The Solar System

112 PART 1^ |^ EXPLORING THE SKY

artifi cial star in the fi eld of view. Th e adaptive optics system can
use the artifi cial star to correct the image of the fainter target.
Today astronomers are planning huge optical telescopes
composed of segmented mirrors tens of meters in diameter. Th ose
telescopes would be almost useless without adaptive optics.

Interferometry

One of the reasons astronomers build big telescopes is to increase
resolving power, and astronomers have been able to achieve very
high resolution by connecting multiple telescopes together to
work as if they were a single telescope. Th is method of synthesiz-
ing a larger telescope is known as interferometry (■ Figure 6-15).
One expert said, “We combine the light from separate telescopes
and fool the waves into thinking they were collected by one big
’scope.” Th e images from such a virtual telescope are not limited
by the diff raction fringes of the individual small telescopes but
rather by the diff raction fringes of the much larger virtual
telescope.
In an interferometer, the light from the separate telescopes
must be combined as if it had been collected by a single large
mirror. Th at means that a network of small, high-precision mir-
rors must bring the light beams together, and the path that each
light beam travels must be controlled so that it does not vary
more than a small fraction of the wavelength. Turbulence in
Earth’s atmosphere constantly distorts the light, and high-speed
computers must continuously adjust the light paths. Recall that

Beams combined to

produce final image

Simulated large-

diameter telescope

Precision optical

paths in tunnels

■ Figure 6-15

In an astronomical interferometer, smaller telescopes can combine their light
through specially designed optical tunnels to simulate a larger telescope
with a resolution set by the separation of the smaller telescopes.

the wavelength of light is very short, roughly 0.0005 mm, so

building optical interferometers is one of the most diffi cult tech-

nical problems that astronomers face. Infrared- and radio-wave-

length interferometers are slightly easier to build because the

wavelengths are longer. In fact, as you will discover later in this

chapter, the fi rst astronomical interferometers were built by radio

astronomers.

Th e VLT shown in Figure 6-12 consists of four 8.2-m tele-

scopes that can operate separately but can also be linked together

through underground tunnels with three 1.8-m telescopes on the

same mountaintop. Th e resulting optical interferometer provides

the resolution (but, of course, not the light gathering power) of

a telescope 200 meters in diameter. Other telescopes, such as the

two Keck 10-m telescopes, can work as interferometers. Th e

CHARA array on Mt. Wilson combines six 1-meter telescopes to

create the resolving power equivalent of a telescope one-fi fth of a

mile in diameter. Th e Large Binocular Telescope shown in

Figure 6-12 can be used as an interferometer.

Although turbulence in Earth’s atmosphere can be partially

averaged out in an interferometer, plans are being made to put

interferometers in space to avoid atmospheric turbulence alto-

gether. Th e Space Interferometry Mission, for example, will work

at visual wavelengths and study everything from the cores of

erupting galaxies to planets orbiting nearby stars.

SCIENTIFIC ARGUMENT

Why do astronomers build observatories at the tops of mountains?

Measurement accuracy is so important that scientists often take

extreme steps to gather accurate information. It certainly isn’t easy

to build a large, delicate telescope at the top of a high mountain,

but it is worth the effort. A telescope on top of a high mountain

is above the densest part of Earth’s atmosphere. There is less air

to dim the light, and there is less water vapor to absorb infra-

red radiation. Even more important, the thin air on a mountaintop

causes less disturbance to the image, and consequently the seeing

is better. A large telescope on Earth’s surface has a resolving power

much better than the distortion caused by Earth’s atmosphere. So,

it is limited by seeing, not by its own diffraction. It really is worth

the trouble to build telescopes atop high mountains.

Astronomers not only build telescopes on mountaintops, they

also build gigantic telescopes many meters in diameter. Revise

your argument to focus on telescope design. What are the prob-

lems and advantages in building such giant telescopes?

Special Instruments

Just looking through a telescope doesn’t tell you much.

A star looks like a point of light. A planet looks like a little disk.

A galaxy looks like a hazy patch. To use an astronomical telescope

to learn about the universe, you must be able to analyze the light

the telescope gathers. Special instruments attached to the tele-

scope make that possible.

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