Observational Astronomy 111 Historical development

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The Greek astronomer Hipparchus (c. 127 bc) used astronomical observations to determine the lengths of the four seasons and the duration of the year to within 6.5 minutes. He also derived the distance to the Moon and the Sun, but his most amazing feat was to notice a small westward drift of the constellations which we now call the precession of the equinoxes. This effect causes the current Pole Star (Polaris) to move away from the North point and circle back after almost 26,000 years! Chinese astronomers recorded the appearance and fading of an exceptionally bright star in 1054 ad in the constellation we now call Taurus, but it was not until the 20th century that Edwin Hubble (1889-1953) associated this event with the supernova explosion which gave rise to the Crab Nebula, also known as Messier 1, the first entry in the list of nebulous objects studied by Charles Messier (1730-1817). Following the invention of the telescope in the early 1600s, Galileo Galilei (1564-1642) and others were finally able to enhance the sensitivity of the only light detector available to them, the human eye, and to resolve details such as craters and mountains on the Moon, the rings of Saturn, moons orbiting Jupiter, and the individual stars in the Milky Way. By making careful drawings (Figure 1.1) of what their eyes could detect during moments of minimum atmospheric turbulence, what astronomers today call moments of "good

Figure 1.1. Hand-drawn sketches of features on the surface of the Moon as might have been made by the first astronomers to use telescopes to enhance the power of the eye.

seeing", the early 17th-century scientists were able to convey pictorially those observations to others.

Better telescopes led to more astronomical discoveries, which in turn stimulated the development of even bigger and better telescopes. Opticians developed color-corrected lenses for telescopes and then, following Isaac Newton (1642-1727), telescopes using reflections from curved mirrors instead of transmission through lenses were gradually introduced. William Herschel (1738-1822), a prolific observer and discoverer of the planet Uranus, pioneered the construction of many reflecting telescopes with long "focal lengths" and large magnifications; in later years the emphasis would move to larger diameter mirrors rather than longer focal lengths. With the invention of the prism spectroscope by Joseph Fraunhofer (1787-1826), the chemical constitution of the Sun and stars became amenable to physical study. In Fraunhofer's early experiments a beam of sunlight was passed through a narrow rectangular slit in a mask and then through a glass prism to produce a colored spectrum in the manner similar to Newton and others (Figure 1.2). The critical addition made by Fraunhofer was a small telescope mounted on a movable arm which could be set to precise angles to view the spectrum. Initially, the light detector was still the human eye. Fraunhofer found that the normal band of colors from violet to red was crossed by numerous dark vertical lines. Eventually the pattern of these Fraunhofer absorption lines (actually images of the entrance slit partially devoid of light) was shown to be characteristic of individual chemical elements. The elements hydrogen, calcium, sodium, and iron were recognized in the spectra of the Sun, and later, the stars. Further spectroscopic observations of the Sun soon led to the discovery by Janssen and Lockyer (in 1868) of an unknown element which we now know to be a major constituent of the universe. This new element, helium, was named after the Greek word for the Sun helios; helium was not discovered on Earth until 1895.

When dry, gelatin-based photographic emulsions became routinely available in the late 19th century, astronomers such as Henry Draper (1837-1882) lost no time in

Figure 1.2. Joseph Fraunhofer's spectroscope and the dark lines in the spectrum of the Sun which now bear his name. This instrument combined with the photographic plate—rather than the human eye—opened the way to a physical understanding of the universe.

putting them to use to catalog the appearance and properties of a wide range of objects in the night sky. The photographic process was unarguably more accurate and more sensitive than the keenest human eye and the most artistic hand. From planets to stars to galaxies, the new observational tools were applied. Still larger telescopes were constructed, each a technical feat for its era, reaching a mirror diameter of 100 inches (2.54 meters) in 1917 with the completion of the Hooker Telescope on Mount Wilson by George Ellery Hale (1868-1938). Just one of the great discoveries which followed was the expansion of the universe by Edwin Hubble and Milton Humason in 1929.

The history of astronomy is marked by such sporadic progress. Each improvement in scientific apparatus, each new development in technology, helps to provide answers to old questions. Inevitably, the new observational methods uncover a host of new questions, which in turn drive the quest for even better measuring equipment! Progress in studying the universe has always been related to "deeper" surveys of the cosmos reaching to ever-fainter objects, or higher resolution yielding more and more fine detail, or larger statistical samples from which generalizations can be made, or broader spectral response to sample all the energy forms passively collected by the Earth. That trend has continued since the Renaissance of the 16th century to the present day in a kind of ever-increasing spiral, with new tools or technologies leading to new discoveries which in turn drive the development of better tools.

A key feature of observational astronomy has been record-keeping, maintaining archives of observations, usually in some pictorial form, for future investigators to compare and consider. In terms of its ability to convert light into a measurable quantity, the photographic plate is actually less sensitive than the human eye. The great advantage of the photographic plate, however, is that it can build up a picture of a faint object by accumulating light on its emulsion for a long period of time. It is therefore called an "integrating" detector. The eye cannot do this to any significant extent. Moreover, the plate provides a permanent record for future study by others.

By using a photographic plate as the recording device in a spectrometer, astronomers could extend their investigations effectively and efficiently into the domain of quantitative astrophysics. Initially, of course, the flood of photographic material was analyzed by human eyes, and those eyes were mostly those of a dedicated group of female assistants hired by the director of the Harvard Observatory College, Edward Charles Pickering (1846-1919), toward the end of the last decade of the 19th century. Over forty women were employed by the observatory during the period of Pickering's tenure as director, and their efforts in handling the torrent of new astronomical data laid the foundations of modern astrophysics. Stellar spectral classifications led to the understanding that the colors of stars was largely a temperature sequence and that stars shine by the energy released in thermonuclear fusion reactions brought about spontaneously by the enormous temperatures and pressures at their centers. Among the most well-known of the Harvard ladies is Henrietta Leavitt (1868-1921) whose work on the class of stars called Cepheid variables, which pulsate in brightness with a period that is proportional to their true or absolute average brightness, led to a distance estimator and an appreciation of the true size and shape of our galaxy. During the first half of the 20th century, these tools inevitably resulted in more discoveries (per year) and a massive increase in the "data rate''; that is, the amount of information being collected, scrutinized, and archived for posterity. But these advances were only the beginning.

Even as the 100-inch (2.54 m) Hooker telescope was discovering the expansion of the universe, plans were being laid to build the great 200-inch (5.08 m) reflecting telescope on Mount Palomar in southern California. That telescope, named after George Ellery Hale, went into operation in 1949 and remained the largest telescope in the world until the construction of the Russian (then Soviet) 6 m Bol'shoi Teleskop Azimutal'ny (BTA) in 1976. Construction of both of these large telescopes was challenging. For the 200-inch, Hale secured a grant in 1928 from the Rockefeller Foundation, but optical figuring of the Pyrex mirror took from 1936 to 1947 with four years off for World War II. The telescope was dedicated in June 1948 ten years after Hale's death, but it was another 16 months before director Ira Bowen (18981973) opened the telescope for full-time use. Weighing about 1,000 tons, the dome of the Hale telescope stands 41 m (135 ft) high and is 42m (137 ft) in diameter. Likewise, the BTA on Mount Pastukhov on the northern side of the Caucasus range has a dome that is 58 m high and a primary mirror of Pyrex weighing 42 tons with so much thermal inertia that it can only tolerate a 2°C change per day if it is to retain its optical figure. Thermal inertia, the large dome, and the site turn out to be limitations on the best image quality that can be delivered. In the years that followed, astronomers would apply those lessons learned.

Building telescopes larger than 5 meters in diameter was going to be difficult, but observational astronomy received multiple boosts in the 1960s partly by the construction of many new optical observatories with 4-meter class telescopes; that is, with mirror diameters of approximately 3 m to 4 m (metric dimensions are preferred, see Appendix B for conversions). Although the telescopes were slightly smaller, these new facilities were well-equipped and located on excellent but somewhat more remote mountain sites in different parts of the world including the Arizona desert, the mountains of northern Chile, and the summit of Mauna Kea on the Big Island of Hawaii. The story so far refers only to "optical" astronomy. Another part of the

1960s expansion was stimulated by the exciting new look at the universe which accompanied the rise of radio astronomy and the discovery of completely new phenomena such as the incredibly luminous and distant quasars, thought to be supermassive black holes at the center of large galaxies, and the remarkable pulsars, now understood to be spinning neutron "stars" embedded in the remnants of a supernova explosion. All of this occurred during the successful development of the Soviet and American space programs which led to satellite astronomy and the opening up of the X-ray, ultraviolet, and infrared regions in the 1960s and 1970s. History shows that the introduction of any new domain results in new discoveries (e.g., Harwit, 2003). Other, more subtle, transformations began to occur around this time too, through the introduction of electronic computing machines and electronic devices which could be used as detectors of light. Photocells and sensitive "night-vision" TV cameras came first, but the steep rise of consumer micro-electronic products through the 1970s was to accelerate the changes rippling through astronomy. Even the telescopes themselves could be improved by the use of electronically encoded computer-controlled drive systems, thereby enabling much faster setup times and more reliable tracking across the sky. The newest radio and optical telescopes were remotely controlled, and the concept of converting measurements into an electronic form readily acceptable to a computer became standard practice. Computer power expanded exponentially, and astronomers eagerly used those capabilities to the full.

Construction of larger telescopes stagnated until the mid-1980s when Jerry Nelson of the University of California broke the paradigm by suggesting the concept of a segmented mirror whose shape was controlled by a computer. Around the same time it was also realized that very large thin mirrors with low thermal inertia could be used if computer-controlled force-actuators maintained their shape throughout the night. Consequently, optical telescopes have now reached gigantic proportions with diameters around 10 m (^394 inches) for the twin telescopes of the W. M. Keck Observatory (WMKO) which began operations in 1993 and 1996, respectively. Moreover, there are now telescopes, both on the ground and in space, to cover far more than the visible light our human eyes are designed to see. Today, computers actively control the shape of optical surfaces in the telescope and in associated instruments, performing thousands of calculations per second to correct the image quality. Smaller, highly automated telescopes survey the entire sky to unprecedented depths and many of these images are immediately available in digital form to all astronomers. This flood of quantitative information is due to strides in the range and sensitivity of electronic detection devices. It is the impact of semiconductor electronic light-sensors attached to the new generation of telescopes (both on the ground and in space) which has had an effect as dramatic as the introduction of the photographic plate itself over one hundred years ago.

There can be little doubt that we are living in a time of rapid technology development. This is the Digital Age, the age of the "microchip". Semiconductor technology, of which the "silicon chip" found in computers is by far the most widely known example, has touched almost every aspect of our daily lives. The mass production of silicon chips has brought Personal Computers (PCs) of incredible power, at relatively low cost, to almost every environment: homes, schools, offices, and industry. The Digital Age is also the age of global electronic communication. There can be few people left who haven't at least heard of the Internet and the World Wide Web! School kids can "download" images from the Hubble Space Telescope web site and "email" messages and pictures to friends half-way around the world almost instantaneously by typing at a computer keyboard.

What is a semiconductor? A semiconductor is a crystalline material with some of the properties of a good conductor of electricity (like copper metal), and some of the properties of an electrical insulator (like glass, for example). Because of its crystalline (solid-state) structure, a slab of such material behaves the same at all points. Semiconductor crystals can be "grown" in a controlled way from a melt, and moreover, the electrical properties can be tailored by introducing so-called impurity atoms into the crystal structure at the atomic level, so that by microscopic sculpting of the semiconductor material, all sorts of tiny electrical components and circuits can be constructed. The final piece, often not much larger than a thumbnail, is referred to as an "integrated circuit'' or more commonly, as a "chip". Besides silicon, there is germanium, gallium arsenide, indium antimonide, and several other materials with these properties. Semiconductors can be used to manufacture a host of low-power micro-electronic components including amplifiers, all sorts of logic units, computer memory, very complex chips called microprocessors capable of many computational functions, and tiny imaging devices of remarkable sensitivity. Silicon is the most well-developed semiconductor so far, but even for silicon the potential for yet smaller and smaller microchips still exists. Astronomy has benefited in this semiconductor revolution because the apparatus needed for scientific experiments and for complex calculations, which were completely impossible before, are now viable with the aid of the latest electronic imaging devices and powerful high-speed electronic computers.

Almost all modern astronomical research is carried out with photo-electronic equipment, by which we mean instrumentation that converts radiant energy (such as light) into electrical signals which can be digitized; that is, converted into numerical form for immediate storage and manipulation in a computer. Usually highly automated and remotely controlled, these instruments, and telescopes to which they are attached, rely heavily on electronics and computers. Computers play an equally crucial role in helping astronomers assimilate, analyze, model, and archive the prodigious quantity of data from the new instruments. The ongoing miniaturization of computers and the ever-increasing availability of large amounts of relatively cheap computer memory means that astronomers can employ fairly complex electronic and computer systems at the telescope which speed up and automate data-gathering. As a result, those astronomical facilities, which may be costly initially, and the data they produce can be available to a much wider range of scientists than would otherwise be possible. Today, a large modern observatory requires an enormous breadth of engineering, scientific, and managerial skills to operate efficiently and produce the very best results.

Many readers will be familiar with sources of current and topical astronomical results, whether these are professional journals (e.g., Nature, the Astrophysical Journal) or popular magazines (e.g., Sky & Telescope) or any of the numerous astronomical sites accessible on the World Wide Web. How are such remarkable observations obtained? Most press releases do not describe in detail the apparatus or the technology used in making the discovery. Of course, it would not be easy to do so because of the "jargon barrier" and the complexity of the technology itself. This is unfortunate, because it underemphasizes an important link between modern technology and the quest for fundamental knowledge embodied in astronomy, a search for answers to the most basic questions about our universe. Our theme throughout this book is to emphasize this link.

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