The Solar System

CHAPTER 6 | LIGHT AND TELESCOPES 109

Adaptive Optics

Not too many years ago, astron-

omers thought it was pointless to

build more large telescopes on

Earth’s surface because of seeing

distortion caused by the atmo-

sphere. In the 1990s, computers

became fast enough to allow

astronomers to correct for some

of that distortion, and that has

made the new generation of giant

telescopes possible.

Adaptive optics uses high-

speed computers to monitor the distortion produced by turbulence

in Earth’s atmosphere and then correct the telescope image to

sharpen a fuzzy blob into a crisp picture. Th e resolution of the

image is still limited by diff raction in the telescope, but removing

much of the seeing distortion produces a dramatic improvement in

the detail visible (■ Figure 6-14). Don’t confuse adaptive optics

with the slower-speed active optics that controls the overall shape

of a telescope mirror.

To monitor the distortion in an image, adaptive optics sys-

tems must look at a fairly bright star in the fi eld of view, and there

isn’t always such a star properly located near a target object such

as a faint galaxy. In that case, astronomers can point a laser at a

spot in the sky very close to their target object, and where the laser

excites gas in Earth’s upper atmosphere, it produces a glowing

Modern computers have revolutionized telescope design and
operation. Nearly all large telescopes are operated by astronomers
and technicians working at computers in a control room, and
some telescopes can be operated by astronomers thousands of
miles from the observatory. Some telescopes are fully automated
and observe without direct human supervision. Th is has made
possible huge surveys of the sky in which millions of objects are
observed. Th e Sloan Digital Sky Survey, for example, mapped the
sky, measuring the position and brightness of 100 million stars
and galaxies at a number of wavelengths. Th e Two-Micron All
Sky Survey (2MASS) has mapped the entire sky at three wave-
lengths in the infrared. Other surveys are being made at other
wavelengths. Astronomers will study those data banks for decades
to come.

If built, the European Extremely

Large telescope (E-ELT) will have

a 42-m diameter mirror composed

of 906 segments. Note the car

at lower left for scale.

Thirty Meter Telescope

Giant Magellan Telescope

The 42-m mirror will

contain 906 segments.

Note the human figure for

scale in this computer

graphic visualization.

■ Figure 6-13

The proposed Giant Magellan Telescope

will have the resolving power of a tele-

scope 24.5 meters in diameter when it

is fi nished in about 2016. The Thirty

Meter Telescope (TMT) is planned to

occupy a specially designed dome. Like

nearly all of the newest large tele-

scopes, the European Extremely Large

Telescope will be on an alt-azimuth

mounting. (GMT: ESO; TMT: TMT: Thirty-

Meter Telescope; E-ELT: ESO)

Adaptive Optics

Off

On

■ Figure 6-14

In these images of the center of our gal-

axy, the adaptive optics system was turned

off for the left image and on for the right

image. Not only are the images of stars

sharper, but because the light is focused into

smaller images, fainter stars are visible. The

laser beam shown leaving one of the Keck

Telescopes produces an artifi cial star in the

fi eld of view, and the adaptive optics sys-

tem uses the laser-produced point of light to

reduce seeing distortion in the entire image.

(left, CFHT; right, Paul Hirst)