The Solar System

CHAPTER 6 | LIGHT AND TELESCOPES 103

Its lens is 1 m (40 in.) in diameter and weighs half a ton. Larger

refracting telescopes are prohibitively expensive.

Th e primary mirrors of refl ecting telescopes are much less

expensive because the light refl ects off the front surface of the mir-

ror. Th is means that only the front surface needs to be ground to

precise shape. Th is front surface is coated with a highly refl ective

surface of an aluminum alloy, and the light refl ects from this front

surface without entering the glass. Consequently, the glass of the

mirror need not be perfectly transparent, and the mirror can be

supported over its back surface to reduce sagging. Most impor-

tant, refl ecting telescopes do not suff er from chromatic aberration

because the light is refl ected before it enters the glass. For these

reasons, every large astronomical telescope built since the begin-

ning of the 20th century has been a refl ecting telescope.

The Powers of a Telescope

Astronomers build large telescopes because a telescope can aid

your eyes in three ways—the three powers of a telescope—and

the two most important of these powers depends on the diameter

of the telescope.

Nearly all of the interesting objects in the sky are faint

sources of light, so astronomers need telescopes that can gather

large amounts of light to produce bright images. Light-gathering

power refers to the ability of a telescope to collect light. Catching

light in a telescope is like catching rain in a bucket—the bigger

the bucket, the more rain it catches (■ Figure 6-7). Light-

gathering power is proportional to the area of the telescope

Blue

image

Single lens

Red

image

Yellow

image

b

a

Achromatic lens Red and

yellow

images

Blue

image

■ Figure 6-6

(a) A normal lens suffers from chromatic aberration because short wave-
lengths bend more than long wavelengths. (b) An achromatic lens, made in
two pieces of two different kinds of glass, can bring any two colors to the
same focus, but other colors remain slightly out of focus.

Primary

lens

Eyepiece

a b

Secondary

mirror

Primary

mirror

Eyepiece

■ Figure 6-5

(a) A refracting telescope uses a primary lens to focus starlight into an image
that is magnifi ed by a lens called an eyepiece. The primary lens has a long focal
length, and the eyepiece has a short focal length. (b) A refl ecting telescope
uses a primary mirror to focus the light by refl ection. A small secondary mirror
refl ects the starlight back down through a hole in the middle of the primary
mirror to the eyepiece.

Light

Light

■ Figure 6-7

Gathering light is like catching rain in a bucket. A large-diameter telescope

gathers more light and has a brighter image than a smaller telescope of the

same focal length.