The Solar System

106 PART 1^ |^ EXPLORING THE SKY

the air fl ows smoothly and is not turbulent. Building an obser-

vatory on top of a high mountain far from civilization is diffi -

cult and expensive, as you can imagine from the photo in Figure

6-10b, but the dark sky and steady seeing make it worth the

eff ort.

Buying a Telescope

Th inking about how to shop for a new telescope will not only help

you if you decide to buy one but will also illustrate some important

points about astronomical telescopes.

Assuming you have a fi xed budget, you should buy the

highest-quality optics and the largest-diameter telescope you

can aff ord. You can’t make the atmosphere less turbulent, but

you can choose good optics. If you buy a telescope from a toy

store and it has plastic lenses, you shouldn’t expect to see very

much. Also, you want to maximize the

light-gathering power of your tele-

scope, so you want to purchase the

largest-diameter telescope you can

aff ord. Given a fi xed budget, that

means you should buy a refl ecting tele-

scope rather than a refracting tele-

scope. Not only will you get more

diameter per dollar, but your telescope

will not suff er from chromatic

aberration.

You can safely ignore magnifi ca-

tion. Department stores and camera

shops may advertise telescopes by quot-

ing their magnifi cation, but it is not an

important number. What you can see is

determined by light-gathering power,

optical quality, and Earth’s atmosphere.

You can change the magnifi cation sim-

ply by changing eyepieces.

Other things being equal, you

should choose a telescope with a solid

mounting that will hold the telescope

steady and allow you to point it at

objects easily. Computer-controlled

big cities and usually on high mountains. Astronomers avoid cit-
ies because light pollution, the brightening of the night sky by
light scattered from artifi cial outdoor lighting, can make it
impossible to see faint objects (■ Figure 6-10a). In fact, many
residents of cities are unfamiliar with the beauty of the night sky
because they can see only the brightest stars. Even far from cities,
nature’s own light pollution, the moon, is sometimes so bright it
drowns out fainter objects, and astronomers are often unable to
observe on the nights near full moon. On such nights, faint
objects cannot be detected even with the largest telescopes on
high mountains.
Astronomers prefer to place their telescopes on carefully
selected high mountains for a number of reasons. Th e air there
is thin, very dry, and more transparent, especially in the infra-
red. For the best seeing, astronomers select mountains where

Astronomers no longer

build large observatories

in populous areas.

A number of major

observatories are located on

mountaintops in the Southwest.

a Visual-wavelength image

b

Paranal Observatory

Altitude: 2635 m (8660 ft)

Location: Atacama desert of northern Chile

Nearest city: Antofagasta 120 km (75 mi)

■ Figure 6-10

(a) This satellite view of the continental

United States at night shows the light pollu-

tion and energy waste produced by outdoor

lighting. Observatories cannot be located near

large cities. (NASA) (b) The domes of four giant

telescopes are visible at upper left at Paranal

Observatory, built by the European Southern

Observatory. The Atacama Desert is believed to

be the driest place on Earth. (ESO)