Even large telescopes have their limitations. For example, according to the discussion in the preceding section, the 5-m Hale telescope should have an angular resolution of around 0.02". In practice, however, it cannot do better than about 1". In fact, apart from instruments using special techniques developed to examine some particularly bright stars, no ground-based optical telescope built before 1990 can resolve astronomical objects to much better than 1". The reason is Earth's turbulent atmosphere, which blurs the image even before the light reaches our instruments. In recent years, great strides have been made in overcoming this obstacle. Telescopes have been placed above the atmosphere, and computers are playing an increasingly important role in both telescope operation and image processing.
As we observe a star, atmospheric turbulence produces continual small changes in the optical properties of the air between the star and our telescope (or eye). 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 5.14 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 5.14 Individual photons from a distant star strike the detector in a telescope at slightly different locations because of turbulence in 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 5.15)—which is why many large telescopes have recently been constructed at those two exceptionally clear locations.
Figure 5.15 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 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 Earth orbit by NASA's space shuttle Discovery in 1990 (see Interlude 5-1.). This telescope has a 2.4-m mirror, with a diffraction limit of only 0.05", giving astronomers a view of the universe as much as 20 times sharper than that normally available from even much larger ground-based instruments.
Computers play an important role in observational astronomy. Most large telescopes today are controlled either by computers or by operators who rely heavily on computer assistance, and images and data are recorded in a form that can be easily read and manipulated by computer programs.
It is becoming rare for photographic equipment to be used as the primary means of data acquisition at large observatories. Instead, electronic detectors known as charge-coupled devices, or CCDs, are in widespread use. Their output goes directly to a computer. A CCD (Figure 5.16) 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, and a two-dimensional image is obtained. A CCD is typically 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 continue to increase. Incidentally, the technology is not limited to astronomy—many home video cameras contain CCD chips similar in basic design to those in use at the great astronomical observatories of the world.
Figure 5.16 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. (a) Detail of a CCD array. (b) A CCD chip mounted for use at the focus of a telescope.
CCDs have two important advantages over photographic plates, which were the staple of astronomers for over a century. First, CCDs are much more efficient than photographic plates, recording as many as 75 percent of the photons striking them, compared with less than 5 percent for photographic methods. This means that a CCD image can show objects 10 to 20 times fainter than can a photograph made using the same telescope and the same exposure time. Alternatively, a CCD can record the same level of detail in less than a tenth of the time required by photographic techniques, or record that detail with a much smaller telescope. Second, CCDs produce a faithful representation of an image in a digital format that can be placed directly on magnetic tape or disk, or even sent across a computer network to an observer's home institution.
Computers are also widely used to reduce background noise in astronomical images. 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 Earth's atmosphere. It can also be caused by electronic "hiss" within the detector. 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 its effects partially removed with the aid of high-speed computers, allowing astronomers to see features that would otherwise remain hidden.
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. Figure 5.17 illustrates 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.
Figure 5.17 (a) Ground-based view of the star cluster R136, a group of stars in the Large Magellanic 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 that is bringing about striking improvements in the resolution of ground-based optical telescopes takes these ideas of computer control and image processing one stage further. If an image can be analyzed while the light is still being collected (a process that can take many minutes, or even hours, in some cases), it is possible to adjust the telescope from moment to moment to correct for the effects of mirror distortion, temperature changes, and bad seeing. In principle, the telescope might even 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 (NTT is the most prominent instrument visible in Figure 5.15). 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 (see Figure 5.18a). The Keck 10-m instruments, one of whose hexagonal mirrors is shown in Figure 5.18b, also employ these methods and may ultimately achieve resolution as fine as 0.25" by these means.
Figure 5.18 (a) These false-color infrared photographs of part of the star cluster R136—the same object shown in Figure 5.17—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. Adaptive optics presents formidable theoretical and technological problems, but the rewards are so great that they are presently the subject of intense research. Declassified SDI ("Star Wars") technology has provided an enormous boost to this effort. In the experimental system shown in Figure 5.19(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.
Figure 5.19 (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 Mount Haleakala in Maui, Hawaii, employing similar technology. The uncorrected image (left) of the double star Castor is a blur spread over several arc seconds, with little hint of its binary nature. With adaptive compensation applied (right), the resolution is improved to a mere 0.1" and the two components are clearly resolved.
Figure 5.19(b) compares the results of a pair of observations of a nearby double star called Castor. Spectroscopic observations long ago revealed the double nature of this object, which appears to be a binary-star system. The image on the left shows the star system as seen through an ordinary, moderate-sized telescope—an oblong blur combining the light from the two stars, spread over several arc seconds. On the right, with the adaptive optics system turned on, one star is clearly distinguishable from the other. The two stars are separated by less than an arc second; the resolution, with adaptive optics turned on, is about 0.1".
Adaptive optics systems are planned for many existing large telescopes. For example (if development goes as planned), the Keck telescopes will, by the year 2000, incorporate adaptive-optics instrumentation capable of producing diffraction-limited (0.02" resolution) images at near-infrared wavelengths. In the next decade, it may well be possible to have the "best of both worlds," achieving with a large ground-based telescope the kind of resolution presently attainable only from space.
