatorial mount, for a 37-foot reflector with a 48-inch mirror, in 1860.
Carl von Steinheil in Germany and Léon Foucault in France began making reflecting telescopes with mirrors of glass with a thin deposit of silver on them in the 1850s. Silvered glass was lighter and more reflective than “speculum” metal and could be recoated rather than having to be repolished. Foucault also devised the knife-edge test to find errors in a mirror surface.
By the 1880s photography had become an important tool for the astronomer, thanks to the work of Henry Draper, Lewis Rutherford, and others. Eventually photography would completely replace visual observation for the professional astronomer. First, though, telescopes needed to be designed specifically for photography, and George Willis Ritchey, an instrument maker at the Yerkes Observatory, did it. Since 1882 it had been known that the speed a telescopic photographic image forms is inversely proportional to the focal ratio of the telescope. Ritchey designed “fast” reflecting telescopes. His masterpiece was the 100-inch Hooker Telescope on Mount Wilson near Pasadena, the telescope Edwin Hubble used to determine that the galaxies then called “spiral nebulae” were millions of light-years away and thus constituted immense star systems like our own Milky Way. Among other innovations the Hooker Telescope had a mirror that had been coated with aluminum in a vacuum, the way all modern telescope mirrors are made.
The last link in modern reflecting telescope design was supplied by Bernard Schmidt, an Estonian optical engineer, with the encouragement of the astronomer Walter Baade. Schmidt tackled the problem of designing a fast telescope with a wide field of view, and he solved it by inventing the “correcting plate,” a glass lens placed at the opening of the reflecting telescope and having just enough curvature to balance the aberration of the spherical mirror. This design was used in the 48-inch Palomar Schmidt, the companion telescope for the 200-inch Hale Telescope on Mount Palomar, which was for twenty-five years the world’s largest. Schmidt’s correcting plate is also the modification of the Cassegrain design that is found in millions of compact reflecting telescopes used by amateurs today.
Larger telescopes were built in the second half of the twentieth century, including the twin Keck Telescopes on Mauna Kea in Hawaii, each about twice as big as the Hale. But the most important developments have not been size. Large telescopes are now built with segmented mirrors made of the ceramic-glass mixture that replaced Pyrex glass because of its even smaller coefficient of expansion with temperature change. They use electronic imaging devices twenty times more sensitive than photographic plates. But the most significant development of the end of the century was clearly adaptive optics, which distort the mirror of a telescope to compensate for atmospheric currents and very nearly reproduce the seeing possible to a telescope in space. This is not to belittle the contributions of telescopes in space; the Hubble, in addition to giving us spectacular, unforgettable images, has enlarged our concept of the universe to a hundred billion galaxies.
Telescopes now “look” in all wavelengths of the electromagnetic spectrum from gamma ray detection to infrared and ultraviolet light to radio emissions. We measure the spectra of stars from beyond the range of any telescope we can make, using as our lenses galaxy clusters, which can bend light as if it were being refracted by a telescope millions of light-years in diameter. Watson predicts that by the middle of the current century we could be using telescopes in space whose reflecting surface will be measured in acres, made up of many cells of delicate reflecting membranes “held in perfect shape in zero gravity by the pressure of light itself, emitted from infrared lasers.”
