The Solar System

CHAPTER 6 | LIGHT AND TELESCOPES 115

moon. Fortunately, radio astronomers can combine two or more

radio telescopes to form a radio interferometer capable of much

higher resolution. Just as in the case of optical interferometers,

the radio astronomer combines signals from two or more widely

separated dishes and “fools the waves” into behaving as if they

were collected by a much bigger radio telescope.

Radio interferometers can be quite large. Th e Very Large

Array (VLA) consists of 27 dish antennas spread in a Y-shape

across the New Mexico desert (■ Figure 6-20). In combination,

they have the resolving power of a radio telescope 36 km (22 mi)

in diameter. Th e VLA can resolve details smaller than 1 arc sec-

ond. Eight new dish antennas being added across New Mexico

will give the VLA ten times better resolving power. Another large

radio interferometer, the Very Long Baseline Array (VLBA), con-

sists of matched radio dishes spread from Hawaii to the Virgin

Islands and has an eff ective diameter almost as large as Earth. Th e

Allen Telescope Array being built in California will eventually

include 350 separate radio dishes. Radio astronomers are now

planning the Square Kilometer Array, which will contain a huge

number of radio dishes totaling a square kilometer of collecting

area and spread to a diameter of at least 6000 kilometers. Th ese

must be scanned over an object to produce a map of the radio
intensity at diff erent points. Such radio maps are usually repre-
sented using contours to mark areas of similar radio intensity,
much like a weather map where contours fi lled with color indi-
cate areas of precipitation (■ Figure 6-19).

Limitations of a Radio Telescope

A radio astronomer works under three handicaps: poor resolu-
tion, low intensity, and interference. Recall that the resolving
power of an optical telescope depends on the diameter of the
objective lens or mirror. It also depends on the wavelength of the
radiation because, at very long wavelengths like those of radio
waves, the diff raction fringes are very large. Th at means a radio
map can’t show fi ne detail. As with an optical telescope, there is
no way to improve the resolving power without building a bigger
telescope. Consequently, radio telescopes generally have large
diameters to minimize the diff raction fringes.
Even so, the resolving power of a radio telescope is not good.
A dish 30 m in diameter receiving radiation with a wavelength of
21 cm has a resolving power of about 0.5°. Such a radio telescope
would be unable to detect any details in the sky smaller than the

Amplifier Computer

Dish reflector

Cable

Antenna

■ Figure 6-18

In most radio telescopes, a dish refl ector concentrates the radio signal on the antenna. The signal is then amplifi ed and recorded. For all but the

shortest radio waves, wire mesh is an adequate refl ector.