3.3 High-Resolution Astronomy

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 about 1''. The reason is Earth's turbulent atmosphere, which blurs the image even before the light reaches our instruments.

Atmospheric Blurring

As we observe a star, atmospheric turbulence produces continuous 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, again and again, and the stellar image dances around on the detector (or retina). This is the cause of the well-known "twinkling" of stars. It occurs for the same basic reason that objects appear to shimmer when viewed across a hot roadway on a summer day.

Figure 3.11 Atmospheric Turbulence Individual photons from a distant star strike a telescope detector at slightly different locations because of turbulence in Earth’s atmosphere. Over time, the 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.

On a good night at the best observing sites, the maximum deflection produced by the atmosphere is slightly less than 1''. Consider taking a photograph of a star under such conditions. After a few minutes' exposure time (long enough for the intervening atmosphere to have undergone many small, random changes), the dancing sharp image of the star has been smeared out over a roughly circular region 10 or so in diameter (Figure 3.11). Astronomers use the term seeing to describe the effects of atmospheric turbulence. The circle over which a star's light is spread is called the seeing disk.

Figure 3.12 Southern Hemisphere Telescope 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 carrying various pieces of support equipment, making this one of the most versatile observatories south of the equator. The New Technology Telescope is inside the square dome at center. (ESO)
To achieve the best possible seeing, telescopes are sited on mountaintops (to get above as much of the atmosphere as possible) in locations 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 in Hawaii (Figure 3.8) and at La Silla and Cerro Paranal in the Andes Mountains of Chile (Figure 3.12), which is why many large telescopes have recently been constructed at those two exceptionally clear sites.

A telescope placed above the atmosphere, in Earth orbit, can achieve resolution close to the diffraction limit, 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 (Figure 3.6b), has a 2.4-m mirror and a diffraction limit of 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. (See Interlude 3-1.)

Image Processing

Figure 3.13 CCD Chip (a)A charge-coupled device consists of hundreds of thousands, or even millions, of tiny light-sensitive cells called 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. (b) A CCD chip mounted at the focus of a telescope. (AURA; R. Wainscoat/Peter Arnold)
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. A CCD (Figure 3.13) 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 are steadily increasing.

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 five percent for photographic methods. This means that a CCD instrument can image objects 10 to 20 times fainter—or the same object 10 to 20 times faster—than can a photographic plate. 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 for analysis.

Using computer processing, astronomers can compensate for known instrumental defects and even correct some effects of bad seeing. In addition, the computer can often carry out many of the tedious and time-consuming chores that must be performed before an image or spectrum reaches its final form. Figure 3.14 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 3.14 Image Processing (a) 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 R136 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. (AURA; NASA)

New Telescope Design

By analyzing the image while the light is still being collected, it is possible to adjust the telescope from moment to moment to reduce the effects of mirror distortion, temperature changes, and bad seeing. Some of these techniques, collectively known as active optics, were first combined in the New Technology Telescope (NTT) at the European Southern Observatory in Chile (Figure 3.12). This 3.5-m instrument 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 and greatly improving image quality (Figure 3.15). The Keck and VLT instruments employ similar methods and may ultimately achieve resolution as fine as 0.25" by these means.

Figure 3.15 Active Optics Infrared images of part of the star cluster R136 contrast the resolution obtained (a) without and (b) with an active optics system. (NASA)

An even more ambitious undertaking is known as adaptive optics. This technique deforms the shape of a mirror's surface, under computer control while the image is being exposed, in order to undo the effects of atmospheric turbulence. In the experimental system shown in Figure 3.16(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. Already, impressive improvements in image quality have been obtained (Figure 3.16b). These developing techniques are also being incorporated into Keck and VLT. In the next decade, it may well be possible to achieve with large ground-based telescopes the kind of resolution presently attainable only from space.

Figure 3.16 Adaptive Optics (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 0.10" and the two components are clearly resolved. (R. Ressmeyer/Starlight Collection, A Division of Corbis © 1994, MIT Lincoln Laboratory)

Concept Check

What steps do optical astronomers take to overcome the obscuring and blurring effects of Earth's atmosphere?