(Background above) Called the "family portrait" of all the many devices aboard the Hubble Space Telescope, this semi-transparent illustration shows some of the main features of this spaceborne observatory. The large blue disk at center of the spacecraft is the primary mirror, and the red gadgets to its rear are the sensors that guide the pointing of the telescope. The open aperture door is at upper left. The huge solar panels are shown in yellow-checkered blue at left and partly obscured at right. Looking inside the aft bay of the vehicle, we can see key components of each of the science instruments--the spectrometers are shown in copper and blue (in the foreground), the cameras in pink, green and lavender (mostly in the background). (Insets A, B, C, and D) The four small insets are computer-rendered views of Hubble in orbit. Despite having the size of a city bus, Hubble is designed to move in space with the grace of a prima ballerina. All these illustrations are taken from video animations made by the astronomy artist and animator Dana Berry.
Studying this chapter will enable you to:
Sketch and describe the basic designs of the major types of optical telescopes used by astronomers.
Explain why very large telescopes are needed for most astronomical study, and specify the particular advantages of reflecting telescopes for astronomical use.
Describe how the Earth's atmosphere affects astronomical observations, and discuss some of the current efforts to improve ground-based astronomy.
Discuss the specific advantages and disadvantages of radio astronomy.
Explain how interferometry can enhance the usefulness of radio observations.
List the other types of nonvisible radiation currently being exploited for astronomical observation and summarize the advantages, limitations, and chief uses of each.
Say why it is important to make astronomical observations in different regions of the electromagnetic spectrum.
Observational knowledge of the cosmos normally advances in three phases: First, astronomers collect and measure radiation from space, using a device known as a telescope. Second, they store the resulting data for future use, usually on photographic film or in digital form. Finally, they analyze and interpret the data. The laws of physics are applied, and a model, or theory, that explains the data is developed and tested. Theoretical work plays a vital role in this process and often suggests what new data need to be collected. At its heart, however, astronomy is an observational science. More often than not, observations of cosmic phenomena precede any clear theoretical understanding of their nature. Our detecting instruments--our telescopes--have evolved to observe as broad a range of wavelengths as possible. Until the middle of the twentieth century, telescopes were limited to visible light. Since then, however, technological advances have broadened our view of the universe to all regions of the electromagnetic spectrum. Whatever the details of its design, a telescope is a device whose basic purpose is to collect electromagnetic radiation and deliver it to a detector for detailed study.
Back In essence, a telescope is a "light bucket," whose primary function is to capture as many photons as possible from a given region of the sky and then to concentrate them into a focused beam for analysis. An optical telescope is one designed specifically to collect the wavelengths that are visible to the human eye. Optical telescopes have a long history, reaching back to the days of Galileo in the early seventeenth century. They are probably also the best-known type of telescope, so it is fitting that we begin our study of astronomical hardware with these devices.
Modern astronomical telescopes have evolved a long way from Galileo's simple apparatus. Their development over the years has seen a steady increase in size for one simple, but very important, reason: Large telescopes can gather and focus more radiation than can their smaller counterparts, allowing astronomers to study fainter objects and to obtain more detailed information about bright ones. This fact has played a central role in determining the design of contemporary instruments.
Optical telescopes fall into two basic categories--reflectors and refractors. Figure 5.1(a) below shows how a reflecting telescope uses a carefully shaped, curved mirror to gather and concentrate a beam of light. The mirror is constructed so that all light rays arriving parallel to its axis, regardless of their distance from that axis, are reflected to pass through a single point, called the prime focus. The distance between the center of the mirror and the prime focus is the focal length.
Figure 5.1 (a) A curved mirror can be used to focus rays of light parallel to the axis to a point. Light rays traveling along the axis of the mirror are simply reflected back along the axis. Off-axis rays are reflected through greater and greater angles the farther they are from the axis, so that they all pass through the same focal point. (b) A lens can be thought of as a series of prisms. A light ray traveling along the axis of the lens is undeflected as it passes through the lens. Rays arriving at progressively greater distances from the axis are deflected by increasing amounts, so all rays parallel to the axis are focused to a point.
A refracting telescope uses a lens to focus the incoming light. Refraction is simply the bending of a beam of light as it passes from one transparent medium (for example, air) into another (such as glass). Consider how a stick in water looks bent. The stick itself remains straight, of course, but the light by which we see it is bent--refracted--as it leaves the water and enters the air. As illustrated in Figure 5.1(b), we can think of a lens as a series of prisms, chosen in such a way that all rays of light striking the lens parallel to the axis are refracted to pass through the focus.
Astronomical telescopes are often used to make images of their field of view. Figure 5.2 illustrates how this is accomplished, in this case by the mirror in a reflecting telescope. (Lenses also form images; however, since all of the world's large optical telescopes are reflectors, we will concentrate on mirrors here.) Light from a distant object (such as a star) reaches us as parallel, or very nearly parallel, rays. Any ray of light entering the instrument parallel to the telescope's axis strikes the mirror and is reflected through the prime focus. Light coming from a slightly different direction--inclined slightly to the axis--is focused to a slightly different point. In this way, an image is formed near the prime focus. Each point on the image corresponds to a different point in the field of view.
Figure 5.2 Formation of an image by a mirror. Rays of light coming from different points on a distant object are focused to slightly different locations. The result is that an image of the object is formed around the prime focus. Notice that the image is inverted (that is, upside down).
The prime-focus images produced by large telescopes are actually quite small--the image of the entire field of view may be as little as 1 centimeter across. Often, the image is magnified with a lens known as an eyepiece before being observed by eye or, more likely, recorded as a photograph or digital image. Figure 5.3(a) shows the basic design of a simple reflecting telescope, illustrating how a small secondary mirror and eyepiece are used to view the image. Figure 5.3(b) shows how a refracting telescope accomplishes the same function. In principle, there is no limit to the magnification that can be achieved just by using more and more powerful eyepieces. However, as we will see in a moment, there are important practical restrictions on how much detail can be extracted in this way.
Figure 5.3 Comparison of (a) reflecting and (b) refracting telescope systems. Both types are used to gather and focus cosmic radiation--to be observed by human eyes or recorded on photographs or in computers. In either case, the image formed at the focus is viewed with a small magnifying lens called an eyepiece.
These two telescope designs--reflecting and refracting--achieve the same basic result: A beam of light, initially close to the axis of the instrument, is focused to form an image. On the face of it, then, it might appear that there is little reason to prefer either mirrors or lenses in telescope construction. However, there are some important factors to consider when deciding which type to buy or build:
Figure 5.4 Chromatic aberration. A prism bends blue light more than it bends red light, so the blue component of light passing through a lens is focused slightly closer to the lens than is the red component. As a result, the image of an object acquires a colored "halo," no matter where we place our detector.
For these reasons, all large modern telescopes use mirrors as their primary light gatherers. Figure 5.5 shows the world's largest refractor, installed in 1897 at the Yerkes Observatory in Wisconsin and still in use today. It has a lens diameter of 1 m (about 40 inches). By contrast, some new reflecting telescopes have mirror diameters in the 10 m range, and larger instruments are on the way.
Figure 5.5 Photograph of the Yerkes Observatory's 1 m diameter refracting telescope.
Figure 5.6 presents a diagram of some basic reflecting telescope designs. Radiation from a star enters the instrument, passes down through the main telescope tube, strikes the curved surface of the primary mirror, and is reflected back toward the prime focus, near the top of the tube. Sometimes astronomers simply place their instruments right at the prime focus, as in Figure 5.6(a). However, it can be very inconvenient, or even impossible, to suspend bulky pieces of equipment there. More often, the light is intercepted on its path to the focus by a smaller secondary mirror and then redirected to a more convenient location. Three such arrangements are shown in Figure 5.6(b) through (d).
Figure 5.6 The essential features of the apparatus used to collect, focus, and record information from cosmic objects. Shown here are four different reflecting telescope designs: (a) prime focus, (b) Newtonian focus, (c) Cassegrain focus, and (d) coudé focus. Each uses a primary mirror at the bottom of the telescope to capture radiation, which is then directed along different paths for analysis. Notice that the secondary mirrors shown in (c) and (d) are actually slightly diverging, moving the focus outside the telescope.
In a Newtonian telescope (named after Sir Isaac Newton, who invented this particular design), the light is intercepted before it reaches the prime focus and is deflected by 90°, usually to an eyepiece at the side of the instrument. This is a particularly popular design for smaller reflecting telescopes, such as those used by amateur astronomers.
Alternatively, astronomers may choose to work on a rear platform where they can use equipment, such as a spectroscope, that is too heavy to hoist to the prime focus. In this case, light reflected by the primary mirror toward the prime focus is intercepted by a smaller secondary mirror, which reflects it back down through a small hole at the center of the primary mirror. This arrangement is known as a Cassegrain telescope (after Guillaume Cassegrain, a French lensmaker), and the point behind the primary mirror where the light from the star finally converges is called the Cassegrain focus.
Another, more complex, observational configuration requires starlight to be reflected by several mirrors. As in the Cassegrain design, light is first reflected by the primary mirror toward the prime focus and reflected back down the tube by a secondary mirror. A third, much smaller mirror then deflects the light into an environmentally controlled laboratory. Known as the coudé room (from the French word for "bent"), this laboratory is separate from the telescope itself, enabling astronomers to use very heavy and finely tuned equipment that could not possibly be lifted to either the prime focus or to an elevated platform at the rear of the telescope. The light path to the coudé focus lies along the axis of the telescope's mount--that is, the axis around which the telescope rotates as it tracks objects across the sky--so that it does not change as the telescope moves.
To illustrate some of the points we have discussed, let us briefly consider an instrument that has been at or near the forefront of astronomical research for much of the last half-century. Figure 5.7 depicts the Hale 5 m (200 inch) diameter optical telescope on California's Palomar Mountain, dedicated in 1948. As the size of the figure in the observer's cage at the prime focus indicates, this is indeed a very large telescope. In fact, for almost three decades the Hale telescope was the largest in the world. Observations can be made at the prime, the Cassegrain, or the coudé focus, depending on the needs of the user. The coudé room itself is out of the picture, to the lower right.
Figure 5.7 (a) An artist's illustration of the 5 m diameter Hale optical telescope on Palomar Mountain in California. (b) A photograph of the telescope. (c) Astronomer Edwin Hubble in the observer's cage at the Hale prime focus.
Large reflectors are good at forming images of narrow fields of view, where all the light that strikes the mirror surface moves almost parallel to the axis of the instrument. However, if the light enters at an appreciable angle, it cannot be accurately focused, degrading the overall quality of the image. The effect (called coma) worsens as we move farther from the center of the field of view. Eventually, the image quality is reduced to the point where it is no longer usable. The distance from the center to where the image becomes unacceptable defines the useful field of view of the telescope--typically, only a few arc minutes for large instruments.
A design that overcomes this problem is the Schmidt telescope, named after its inventor, Bernhard Schmidt, who built the first such instrument in the 1930s. The telescope uses a correcting lens, which sharpens the final image of the entire field of view. Consequently, a Schmidt telescope is well suited to producing wide-angle photographs, covering several degrees of the sky. Because the design of the Schmidt telescope results in a curved image that is not suitable for viewing with an eyepiece, the image is recorded on a specially shaped piece of photographic film. For this reason, the instrument is often called a Schmidt camera (Figure 5.8). The Palomar Observatory Schmidt camera, one of the largest in the world (with a 1.8 m mirror and a 1.2 m lens), performed a survey of the entire northern sky in the 1950s. The Palomar Observatory Sky Survey, as it is known, has for decades been an invaluable research tool for professional observers. A second, digital survey is now nearing completion.
Figure 5.8 The Schmidt camera of the European Southern Observatory in Chile.
When a photographic plate is placed at the focus to record an image of the field of view, the telescope is acting in effect as a high-powered camera. However, this is by no means the only light-sensitive device that can be placed at the focus to analyze the radiation received from space. When very accurate and rapid measurements of light intensity are required, a device known as a photometer is used. A photometer measures the total amount of light received in all or part of the image. When only part of the image is under study, the region of interest is selected simply by masking out the rest of the field of view. Using a photometer often means "throwing away" spatial detail, but in return more information is obtained about the intensity and time variability of a particular source, such as a pulsating star or a supernova explosion.
Often, astronomers want to study the spectrum of the incoming light. Large spectrometers frequently work in tandem with optical telescopes. Light radiation collected by the primary mirror may be redirected to the underground coudé room, defined by a narrow slit, passed through a prism, and projected onto a screen--a process not so different from the operation of the simple spectroscope described in Chapter 4. The spectrum can be studied in real time (that is, as it happens) or stored on a photographic plate (or, more commonly nowadays, on a computer disk) for later analysis.
Back As we have already noted, astronomers generally prefer large telescopes over small ones. This preference has to do both with the amount of light the telescopes collect and with the amount of detail that can be seen.
For optical work, the main reason for using a larger telescope is simply that it has a greater collecting area--the total area of a telescope capable of capturing radiation. The larger the telescope's reflecting mirror or refracting lens, the more light it collects, and the easier it is to measure and study an object's radiative properties. Astronomers spend a large fraction of their time observing very distant--and hence, by the inverse-square law, very faint--cosmic sources. If we wish to make detailed observations of objects far from our own cosmic neighborhood, very large telescopes are essential tools. Figure 5.9 below illustrates the effect of increasing telescope size by comparing images of the Andromeda Galaxy taken with different instruments.
Figure 5.9 Effect of increasing telescope size on an image of the Andromeda Galaxy. Both photographs had the same exposure time; the bottom image was taken with a telescope twice the size of that used to make the top image. Fainter detail can be seen as the diameter of the telescope mirror increases because larger telescopes are able to collect more photons per unit time.
The observed brightness of an astronomical object is directly proportional to the area of our telescope's mirror, and therefore to the square of the mirror diameter. Thus, a 5 m telescope will produce an image 25 times as bright as a 1 m instrument because a 5 m mirror has 52 = 25 times the collecting area of a 1 m mirror. We can also think of this relationship in terms of the length of time required for a telescope to collect enough energy to create a recognizable image on a photographic plate. Our 5 m telescope will produce an image 25 times faster than the 1 m device because it gathers energy at a rate 25 times greater. Expressed in another way, a 1-hour time exposure with a 1-m telescope is roughly equivalent to a 2.4-minute time exposure with a 5 m instrument.
A second advantage of large telescopes is their resolution. Angular resolution refers to the ability to distinguish between two adjacent objects in the sky. The finer the angular resolution, the better we can make such a distinction, and the better we can see the details of any given object. Figure 5.10 illustrates how the appearance of two nearby objects might change as the angular resolution varies.
Figure 5.10 Two comparably bright light sources become progressively clearer when viewed at finer and finer angular resolution. When the angular resolution is much poorer than the separation of the objects, as at the top, the objects appear as a single fuzzy "blob." As the resolution improves, the two sources become discernible as separate objects.
What limits a telescope's resolution? One important factor is diffraction, the tendency of light, and all other waves for that matter, to bend around corners. As light enters the telescope, the rays are bent slightly, and this bending makes it impossible to focus the light to a sharp point, even with a perfectly constructed mirror. The angle through which the beam bends is proportional to the wavelength of the radiation divided by the width of the opening (in this case, the diameter of the mirror). In convenient units,
angular resolution (arc sec) = 0.25 ,
where 1 (micron) = 10-6 m. This angle determines the angular resolution of the telescope.
Thus, for a given telescope size, the amount of diffraction increases in proportion to the wavelength used, so that observations in the infrared or radio range are often limited by its effects. For example, in an otherwise perfect observing environment, the best possible angular resolution of blue light (with a wavelength of 400 nm) that could be obtained using a 1-m telescope observing would be about 0.1´´. This quantity is known as the diffraction-limited resolution of the telescope. But if we were to use our 1-m telescope to make observations in the near infrared range, at a wavelength of 10 microns, the best resolution we could obtain would be only 2.5´´.
A 1 m radio telescope operating at a wavelength of 1 cm would have an angular resolution of slightly under 1°.
Similarly, for light of any given wavelength, large telescopes produce less diffraction than small ones. A 5 m telescope observing in blue light would have a diffraction-limited resolution five times finer than the 1-m telescope just discussed--about 0.02´´. A 0.1 m (10 cm) telescope would have a diffraction limit of 1´´ and so on. For comparison, the angular resolution of the human eye is about 0.5´.
Figure 5.11 shows how the Andromeda Galaxy would appear in greater detail with progressively higher resolution, when viewed in visible light through a hypothetical series of telescopes. In fact, as we will see in a moment, no large ground-based telescope actually comes close to its diffraction limit, because of the blurring effects of Earth's atmosphere.
Figure 5.11 Detail becomes clearer in the Andromeda Galaxy as the angular resolution is improved some 600 times, from (a) 10´, to (b) 1´, (c) 5´´, and (d) 1´´.
We have seen that in the push toward larger and larger telescopes, refractors were abandoned long ago, in large part because of the difficulty in constructing them. But, while large refracting telescopes are certainly hard to build, large reflectors are not exactly easy. Conventional telescope mirrors are made from large blocks of quartz, glass, or some other type of polishable material capable of withstanding large temperature changes with little expansion or contraction. Workers begin the construction process by pouring the molten material into a large cast, then cooling it slowly over the course of several years to keep it from cracking or developing internal stresses while it changes from liquid to solid. It takes years more to grind and polish the surface to the required curvature. Finally, the surface is coated with a thin film of aluminum to provide a reflecting surface.
All these stages are slow, painstaking, and, unfortunately, not always successful. Engineers encounter
severe difficulties in building very large telescopes by these means. Indeed, since the construction of the 5
m Palomar instrument in 1948, only one larger single-mirror telescope (a
6 m instrument in Russia) has been completed; this telescope, regrettably, suffers from several optical defects and does not focus light well, resulting in poor-quality images.
Until the 1980s, the conventional wisdom was that telescopes with mirrors larger than 5 or 6 m in diameter were simply too expensive and impractical to build. However, new manufacturing techniques, coupled with radically new mirror designs, now make the construction of telescopes in the 8- to 12-m range almost a routine matter. Experts can now make large mirrors much lighter for their size than had previously been believed feasible, and can combine many smaller mirrors into the equivalent of a much larger single-mirror telescope. Several large-diameter instruments are now under construction, and many more are planned.
The California Institute of Technology and the University of California are cooperating to build two 10 m telescopes atop Mauna Kea, in Hawaii (see Figure 5.26 later in this chapter). These telescopes, known as the Keck telescopes, and shown in Figure 5.12, employ a segmented design, each combining 36 1.8 m six-sided mirrors into the equivalent area of a single 10 m reflector. The first Keck telescope saw "first light" in 1990. It became fully operational in 1992.
Figure 5.12 (a) Photograph of the twin Keck telescopes, one still under construction as of mid-1995. (b) A bird's-eye view of the segmented mirror inside the completed dome. (c) A closer view of the mirror. Note the technician in orange coveralls at the center. Taking the Deepest View of the Heavens
Pushing the Envelope toward Better Optics
Back In recent years, new technologies have made it possible to improve both light collection and image formation. Telescopes are being moved out beyond much of the atmosphere, and computers are playing an increasingly important role in astronomy.
Even large telescopes have their limitations. For example, consider again the 5 m optical telescope on Palomar Mountain. According to our earlier discussion of diffraction, this telescope should achieve an angular resolution of around 0.02´´.
In practice, it cannot exceed 1´´.
In fact, apart from instruments using special techniques developed to examine some bright stars, no ground-based optical telescope built before 1990 can presently resolve astronomical objects to much better than an arc second. Why?
As we observe a star, atmospheric turbulence produces continual small changes in the optical properties of the air between the star and our telescope. The light from the star is refracted slightly, and the stellar image dances around on the detector (or on our retina). This continual deflection is the cause of the well-known "twinkling" of stars. It occurs for the same reason that objects appear to shimmer when viewed across a hot roadway on a summer day.
On a good night at the best observing sites, the maximum amount of deflection produced by the atmosphere is slightly less than 1´´. Consider taking a photograph of a star. After a few minutes' exposure time (long enough for the intervening atmosphere to have undergone many small, random changes), the image of the star has been smeared out over a roughly circular region an arc second or so in diameter. Astronomers use the term seeing to describe the effects of atmospheric turbulence. The circle over which a star's light (or the light from any other astronomical source) is spread is called the seeing disk. Figure 5.13 illustrates the formation of the seeing disk for a small telescope.*
*In fact, for a large instrument--more than about 1 m in diameter--the situation is more complicated, because rays striking different parts of the mirror have actually passed through different turbulent atmospheric regions. The end result is still a seeing disk, however.
Figure 5.13 Individual photons from a distant star strike the detector in a telescope at slightly different locations because of turbulence in the Earth's atmosphere. Over time, the individual photons cover a roughly circular region on the detector, and even the pointlike image of a star is recorded as a small disk, called the seeing disk.
To achieve the best possible seeing, telescopes are sited on mountaintops (to get above as much of the atmosphere as possible) in regions of the world where the atmosphere is known to be fairly stable and relatively free of dust, moisture, and light pollution from cities. In the continental United States, these sites tend to be in the desert Southwest. The U.S. National Observatory for optical astronomy in the Northern Hemisphere, completed in 1973, is located high on Kitt Peak near Tucson, Arizona. The site was chosen because of its many dry, clear nights. Seeing of 1´´ from such a location is regarded as good, and seeing of a few arc seconds is tolerable for many purposes. Even better conditions are found on Mauna Kea, Hawaii, and at Cerro Tololo and La Silla in the Andes Mountains of Chile (Figure 5.14)--which is why many large telescopes have recently been located at those two exceptionally clear locations.
Figure 5.14 Located in the Andes Mountains of Chile, the European Southern Observatory at La Silla is run by a consortium of European nations. Numerous domes house optical telescopes of different sizes, each with varied support equipment, making this one of the most versatile observatories south of the equator.
An optical telescope placed in orbit about the Earth or on the Moon could obviously overcome the limitations imposed by the atmosphere on ground-based instruments. Without atmospheric blurring, extremely fine resolution--close to the diffraction limit--can be achieved, subject only to the engineering restrictions of building or placing large structures in space. The Hubble Space Telescope (HST) (named for one of America's most notable astronomers, Edwin Hubble) was launched into the Earth's orbit by NASA's space shuttle Discovery in 1990. This telescope has a 2.4 m mirror, with a diffraction limit of only 0.05´´. Thus, this orbiting observatory can give us a view of the universe as much as 20 times sharper than is normally available from even the much larger ground-based instruments. (See Interlude 5-1 below)
Most large telescopes today are controlled by computers or by operators who rely heavily on their assistance, and the images themselves are recorded in a form that can be easily read and manipulated by computer programs. It is becoming fairly rare for photographic equipment to be used as the primary means of data acquisition at large observatories. Rather, electronic detectors known as charge-coupled devices, or CCDs, are now in widespread use. Their output goes directly to a computer.
A CCD (see Figure 5.15) consists of a wafer of silicon divided into a two-dimensional array of many tiny picture elements, known as pixels. When light strikes a pixel, an electric charge builds up on the device. The amount of charge is directly proportional to the number of photons striking each pixel--in other words, to the intensity of the light at that point. The charge buildup is monitored electronically, so that a two-dimensional image can be obtained. The entire device is typically only a few square centimeters in area and may contain several million pixels, generally arranged on a square grid. As the technology improves, both the areas of CCDs and the number of pixels they contain are steadily increasing. Incidentally, the technology is not limited to astronomy--many home video cameras contain CCD chips quite similar in basic design to those in use at the great astronomical observatories of the world.
Figure 5.15 A charge-coupled device consists of hundreds of thousands, or even millions, of tiny light-sensitive cells, or pixels, usually arranged in a square array. Light striking a pixel causes an electrical charge to build up on it. By electronically reading out the charge on each pixel, a computer can reconstruct the pattern of light--the image--falling on the chip. Photograph (a) is a detail of a CCD array; (b) shows a CCD chip mounted for use at the focus of a telescope.
Charge-coupled devices have many advantages over photographic plates, which were the staple of astronomers for over a century. CCDs are much more efficient than photographs. They record as many as 75 percent of the photons striking them, while photographic methods record less than 5 percent. This fact alone means that a CCD image can show objects ten to twenty times fainter as can a photograph taken using the same telescope and the same exposure time. Alternatively, CCDs can record the same level of detail in less than a tenth of the time required by photographs, or record that detail with a much smaller telescope. CCDs also produce a faithful representation of an image in a digital format that can be stored on magnetic tape, or disk, or even sent directly across a computer network to an observer's home institution for detailed study.
With the aid of high-speed computers, the background noise found in the "raw" image from a telescope can be greatly reduced, allowing astronomers to see features that would otherwise remain hidden. Noise is anything that corrupts the integrity of a message, such as static on an AM radio or "snow" on a television screen. The noise corrupting telescopic images has many causes. In part, it results from faint, unresolved sources in the telescope's field of view and from light scattered into the line of sight by the Earth's atmosphere. It can also be caused by electronic "hiss" within the detector itself. Whatever the origin of noise, its characteristics can be determined (for example, by observing a part of the sky where there are no known sources of radiation), and then the noise can be partially removed.
Using computer processing, astronomers can also compensate for known instrumental defects and even correct some effects of bad seeing. In addition, the computer can often carry out many of the relatively simple, but tedious and time-consuming, chores that must be performed before an image (or spectrum) reaches its final "clean" form. Figures 5.16(b) and (c) illustrate how computerized image-processing techniques were used to correct for known instrumental problems in the Hubble Space Telescope, allowing much of the planned resolution of the telescope to be recovered even before its repair in 1993. For comparison, Figure 5.16(a) shows the best image obtainable from the ground.
Figure 5.16 (a) A ground-based view of the star cluster R136, a group of stars in the Large Magellenic Cloud (a nearby galaxy). (b) The "raw" image of this same region as seen by the Hubble Space Telescope in 1990, before the repair mission. (c) The same image after computer processing that partly compensated for imperfections in the mirror. (d) The same region as seen by the repaired HST in 1994.
An exciting development, promising to bring about striking improvements in the resolution of ground-based optical telescopes, takes these ideas of computer control and image processing one stage farther. If an image could be analyzed while the light was still being collected (a process that can take many minutes, or even hours, in some cases), it might be possible to adjust the telescope from moment to moment to correct for the effects of mirror distortion, temperature changes, and bad seeing. Perhaps the telescope could come close to the theoretical (diffraction-limited) resolution. Some of these techniques, collectively known as active optics, are already in use in the New Technology Telescope (NTT), located at the European Southern Observatory in Chile (the most prominent instrument in Figure 5.14). This 3.5 m instrument, employing the latest in real-time telescope controls, achieves resolution of about 0.5´´ by making minute modifications to the tilt of the mirror as its temperature and orientation change, thus maintaining the best possible focus at all times. From its very first observing run, NTT became the highest-resolution optical telescope on Earth (see Figure 5.17a). The Keck 10-m instruments, one of whose mirrors is shown in Figure 5.17b, also employ these methods, and may ultimately achieve resolution as fine as 0.25´´.
Figure 5.17 (a) These false-color infrared photographs of part of the star cluster R136--the same object shown in Figure 5.16 above--contrast the resolution obtained without the active optics system (left image) with that achievable when the active optics system is in use (right image). (b) A hexagonal mirror segment destined for one of the Keck telescopes undergoes shaping and polishing. The unusually thin glass will be backed by push-pull pistons that can adjust the precise configuration of the segment during observations so as to attain improved resolution.
An even more ambitious undertaking is known as adaptive optics. This technique actually deforms the shape of the mirror's surface, under computer control, while the image is being exposed. The intent is to undo the effects of atmospheric turbulence. In the experimental system shown in Figure 5.18(a), lasers probe the atmosphere above the telescope, returning information about the air's swirling motion to a computer that modifies the mirror thousands of times per second to compensate for poor seeing. Adaptive optics presents formidable theoretical and technological problems, but the rewards are so great that they are presently the subject of intense research. Recently declassified SDI ("Star Wars") technology has provided an enormous boost to this effort. Already, impressive improvements in image quality have been obtained (see Figure 5.18b). In the next decade, it may well be possible to have the "best of both worlds" by achieving with a large ground-based telescope the kind of resolution presently attainable only from space.
Figure 5.18 (a) Until mid-1991, the Starfire Optical Range at Kirtland Air Force Base in New Mexico was one of the U.S. Air Force's most closely guarded secrets. Here, beams of laser light probe the atmosphere above the system's 1.5 m telescope, allowing minute computer-controlled changes to be made to the mirror surface thousands of times each second. (b) The improvement in seeing produced by such systems can be dramatic, as can be seen in these images acquired at another military observatory atop Mt. Haleakala in Maui, Hawaii, employing similar technology. The uncompensated image (left) of the bright star Procyon is a blur spread over several arc seconds. With adaptive compensation applied (right), the resolution is improved to a mere 0.2 arc second.
Back Optical telescopes are limited to those wavelengths that the human eye can detect. However, information also comes to us at other wavelengths. In addition to the visible radiation that normally penetrates the Earth's atmosphere on a clear day, radio radiation also reaches the ground. In fact, the radio window in the electromagnetic spectrum is much wider than the optical window, as indicated in Figure 3-9. As the atmosphere is no hindrance to long-wavelength radiation, radio astronomers have built many ground-based radio telescopes capable of detecting cosmic radio waves. These devices have all been constructed since the 1950s--radio astronomy is a much younger subject than optical astronomy.
The field originated with the work of Karl Jansky at Bell Labs in 1931, but only after the technological push of World War II did it grow into a distinct branch of astronomy. Jansky was engaged in a study of shortwave radio interference when he discovered a faint static "hiss" that had no apparent terrestrial source. He noticed that the strength of the hiss varied in time and that its peak occurred about 4 minutes earlier each day. He soon realized that the peaks were coming exactly one sidereal day apart, and correctly inferred that the hiss was not of terrestrial origin but came from a definite direction in space. That direction is now known to correspond to the center of our Galaxy. It took over a decade, and the realization by astronomers that interstellar gas could actually be observed at radio wavelengths, for the full importance of his work to be appreciated, but today Jansky is widely regarded as the father of radio astronomy.
Figure 5.19 shows a fairly typical radio telescope, the large 43 m (140 foot) diameter telescope located at the National Radio Astronomy Observatory in West Virginia.
Figure 5.19 The 43-m-diameter radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia.
Although much larger than any reflecting optical telescope, most radio telescopes are built in basically the same way. They have a large horseshoe-shaped mount that supports the large, curved metal "dish," or mirror. The collecting area captures cosmic radio waves and reflects them to the focus, where a receiver detects the signals and channels them to the computer. The signals may be partially analyzed before being stored digitally for later processing and full analysis. Unlike optical instruments, which can simultaneously detect all visible wavelengths, radio detectors normally register only a narrow band of wavelengths at any one time. To observe radiation at another radio frequency, we must retune the equipment, much as we tune a television set to a different channel.
Large radio telescopes are very sensitive instruments and can detect even very faint radio sources. Indeed, the total amount of radio energy detected by all the radio telescopes on Earth since the first receiver was built would barely be enough to keep a 100-W light bulb burning for just 10 billionths of a second. However, their angular resolution is generally poor compared to their optical counterparts. The major disadvantage of all radio telescopes is their relatively low resolving power, despite the enormous size of many radio dishes. It is not our atmosphere that is to blame--the radio wavelengths normally studied pass through air without any significant distortion. The problem is that the typical wavelengths of radio waves are about a million times longer than those of visible light, and these longer wavelengths impose a corresponding crudeness in angular resolution because of the effects of diffraction. Recall from Section 5.2 that the longer the wavelength, the greater the diffraction.
The best angular resolution obtainable with a single radio telescope is about 10´´ (for the largest instruments operating at millimeter wavelengths)--at least 10 times coarser than the capabilities of the largest optical mirrors. The resolution varies widely, depending on the wavelength being observed. The 43 m radio telescope shown in Figure 5.19 can achieve resolution of about 1´ when receiving radio waves having wavelengths of around 1 cm. However, it was designed to operate most efficiently (that is, it is most sensitive to radio signals) at wavelengths closer to 5 cm, where the resolution is only about 6´, or 0.1°.
Radio telescopes are large not only because that is the only way they can achieve good resolution, but also because the total amount of energy arriving at the Earth in the form of radio radiation is extremely small--less than a trillionth of a watt spread over the entire surface of our planet. Radio telescopes can be built so much larger than their optical counterparts because their reflecting surface need not be as smooth as is needed for shorter-wavelength light waves. Provided that surface irregularities (dents, bumps, and the like) are much smaller than the wavelength of the waves to be detected, the surface will reflect them without distortion. Because the wavelength of visible radiation is short (approximately 10-6 m), very smooth mirrors are needed to reflect the waves properly, and it is difficult to construct very large mirrors to such exacting tolerances.
For example, when visible light shines on a curved surface having irregularities of even a fraction of a millimeter, the light scatters and does not focus well; the image of the light source is severely distorted. You could test this statement by looking at your own blurred reflection in a piece of unpolished metal. But radio waves of a centimeter or longer wavelength are not scattered at all by slightly rough surfaces. Instead, they are reflected to an accurate focus. Very long radio waves of, say, l m wavelength can reflect perfectly well from surfaces having irregularities even as large as your fist. The situation is somewhat analogous to trying to bounce a ball off an irregular surface. If the surface irregularities are much smaller than the radius of the ball (the wavelength of the photon, in this analogy), the bounce will be true, and the ball will travel in the intended direction (that is, the photon will be reflected toward a focal point). However, if the irregularities are comparable in size to the radius of the ball, the bounce will be unpredictable and erratic (that is, the focus will be blurred).
Figure 5.20 shows the world's largest radio telescope, located in Arecibo, Puerto Rico. Approximately 300 m (1000 feet) in diameter, the surface of the Arecibo telescope spans nearly 20 acres. Constructed in 1963 in a natural depression in the hillside, the dish was originally surfaced with chicken wire, which was lightweight and cheap. Although fairly rough, the chicken wire was adequate for proper reflection because the openings between adjacent strands of wire were much smaller than the long-wavelength radio waves to be detected. The entire Arecibo dish was resurfaced in 1974 with thin metal plates, so it can now be used to study shorter-wavelength radio radiation. Even so, useful observations are still restricted to radiation of wavelength greater than about 10 cm, making the telescope's angular resolution no better than that of some smaller radio telescopes, despite its enormous size. The huge size of the dish, in fact, creates one distinct disadvantage: The Arecibo telescope cannot be pointed very well to follow cosmic objects across the sky. The dish is literally strung among several limestone hills, restricting its observations to those objects that happen to pass within about 20° of overhead.
Figure 5.20 An aerial photograph of the 300 m diameter dish at the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico. The receivers that detect the focused radiation are suspended nearly 300 m (about 80 stories) above the center of the dish. One insert shows a close-up of the radio receivers hanging high above the dish. The other insert shows technicians adjusting the surface of the dish.
Arecibo is an example of a roughly surfaced telescope capable of detecting long-wavelength radio radiation. At the other extreme, Figure 5.21 shows the 36 m diameter Haystack dish in northeastern Massachusetts. It is constructed of polished aluminum and maintains a parabolic curve to an accuracy of about a millimeter all the way across its solid surface. It can reflect and accurately focus radio radiation with a wavelength as short as a few millimeters. The telescope is contained within the protective shell, or radome, which protects the surface from the harsh wind and weather of New England. It acts much like the protective dome of an optical telescope, except that there is no slit through which the telescope "sees." Incoming cosmic radio signals pass virtually unimpeded through the radome's fiberglass construction.
Figure 5.21 Photograph of the Haystack dish, taken from inside the radome. For scale, note the engineer standing at the bottom. Also note the dull shine on the telescope surface, indicating its smooth construction. Haystack is a poor optical mirror but a superb radio telescope. Accordingly, it can be used to reflect and accurately focus radiation having short radio wavelengths, even as small as a fraction of a centimeter.
Despite the inherent disadvantage of relatively poor angular resolution, radio astronomy enjoys many advantages. Radio telescopes can observe 24 hours a day--darkness is not needed for receiving radio signals. The reason for this is simply that the Sun is a relatively weak source of radio energy, so its emission does not swamp radio signals from elsewhere. Astronomers can often make radio observations through cloudy skies, and radio telescopes can detect the longest-wavelength radio waves even during rain or snow storms. Poor weather causes few problems because the wavelength of most radio waves is much larger than the typical size of atmospheric raindrops or snowflakes. Optical astronomy cannot be done under these conditions because the wavelength of visible light is smaller than a raindrop, a snowflake, or even a minute water droplet in a cloud.
However, perhaps the greatest value of radio astronomy (and, in fact, of all other invisible astronomies) is that it opens up a whole new window on the universe. Objects that are bright in the optical region of the spectrum are not necessarily strong radio emitters, and very often, strong radio sources are completely undetectable in visible wavelengths. Thus, radio observations do not just afford us the opportunity of studying the same objects at different wavelengths. They have also allowed us to see new classes of objects that had been completely unknown.
For example, Figure 5.22(a) shows an optical photograph of the Orion Nebula (a huge cloud of interstellar gas) taken with the 4 m telescope on Kitt Peak. Figure 5.22(b) shows a radio contour map of the same region superimposed on the optical image. By aiming a radio telescope at the nebula, radio astronomers determine the strength of its radio emission. Scanning back and forth across the nebula and taking many measurements, the astronomers construct a radio map of the entire region. The map is drawn as a series of contour lines connecting locations of equal radio brightness. These radio contours are similar to pressure contours drawn by meteorologists on weather maps and height contours drawn by cartographers on topographic maps. The inner contours usually represent relatively strong radio signals, the outside contours weak signals.
Figure 5.22 (a) Optical photograph of the Orion Nebula, a star-forming region some 1500 light years distant. The bright regions in this photograph are stars and clouds of glowing gas. The dark regions are not empty but are simply obscured by interstellar matter. The nebula can be seen in Figure 1.6 of the hunter's sword in the constellation Orion. (b) Superposition of a radio contour map onto the optical photograph. Each curve represents a different intensity of radio emission. The resolution of the optical image is about 1 arc second; that of the radio map 1 arc minute.
The radio map shown in Figure 5.22(b) has many similarities to the visible image of the nebula--the radio emission is clearly strongest near the center of the optical image and declines toward the nebular edge. But there are also some subtle differences between the radio and optical images. The two maps differ mainly toward the upper left of the main cloud, where visible light seems to be absent, despite the existence of radio waves. How can radio waves be detected from locations not showing any light emission? This particular nebular region is known to be especially dusty in its top left quadrant. The dust obscures the short-wavelength visible radiation but not the long-wavelength radio radiation.
Thus, our radio map allows us to see the true extent of this cosmic source. Optical images are often distorted by intervening dust somewhere along our line of sight. In fact, many important objects and regions of the universe cannot be seen at all by optical astronomy; the very center of our Milky Way Galaxy is a prime example of a totally invisible region. Our knowledge of such regions results almost entirely from analyses of their longer-wavelength radio and infrared emissions.
Back The main disadvantage of radio astronomy compared with optical work is its lack of good angular resolution. However, radio astronomers have invented ways to overcome this problem. By using a technique known as interferometry, they can improve the angular resolution of some radio maps enormously. In fact, using interferometry, it is actually possible to produce radio images of much higher angular resolution than can be achieved with even the best optical telescopes.
In interferometry, two or more radio telescopes are used in tandem to observe the same object at the same wavelength and at the same time. The combined instruments together make up an interferometer. Figure 5.23 shows an interferometer--several separate radio telescopes working together as a team. By means of electronic cables or radio links, the signals each antenna receives are sent to a central computer that analyzes how the waves interfere with each other when added together. If the detected waves are in step when added, they combine positively to form a strong radio signal. If the signals are not in step, they destructively interfere and cancel each other. As the antennas track their target, a pattern of peaks and troughs emerges, which, after extensive computer processing, translates into an image of the observed object.
Figure 5.23 This large interferometer is made up of 27 separate dishes spread along a Y-shaped pattern about 30 km across on the Plain of San Augustin in New Mexico. The most sensitive radio device in the world, it is called the Very Large Array or VLA, for short. (b) A close-up view from ground level of some of the VLA antennas. Notice that the dishes are mounted on railroad tracks so that they can be repositioned easily.
An interferometer is essentially a substitute for a single huge antenna. With respect to resolving power, the effective telescope diameter of an interferometer equals the distance between its outermost dishes. In other words, two small dishes can act as opposite ends of an imaginary but huge single radio telescope, dramatically improving the angular resolution. For example, resolution of a few arc seconds can be achieved at typical radio wavelengths (such as 10 cm), either by using a single radio telescope 5 km in diameter (which is quite impossible to build) or by using two or more much smaller dishes separated by 5 km and connected electronically.
Large interferometers made up of many dishes, like the instrument shown in Figure 5.23, now routinely attain radio resolution comparable to that of optical images. Figure 5.24 compares an interferometric radio map of a nearby galaxy with a photograph of that same galaxy made using a large optical telescope. The radio clarity is superb--much better than the radio map of Figure 5.22(b).
Figure 5.24 VLA radio "photograph" (or radiograph) of the spiral galaxy M51, observed at radio frequencies with an angular resolution of a few arc seconds (a); shows nearly as much detail as an actual (light) photograph of that same galaxy (b) made with the 4 m Kitt Peak optical telescope.
The larger the distance separating the telescopes--the longer the baseline of the interferometer--the better the resolution attainable. Astronomers have created radio interferometers spanning very great distances, first across North America and later between continents. A typical very-long-baseline interferometry experiment (usually known by the acronym VLBI) might use radio telescopes in North America, Europe, Australia, and Russia to achieve angular resolution on the order of 0.001´´, about 1000 times better than images produced by most current optical telescopes. It seems that even the Earth's diameter is no limit. Radio astronomers have successfully used an antenna in orbit, together with several antennas on the ground, to construct an even longer baseline and achieve still better resolution. Proposals even exist to place interferometers entirely in Earth orbit, and even on the Moon.
Nowadays, interferometry is no longer restricted to the radio domain. Radio interferometry became feasible when electronic equipment and computers achieved speeds great enough to combine and analyze radio signals from separate radio detectors without loss of data. As the technology has improved, it has become possible to apply the same methods to higher-frequency radiation. Millimeter-wave interferometry has already become an established and important observational technique, and it is very likely that infrared interferometry will become commonplace in the coming few years. Interferometry is not yet widely used in optical work because of the technical difficulties involved, but optical interferometry is the subject of intensive research. The new Keck telescopes on Mauna Kea will be used for infrared--and perhaps someday for optical--interferometric work.
Acts of Violence Caught by a Satellite
Back The electromagnetic spectrum consists of far more than just visible light and radio waves. Optical and radio astronomy are the oldest and best-established branches of astronomy, but since the 1970s there has been a virtual explosion of observational techniques covering the many other types of electromagnetic radiation. Today all portions of the spectrum are studied, from radio waves to gamma rays, to maximize the amount of information available about astronomical objects.
As we have already noted in the context of radio astronomy, the types of astronomical objects that can be observed may differ quite markedly from one wavelength range to another. Thus, full-spectrum coverage is essential, not only to see things more clearly, but even to see some things at all.
Because of the transmission characteristics of the Earth's atmosphere, astronomers must study most wavelengths (other than optical and radio) from space. The rise of these "other astronomies" has therefore been closely tied to the development of the space program.
Infrared studies are an important component of modern observational astronomy. Generally, infrared telescopes resemble optical telescopes (indeed, many optical telescopes are also used for infrared work), but their detectors are sensitive to longer-wavelength radiation. Although most infrared radiation is absorbed by the atmosphere (primarily by water vapor), in a few windows in the high-frequency part of the infrared spectrum (see Figure 3.9) the opacity is low enough to allow ground-based observations. Indeed, some of the most useful infrared observing is done from the ground, even though the radiation is somewhat diminished in intensity by our atmosphere.
As with radio observations, the longer wavelength of infrared radiation often enables us to perceive objects partially hidden from optical view. As an example of the penetrating properties of infrared radiation, Figure 5.25 shows a dusty and hazy region in California, hardly viewable optically, but easily seen using infrared radiation.
Figure 5.25 An optical photograph (a) taken near San Jose, California, and an infrared photo (b) of the same area taken at the same time. Longer wavelength infrared radiation can penetrate smog much better than short-wavelength visible light.
Figure 5.26 is a photograph of the world's highest ground-based observatory, perched more than 4 km (about 14,000 feet) above sea level on top of an extinct volcano at Mauna Kea, Hawaii. Despite its remoteness, this site draws a full schedule of astronomers throughout the year. The thin air at this high altitude guarantees less atmospheric absorption of incoming radiation, and hence a clearer view, than is possible from sea level. Mauna Kea is one of the finest locations on the Earth for ground-based optical and infrared astronomy, but the air is so thin that astronomers must occasionally wear oxygen masks while performing their observations.
Figure 5.26 Photograph of the world's highest ground-based observatory at Mauna Kea, Hawaii. Among the domes visible in the picture are those that house the CanadaFranceHawaii 3.6 m telescope, the 2.2 m telescope of the University of Hawaii, Britain's 3.8 m infrared facility, and the twin Keck telescopes (see Figure 5.12).
Astronomers can make still better infrared observations if they can place their instruments above most or all of the Earth's atmosphere. Improvements in balloon-, aircraft-, rocket-, and satellite-based telescope technologies have made infrared research a very powerful tool with which to study the universe (see Figure 5.27). As might be expected, the infrared telescopes that can be carried above the atmosphere are considerably smaller than massive ground-based instruments.
Figure 5.27 (a) A gondola containing a 1-m infrared telescope (lower left) is readied for its balloon-borne ascent to an altitude of about 30 km (100,000 feet), where it will capture infrared radiation that cannot penetrate the atmosphere. (b) An artist's conception of the Infrared Astronomy Satellite, placed in orbit in 1983. This 0.6 m telescope surveyed the infrared sky at wavelengths ranging from 10 to 100 µm. During its 10 months of operation, it greatly increased astronomers' understanding of many different aspects of the universe, from the formation of stars and planets to the evolution of galaxies.
The most advanced facility to function in this part of the spectrum is the Infrared Astronomy Satellite (IRAS), shown in Figure 5.27(b). Launched into Earth orbit in 1983 but now inoperative, this British-Dutch-U.S. satellite housed a 0.6 m mirror with an angular resolution as fine as 30´´. (As usual, the resolution depended on the precise wavelength observed.) Its sensitivity was greatest for radiation in the 10 to 100 µm range. During its 10-month lifetime (and long afterwards--the data archives are still heavily used even today), IRAS contributed greatly to our knowledge of clouds of galactic matter that seem destined to become stars, and possibly planets. These regions are composed of warm gas that cannot be seen with optical telescopes or adequately studied with radio telescopes. Throughout the text we will encounter many findings made by this satellite about comets, stars, galaxies, and the scattered dust and rocky debris found between the stars. All these objects "glow" in the infrared. For example, because much of the material between the stars has a temperature between a few tens and a few hundred kelvins, Wien's law (Section 3.4) tells us that the infrared domain is the natural portion of the spectrum in which to study it.
Figure 5.28(a) shows an IRAS image of the Orion Nebula. At about 1´ angular resolution, the fine details of Orion visible in the earlier optical image (Figure 5.22) cannot be perceived. Nonetheless, astronomers can extract useful information about this object and others like it from such observations. For example, clouds of warm dust and gas, believed to play a critical role in the process of star formation, and extensive groups of bright young stars, completely obscured at visible wavelengths, are seen.
Figure 5.28 (a) This infrared image of the Orion Nebula and its surrounding environment was made by the Infrared Astronomy Satellite. The whiter regions denote greater strength of infrared radiation; the false colors denote different temperatures, descending from white to red to black. (b) The same region photographed in visible light.
Unfortunately, by Wien's law, telescopes themselves also radiate strongly in the infrared unless they are cooled to nearly absolute zero. The end of IRAS's mission came not because of any equipment malfunction or unexpected mishap but simply because its supply of liquid helium coolant ran out. IRAS's own thermal emission then overwhelmed the radiation it was built to detect. The European Space Agency (ESA) and NASA both plan to launch infrared instruments into Earth orbit in the late 1990s. The first of these, ESA's Infrared Space Observatory (ISO), is now in orbit, refining and extending the groundbreaking work begun by IRAS.
On the short-wavelength side of the visible spectrum lies the ultraviolet domain. This region of the spectrum, extending in wavelength from 400 nm (4000 , blue light) down to a few nanometers ("soft" X rays), has only recently begun to be explored. Because Earth's atmosphere is partially opaque to radiation below 400 nm and is totally opaque below about 300 nm (in part because of the ozone layer), astronomers cannot conduct any useful ultraviolet observations from the ground, not even from the highest mountaintop. Rockets, balloons, or satellites are therefore essential to any ultraviolet telescope--a device designed to capture and analyze this high-frequency radiation.
One of the most successful ultraviolet space missions is the International Ultraviolet Explorer, called IUE for short. This satellite was placed in Earth orbit in 1978 and is still functioning as designed (see Interlude 18-1). Like all ultraviolet telescopes, its basic appearance and construction are quite similar to optical and infrared devices. Several hundred astronomers from all over the world have used IUE to explore a variety of phenomena in planets, stars, and galaxies. In subsequent chapters, we will learn what this relatively new window on the universe has shown us about the activity and even the violence that seems to pervade the cosmos. The Hubble Space Telescope, described in Interlude 5-1, is also a superb ultraviolet instrument.
An alternative means of placing astronomical payloads into (temporary) Earth orbit is provided by NASA's space shuttle. In December 1990 and March 1995, a shuttle carried aloft the Astro package of three ultraviolet telescopes (see Figure 5.29). Astronomical shuttle missions offer a potentially very flexible way for astronomers to get instruments into space, without the long lead times and great expense of permanent satellite missions like the Hubble telescope.
Figure 5.29 (a) The Astro payload, carried by the space shuttle in 1990 and 1995, performed ultraviolet and X-ray observations from orbit for the 10-day duration of each mission. (b) This false-color image of the spiral galaxy M74 was made by an ultraviolet telescope aboard Astro.
High-energy astronomy studies the universe as it presents itself to us in X rays and gamma rays--the types of radiation whose photons have the highest frequencies, and hence the greatest energies. How do we detect radiation of such short wavelengths? First, it must be captured high above the Earth's atmosphere because none of it reaches the ground. Second, its detection requires the use of equipment basically different in design from that used to capture the relatively low-energy radiation discussed up to this point.
The basic difference in the design of high-energy telescopes comes about because X and gamma rays cannot be reflected easily by any kind of surface. Rather, these rays tend to pass straight through, or be absorbed by, any material they strike. When X rays barely graze a surface, however, they can be reflected from it in a way that yields an image, although the mirror design is fairly complex (see Figure 5.30). For gamma rays (with wavelengths less than about 0.01 nm), no such method of producing an image has yet been devised. Present-day gamma-ray telescopes simply point in a specified direction and count photons received.
In addition, X-ray and gamma-ray detection methods using photographic plates or CCD devices do not work well. Instead, individual X-ray and gamma-ray photons are counted by electronic detectors on board an orbiting device, and the results are then transmitted to the ground for further processing and analysis. Furthermore, the number of photons in the universe seems to be inversely related to frequency. Billions of visible (starlight) photons arrive at the Earth each second, but hours or even days are often needed for a single gamma-ray photon to be recorded. Not only are these photons hard to focus and measure, they are also few and far between.
Figure 5.30 The arrangement of mirrors in an X-ray telescope allows X rays to be reflected at grazing angles and focused into an image.
Toward the end of the 1970s, a new generation of X-ray and gamma-ray telescopes was launched into Earth orbit. Called the High-Energy Astronomy Observatories, or HEAO for short, these spacecraft made major advances in our understanding of high-energy phenomena throughout the universe. Having greater accuracy and sensitivity than all earlier high-energy satellites, these spacecraft did for X-ray astronomy what the first large optical and radio telescopes did for longer-wavelength radiation. Figure 5.31 is a photograph of the HEAO-2 spacecraft, the first X-ray telescope capable of forming an image of its field of view. In 1979, the year when it first came on-line, the satellite was renamed the Einstein Observatory, in honor of the birth centenary of the great scientist.
Figure 5.31 HEAO-2, also known as the Einstein Observatory, the first imaging X-ray telescope.
Although the collecting diameter of Einstein was only 0.6 m, its angular resolution was a mere 3´´. Accordingly, this spacecraft could produce images of quality comparable to that of optical photographs. Figures 5.32(b) and (c) are Einstein X-ray images, showing some of the many hot regions in and around the center of the Andromeda Galaxy. In this image, the hottest regions stand out most clearly because the black-body curve of their emission peaks well into the high-energy domain. Thus, they shine brightly in X rays compared to the much cooler surrounding material, which emits primarily in the infrared and visible regions of the spectrum and hardly at all at higher energies.
The most recent major X-ray satellite is the German ROSAT (short for Röntgen Satellite, after Wilhelm Röntgen, the discoverer of X rays). Launched in 1990 by a European Ariane rocket, it began its mission with a detailed survey of the X-ray sky and is now making detailed observations of specific astronomical objects (see Figure 5.33). With more sensitivity, a wider field of view, and better resolution than Einstein, ROSAT is providing high-energy astronomers with new levels of observational detail. Even more powerful will be NASA's planned Advanced X-ray Astrophysics Facility (AXAF), another long-duration orbiting observatory (in the spirit of IUE and HST) that may become operational by the end of the 1990s.
Figure 5.32 (b), (c) These X-ray images of the Andromeda Galaxy highlight the galaxy's hottest regions. By contrast, few of the galaxy's stars and gas clouds, detectable at optical, radio, or infrared wavelengths, are hot enough to emit X rays. For comparison, the corresponding visible-light image of the galaxy is shown in (a).
The youngest entrant into the observational arena is gamma-ray astronomy. As mentioned earlier, true imaging gamma-ray telescopes do not exist, so only fairly coarse (1° resolution) observations can be made. Nevertheless, even at that resolution, there is much to be learned. Cosmic gamma rays were originally detected in the 1960s by the U.S. Vela series of satellites, whose primary mission was to monitor illegal nuclear detonations on Earth. Since then, several X-ray telescopes have also been equipped with gamma-ray detectors. By far the most advanced instrument is the Gamma Ray Observatory (GRO), launched by the space shuttle in 1991. This satellite can scan the sky and study individual objects in much greater detail than previously attempted. Figure 5.34 shows GRO on station in low Earth orbit, along with a (false color) gamma-ray image of a highly energetic outburst in the nucleus of a distant galaxy. Figure 5.35(e) below shows another GRO image, this time of our own galaxy.
Figure 5.33 An X-ray image of the Orion region, taken by the ROSAT X-ray satellite. (Compare with Figures 1.6, 5.22, and 5.28.) Note the three stars of Orion's belt and the glowing nebula below them at bottom left in the photograph.
Figure 5.34 (a) This photograph of the 17-ton Gamma-Ray Observatory (also called the Compton Observatory, after an American gamma-ray pioneer) was taken by an astronaut during the satellite's deployment from the space shuttle Atlantis over the Pacific Coast of the United States. (b) A typical false-color gamma-ray image--this one showing a violent event in the distant galaxy 3C279, also known as a "gamma-ray blazar."
Back In this chapter we have studied some of the basic techniques and equipment used by astronomers to study the universe. Besides the familiar optical telescopes that collect light from cosmic objects, other tools are needed to capture the invisible radiation emitted by a variety of celestial sources. Often, these "invisible astronomies" are crucial in our study of objects that are totally obscured from view in the visible range or simply do not emit any visible light. Radio astronomy is the oldest of the nonvisible subjects, high-energy astronomy the newest. In the end, they all supplement one another, helping us accumulate a growing store of astronomical knowledge. Table 5-1 lists the basic regions of the electromagnetic spectrum and describes some of the objects that are typically studied in each frequency range. Bear in mind, though, that the list is far from exhaustive--many important astronomical objects are now routinely observed at many different electromagnetic wavelengths, and are not listed here.
As we proceed through the text, we will discuss more fully the wealth of information that high-precision astronomical instruments can provide us. It is reasonable to suppose that the future holds many further improvements in both the quality and the availability of astronomical data and that many new discoveries will be made. The current and proposed pace of technological progress presents us with the following very exciting prospect: In the twenty-first century, if all goes according to plan, it will be possible, for the first time ever, to make simultaneous high-quality measurements of any astronomical object at all wavelengths, from radio to gamma ray. The consequences of this development for our understanding of the workings of the universe may be little short of revolutionary.
As a preview of the sort of comparison that full-spectrum coverage allows, Figure 5.35 shows a series of images of our own Milky Way Galaxy. They were made by several different instruments, at wavelengths ranging from radio to gamma ray, over a period of about five years. By comparing the features visible in each, we immediately see how multiwavelength observations can complement each other, greatly extending our perception of the universe around us.
Figure 5.35 The Milky Way Galaxy, as it appears (from top to bottom) at (a) radio, (b) infrared, (c) visible, (d) X-ray, and (e) gamma-ray wavelengths.