The Solar System

116 PART 1^ |^ EXPLORING THE SKY

giant radio interferometers depend on state-of-the-art, high-

speed computers to combine signals and create radio maps.

Th e second handicap radio astronomers face is the low

intensity of the radio signals. You learned earlier that the energy

of a photon depends on its wavelength. Photons of radio energy

have such long wavelengths that their individual energies are

quite low. To get detectable signals focused on the antenna, the

radio astronomer must build large collecting areas either as single

large dishes or arrays of smaller dishes.

Th e largest fully steerable radio telescope in the world is at

the National Radio Astronomy Observatory in Green Bank,

West Virginia (■ Figure 6-21a). Th e telescope has a refl ecting

surface 100 meters in diameter, big enough to hold an entire

football fi eld, and can be pointed anywhere in the sky. Its surface

consists of 2004 computer-controlled panels that adjust to main-

tain the shape of the refl ecting surface.

Th e largest radio dish in the world is 300 m (1000 ft) in

diameter. So large a dish can’t be supported in the usual way, so

it is built into a mountain valley in Arecibo, Puerto Rico. Th e

refl ecting dish is a thin metallic surface supported above the val-

ley fl oor by cables attached near the rim, and the antenna hangs

above the dish on cables from three towers built on three moun-

tain peaks that surround the valley (Figure 6-21b). By moving

the antenna above the dish, radio astronomers can point the

telescope at any object that passes within 20 degrees of the zenith

as Earth rotates. Since completion in 1963, the telescope has

been an international center of radio astronomy research.

Th e third handicap the radio astronomer faces is interfer-

ence. A radio telescope is an extremely sensitive radio receiver

listening to faint radio signals. Such weak signals are easily

drowned out by interference that includes everything from poorly

designed transmitters in Earth satellites to automobiles with

faulty ignition systems. A few narrow radio bands in the electro-

magnetic spectrum are reserved for radio astronomy, but even

those are often contaminated by radio noise. To avoid interfer-

ence, radio astronomers locate their telescopes as far from civiliza-

tion as possible. Hidden deep in mountain valleys, they are able

to listen to the sky protected from human-made radio noise.

Radio energy map

Red strongest

Violet weakest

PartlyPartly

cloudycloudy

Rain/Rain/

windwind

Rain/Rain/

iceice

MixMix

IsolatedIsolated

WindyWindyT-stormsT-storms

SnowSnow

showersshowers

FewFew

showersshowers

ShowersShowers

Partly

cloudy

Rain/

wind

Rain/

ice

Mix

Isolated

WindyT-storms

Snow

showers

Few

showers

Showers

a

b

■ Figure 6-20

(a) The Very Large Array uses 27 radio

dishes, which can be moved to different

positions along a Y-shaped set of tracks

across the New Mexico desert. They

are shown here in the most compact

arrangement. Signals from the dishes are

combined to create very-high- resolution

radio maps of celestial objects. (NRAO)

(b) The proposed Square Kilometer Array

will have a concentration of detectors

and radio dishes near its center with

more dishes scattered up to 3000 km

away. (Xilostudios/SKA Program Development

Offi ce)

■ Figure 6-19

(a) A typical weather map uses contours with added color to show which
areas are likely to receive precipitation. (b) A false-color-image radio map
of Tycho’s supernova remnant, the expanding shell of gas produced by the
explosion of a star in 1572. The radio contour map has been color-coded to
show intensity. (Courtesy NRAO)

a b