5.2 Telescope Size

Astronomers generally prefer large telescopes over small ones, for two main reasons. The first has to do with the amount of light a telescope can gather—its light-gathering power. The second is related to the amount of detail that can be seen—the telescope’s resolving power.

LIGHT-GATHERING POWER

One important reason for using a larger telescope is simply that it has a greater collecting area, which is the total area of a telescope capable of gathering 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 much of their time observing very distant—and hence very faint—cosmic sources. In order to make detailed observations of such objects, very large telescopes are essential. Figure 5.10 illustrates the effect of increasing telescope size by comparing images of the Andromeda Galaxy taken with two different instruments. A large collecting area is particularly important for spectroscopic work, as the received radiation in that case must be split into its component wavelengths for further analysis.

Figure 5.10 Sensitivity Effect of increasing telescope size on an image of the Andromeda Galaxy. Both photographs had the same exposure time; Image (b) was taken with a telescope twice the size of that used to make (a). 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. (AURA)

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. Put another way, a 1-hour exposure with a 1-m telescope is roughly equivalent to a 2.4-minute exposure with a 5-m instrument.

Until the 1980s the conventional wisdom was that telescopes with mirrors larger than five or six meters in diameter were simply too expensive and impractical to build. The problems involved in casting, cooling, and polishing a huge block of quartz or glass to very fine tolerances (typically less than the width of a human hair) were just too great. However, new high-tech manufacturing techniques, coupled with radically new mirror designs, 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 now exist, and many more are planned.

Currently, the largest operating optical telescopes are the twin Keck instruments atop Mauna Kea in Hawaii (see Figure 5.11), administered jointly by the California Institute of Technology and the University of California. Each telescope combines 36 hexagonal 1.8-m mirrors into the equivalent collecting area of a single 10-m reflector. The first Keck telescope became fully operational in 1992; the second was completed in 1996. The high altitude and large size of these devices make them particularly well suited for detailed spectroscopic studies of very faint objects, in both the optical and infrared parts of the spectrum. (Mauna Kea’s 4-km altitude minimizes atmospheric absorption of infrared radiation, making this site one of the finest locations on Earth for infrared astronomy.)

Figure 5.11 Mauna Kea Observatory  (a) The world’s highest ground-based observatory, at Mauna Kea, Hawaii, is perched atop an extinct volcano more than 4 km above sea level. Among the domes visible in the picture are those that house the Canada-France-Hawaii 3.6-m telescope, the 8.1-m Gemini North instrument, the 2.2-m telescope of the University of Hawaii, Britain’s 3.8-m infrared facility, and the twin 10-m Keck telescopes. To the right of the twin Kecks is the Japanese 8.3-m Subaru telescope. Several of the largest telescopes are marked. The thin air at this 4-km-altitude site guarantees less atmospheric absorption of incoming radiation and hence a clearer view than at sea level, but the air is so thin that astronomers must occasionally wear oxygen masks while working. (b) The 10-m mirror in the first Keck telescope. Note the technician in orange coveralls at center. (R. Wainscoat; R. Underwood/W. M. Keck Observatory)

Numerous other large telescopes can be seen in Figure 5.11. Some are designed exclusively for infrared work; others, like Keck, operate in both the optical and the infrared. To the right of the Keck domes is the 8.3-m Subaru (the Japanese name for the Pleiades) telescope, operated by the National Astronomical Observatory of Japan. It saw “first light” in 1999. In the distance is the 8.1-m Gemini North instrument, also completed in 1999 by a consortium of seven nations including the U.S. Its twin, Gemini South, in the Chilean Andes, should become operational in 2001. In terms of total available collecting area, the largest telescope currently available is the European Southern Observatory’s optical-infrared Very Large Telescope (VLT), located at Cerro Paranal, in Chile (Figure 5.12). It consists of four separate 8.2-m mirrors that can function as a single instrument. The third mirror was completed in 2000, the fourth in 2001.

Figure 5.12 VLT Observatory Located at the Paranal Observatory in Atacama, Chile, the European Southern Observatory’s Very Large Telescope (VLT) is the world’s largest optical telescope. It comprises four 8.2-m reflecting telescopes, which can be used in tandem to create the effective area of a single 16-m mirror. (ESO)

Figure 5.13 Resolving Power 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.
RESOLVING POWER

A second advantage of large telescopes is their finer angular resolution. In general, resolution refers to the ability of any device, such as a camera or telescope, to form distinct, separate images of objects lying close together in the field of view. The finer the resolution, the better we can distinguish the objects and the more detail we can see. In astronomy, where we are always concerned with angular measurement, “close together” means “separated by a small angle on the sky,” so angular resolution is the factor that determines our ability to see fine structure. Figure 5.13 illustrates how the appearance of two objects—stars, say—might change as the angular resolution of our telescope varies. Figure 5.14 illustrates the result of increasing resolving power with views of the Andromeda Galaxy at several different resolutions.

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. (Discovery 3-1) Because of diffraction, when a parallel beam of light enters a telescope, the rays spread out slightly, making it impossible to focus the beam to a sharp point, even with a perfectly constructed mirror. Diffraction introduces a certain “fuzziness,” or loss of resolution, into the optical system. The degree of fuzziness—the minimum angular separation that can be distinguished—determines the angular resolution of the telescope. The amount of diffraction is proportional to the wavelength of the radiation divided by the diameter of the telescope mirror. As a result we can write, in convenient units,

where 1 µm (1 micron) = 10-6 m (see Appendix 2). Thus, for a given telescope size, the amount of diffraction increases in proportion to the wavelength used. Observations in the infrared or radio range are often limited by its effects. For light of any given wavelength, large telescopes produce less diffraction than small ones.

Figure 5.14 Resolution 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". (AURA)

For a given telescopic size, the amount of diffraction increases in proportion to the wavelength used. Observations in the infrared or radio range are often limited by its effects. For example, according to the formula above, in an otherwise perfect observing environment, the best possible angular resolution of blue light (with a wavelength of 400 nm) that can be obtained using a 1-m telescope is about 0.25" (0.4/1) = 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, at a wavelength of 10 µm (10,000 nm), 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 just under 1º.

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 in the middle of the visual range is about 0.5'.


Concept Check

Give two reasons why astronomers need to build very large telescopes.