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.
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.
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.
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.)
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 fainteror the same object 10 to 20 times fasterthan 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.
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.
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.
What steps do optical astronomers take to overcome the obscuring and blurring effects of Earth's atmosphere?