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 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. Later we will turn our attention to telescopes designed to capture and analyze radiation in other, invisible, regions of the electromagnetic spectrum.
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 shows how a reflecting telescope uses a curved mirror to gather and concentrate a beam of light. The mirror, usually called the primary mirror because telescopes often contain more than one mirror, is constructed so that all light rays arriving parallel to its axis (the imaginary line through the center of and perpendicular to the mirror), regardless of their distance from that axis, are reflected to pass through a single point, called the focus. The distance between the primary mirror and the focus is the focal length. In astronomical contexts, the focus of the primary mirror is referred to as the prime focus.
Figure 5.1 A curved mirror can be used to focus to a single point all rays of light arriving parallel to the mirror axis. Light rays traveling along the axis are reflected back along the axis, as indicated by the arrowheads pointing in both directions. 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 point—the focus.
A refracting telescope uses a lens to focus the incoming light. Refraction is the bending of a beam of light as it passes from one transparent medium (for example, air) into another (such as glass). For example, consider how a pencil half immersed in a glass of water looks bent. The pencil is straight, of course, but the light by which we see it is bent—refracted—as that light leaves the water and enters the air. When that light then enters our eyes, we perceive the pencil as being bent. Figure 5.2(a) illustrates the process and shows how a prism can be used to change the direction of a beam of light. As illustrated in Figure 5.2(b), we can think of a lens as a series of prisms combined in such a way that all light rays striking the lens parallel to the axis are refracted to pass through the focus.
Figure 5.2 (a) Refraction by a prism changes the direction of a light ray by an amount that depends on the angle between the faces of the prism. (b) A lens can be thought of as a series of prisms. A light ray traveling along the axis of a lens is unrefracted as it passes through the lens. Parallel rays arriving at progressively greater distances from the axis are refracted by increasing amounts, in such a way that all are focused to a single point.
Astronomical telescopes are often used to make images of their field of view. Figure 5.3 illustrates how this is accomplished, in this case by the mirror in a reflecting telescope. Light from a distant object (in this case, a comet) 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 5.3 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 cm 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 5.4(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 5.4(b) shows how a refracting telescope accomplishes the same function.
Figure 5.4 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 both cases the image formed at the focus is viewed with a small magnifying lens called an eyepiece.
The two telescope designs shown in Figure 5.4 achieve the same result—light from a distant object is captured and focused to form an image. On the face of it, then, it might appear that there is little to choose between the two in deciding which type to buy or build. However, as telescope size has steadily increased over the years (for reasons to be discussed in Section 5.2), a number of important factors have tended to favor reflecting instruments over refractors:
| 1. | The fact that light must pass through the lens is a major disadvantage of refracting telescopes. Large lenses cannot be constructed in such a way that light passes through them uniformly. Just as a prism disperses white light into its component colors, the lens in a refracting telescope focuses red and blue light differently. This deficiency is known as chromatic aberration. Figure 5.5 shows how chromatic aberration occurs and indicates how it affects the image of a star. Careful design and choice of materials can largely correct chromatic aberration, but it is very difficult to eliminate entirely. | |
| 2. | As light passes through the lens, some of it is absorbed by the glass. This absorption is a relatively minor problem for visible radiation, but it can be severe for infrared and ultraviolet observations because glass blocks most of the radiation coming from those regions of the electromagnetic spectrum. This problem obviously does not affect mirrors. | |
| 3. | A large lens can be quite heavy. Because it can be supported only around its edge (so as not to block the incoming radiation), the lens tends to deform under its own weight. A mirror does not have this drawback because it can be supported over its entire back surface. | |
| 4. | A lens has two surfaces that must be accurately machined and polished—which can be very difficult—but a mirror has only one. |
Figure 5.5 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 5.6 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 5.6 Photograph of the Yerkes Observatory's 1-m-diameter refracting telescope.
Figure 5.7 shows some basic reflecting telescope designs. Radiation from a star enters the instrument, passes down the main tube, strikes the primary mirror, and is reflected back toward the prime focus, near the top of the tube. Sometimes astronomers place their recording instruments at the prime focus. 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 secondary mirror and redirected to a more convenient location, as in Figure 5.7(b) through (d).
Figure 5.7 Four 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, so that they move 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). The point behind the primary mirror where the light from the star finally converges is called the Cassegrain focus.
A 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 reflects 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 the Cassegrain focus. The light path to the coudé room 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 the light path does not change as the telescope moves.
To illustrate some of these points, Figure 5.8 depicts the Hale 5-m-diameter optical telescope on California's Mount Palomar. As the size of the person drawn in the observer's cage at the prime focus indicates, this is indeed a very large telescope. In fact, for almost three decades after its dedication in 1948, the Hale telescope was the largest in the world. It has been at or near the forefront of astronomical research for much of the last half century. Observations can be made at the prime, the Cassegrain, or the coudé focus, depending on the needs of the user.
Figure 5.8 (a) Artist's illustration of the 5-m-diameter Hale optical telescope on Mount Palomar 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 5.9). 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 new, and much more detailed, digital sky survey is now being carried out by a consortium of U.S. research institutions. It should be completed by 2004.)
Figure 5.9 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 collected by the primary mirror may be redirected to the underground coud éroom, defined by a narrow slit, passed through a prism or a diffraction grating, and projected onto a screen—a process not so different from the operation of the simple spectroscope described in Chapter 4. 
(Sec. 4.1) 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.
