120 PART 1^ |^ EXPLORING THE SKY
Such thin mirrors weigh less, are easier to support, and cool faster at
nightfall.
▶ High-speed adaptive optics (p. 109) can monitor distortions caused
by turbulence in Earth’s atmosphere and partially cancel out the blur-
ring caused by seeing.
▶ Interferometry (p. 112) refers to connecting two or more separate
telescopes together to act as a single large telescope that has a
resolution equivalent to that of a telescope as large in diameter as the
separation between the telescopes.
▶ For many decades astronomers used photographic plates to record
images at the telescope, but modern electronic systems such as charge-
coupled devices (CCDs) (p. 113) have replaced photographic plates
in most applications.
▶ Astronomical images in digital form can be computer enhanced and
reproduced as false-color images (p. 113) to bring out subtle details.
▶ Spectrographs (p. 113) using prisms or a grating (p. 113) spread
starlight out according to wavelength to form a spectrum revealing
hundreds of spectral lines (p. 113) produced by atoms in the object
being studied. A comparison spectrum (p. 113) containing lines
of known wavelength allows astronomers to measure wavelengths in
spectra of astronomical objects.
▶ Astronomers use radio telescopes for three reasons: They can detect
cool hydrogen and other atoms and molecules in space; they can see
through dust clouds that block visible light; and they can detect cer-
tain objects invisible at other wavelengths.▶ (^) Most radio telescopes contain a dish refl ector, an antenna, an amplifi er,
and a data recorder. Such a telescope can record the intensity of the
radio energy coming from a spot on the sky. Scans of small regions are
used to produce radio maps.
▶ (^) Because of the long wavelength, radio telescopes have very poor reso-
lution, and astronomers often link separate radio telescopes together
to form a radio interferometer (p. 115) capable of resolving much
fi ner detail.
▶ (^) Earth’s atmosphere absorbs gamma rays, X-rays, ultraviolet, and far-
infrared. To observe at these wavelengths, telescopes must be located
in space.
▶ (^) Earth’s atmosphere distorts and blurs images. Telescopes in orbit are
above this seeing distortion and are limited only by diffraction in their
optics.
▶ (^) Cosmic rays (p. 119) are not electromagnetic radiation; they are sub-
atomic particles such as electrons and protons traveling at nearly the
speed of light. They can best be studied from above Earth’s atmosphere.
Review Questions
- Why would you not plot sound waves in the electromagnetic spectrum?
- If you had limited funds to build a large telescope, which type would
you choose, a refractor or a refl ector? Why? - Why do nocturnal animals usually have large pupils in their eyes? How
is that related to astronomical telescopes? - Why do optical astronomers often put their telescopes at the tops of
mountains, while radio astronomers sometimes put their telescopes in
deep valleys? - Optical and radio astronomers both try to build large telescopes but for
different reasons. How do these goals differ? - What are the advantages of making a telescope mirror thin? What
problems does this cause?
Summary
▶ (^) Light is the visible form of electromagnetic radiation (p. 99), an
electric and magnetic disturbance that transports energy at the speed
of light. The electromagnetic spectrum (p. 100) includes gamma rays,
X-rays, ultraviolet radiation, visible light, infrared radiation, and radio
waves.
▶ (^) You can think of a particle of light, a photon (p. 100), as a bundle of
waves that sometimes acts as a particle and sometimes acts as a wave.
▶ (^) The energy a photon carries depends on its wavelength (p. 99). The
wavelength of visible light, usually measured in nanometers (p. 100)
(10−9 m) or angstroms (p. 100) (10−10 m), ranges from 400 nm to
700 nm (4 000 to 7 000 Å). Radio and infrared radiation (p. 100) have
longer wavelengths and carry less energy. X-ray, gamma-ray, and ultra-
violet radiation (p. 100) have shorter wavelengths and more energy.
▶ (^) Frequency (p. 99) is the number of waves that pass a stationary
point in 1 second. Wavelength equals the speed of light divided by the
frequency.
▶ (^) Earth’s atmosphere is fully transparent in only two atmospheric
windows (p. 101)—visible light and radio.
▶ (^) Astronomical telescopes use a primary lens (p. 101) or mirror
(pp. 101–102) (also called an objective lens or mirror [pp. 101–102])
to gather light and focus it into a small image, which can be magnifi ed by
an eyepiece (p. 102). Short-focal-length (p. 102) lenses and mirrors
must be more strongly curved and are more expensive to grind to shape.
▶ (^) A refracting telescope (p. 101) uses a lens to bend the light and
focus it into an image. Because of chromatic aberration (p. 102),
refracting telescopes cannot bring all colors to the same focus, result-
ing in color fringes around the images. An achromatic lens (p. 102)
partially corrects for this, but such lenses are expensive and cannot be
made much larger than about 1 m in diameter.
▶ (^) Refl ecting telescopes (p. 101) use a mirror to focus the light and are
less expensive than refracting telescopes of the same diameter. Also,
refl ecting telescopes do not suffer from chromatic aberration. Most
large telescopes are refl ectors.
▶ (^) Light-gathering power (p. 103) refers to the ability of a telescope to
produce bright images. Resolving power (p. 104) refers to the ability
of a telescope to resolve fi ne detail. Diffraction fringes (p. 104) in
the image limit the detail visible. Magnifying power (p. 105), the
ability to make an object look bigger, is less important because it can
be changed by changing the eyepiece.
▶ (^) Astronomers build observatories on remote, high mountains for two
reasons. Turbulence in Earth’s atmosphere blurs the image of an astro-
nomical telescope, a phenomenon that astronomers refer to as seeing
(p. 104). Atop a mountain, the air is steady, and the seeing is better.
Observatories are located far from cities to avoid light pollution
(p. 106).
▶ (^) In a refl ecting telescope, light fi rst comes to a focus at the prime
focus (p. 110), but secondary mirrors (p. 110) can direct light
to other focus locations such as a Cassegrain focus (p. 110) or a
Newtonian focus (p. 110). The Schmidt-Cassegrain focus (p. 110)
is popular for small telescopes.
▶ (^) Because Earth rotates, telescopes must have a sidereal drive (p. 111)
to follow the stars. An equatorial mounting (p. 111) with a polar
axis (p. 111) makes this possible, but alt-azimuth mountings
(p. 111) are becoming more popular.
▶ (^) Very large telescopes can be built with active optics (p. 111),
maintaining the shape of fl oppy mirrors that are thin or in segments.