The Astronomy Book

327

See also: Galileo’s telescope 56–63 ■ Gravitational theory 66–73 ■ Space telescopes 188–95 ■
Studying distant stars 304–05

THE TRIUMPH OF TECHNOLOGY

funding for the European Extremely
Large Telescope (E-ELT). When
completed in 2024, this will be
the largest optical telescope ever
built, with a resolution 15 times
sharper than the Hubble Space
Telescope (pp.172–77).

Giant mirror
The E-ELT has an unusual five-
mirror design housed inside a
dome half the size of a football
stadium. The primary mirror (M1),
which collects the visible light
(and near infrared) is built from
798 hexagonal segments that are
4 ft 10 in (1.45 m) wide. Together
they will make a mirror that is 129 ft
(39.3 m) across. In contrast, the
Hubble’s primary mirror is just
7 ft 11 in (2.4 m) wide; even the
E-ELT’s secondary mirror (M2) is
larger than that, at 13 ft 10 in (4.2 m).
The shape of M1 can be fine-
tuned to account for distortions
caused by temperature changes,
and by the gravitational effect as
the telescope swings into different
positions. M2 directs the light from
M1 through a hole in the fourth
mirror (M4) onto the third mirror
(M3). From there light is reflected
back onto M4, the adaptive optics
mirror, which greatly reduces
atmospheric blurring of the image.
M4 follows the twinkling of an

artificial “star,” which is created

by firing a laser into the sky. M4

can alter its shape 1,000 times a

second using 8,000 pistons housed

underneath. In other words, the 798

segments of this astonishing mirror

can ripple and warp in real time

to counteract any atmospheric

distortions. Finally, M5 directs

the image into the camera.

The E-ELT will pick up a

narrower band of the spectrum

than space telescopes, but it can

do so on a much larger scale. As

a result, the E-ELT will be able

to see exoplanets, protoplanetary

discs (including their chemistry),

black holes, and the first galaxies

in greater detail than ever before. ■

At the heart of the E-ELT’s complex arrangement of mirrors is the huge

dish of the primary mirror. It will gather 13 times more light than the largest

existing optical telescopes, and will be aided by six laser guide star units.

The E-ELT’s dome is shown opening

as the sun sets over the desert in this

artist’s impression. The completed

structure will be 256 ft (78 m) high.

Primary

mirror

(M1)

Secondary

mirror (M2)

Third

mirror

(M3)

Fifth

mirror

(M5)

Fourth

mirror (M4)