CHAPTER 6 | LIGHT AND TELESCOPES 113
parallel grooves scribed onto its surface. Diff erent wavelengths of
light refl ect from the grating at slightly diff erent angles, so white
light is spread into a spectrum. You have probably noticed this
eff ect when you look at the closely spaced lines etched onto a
compact disk; as you tip the disk, diff erent colors fl ash across its
surface. You could build a modern spectrograph by using a high-
quality grating to spread light into a spectrum and a CCD cam-
era to record the spectrum.
Th e spectrum of an astronomical object can contain hun-
dreds of spectral lines—dark or bright lines that cross the
spectrum at specifi c wavelengths. Th e sun’s spectrum, for
instance, contains hundreds of dark spectral lines produced by
the atoms in the sun’s hot gases. To measure the precise wave-
lengths of individual lines and identify the atoms that produced
them, astronomers use a comparison spectrum as a calibra-
tion. Special bulbs built into the spectrograph produce bright
lines given off by such atoms as thorium, argon, or neon. Th e
wavelengths of these spectral lines have been measured to high
precision in the laboratory, so astronomers can use spectra of
these light sources as guides to measure wavelengths and iden-
tify spectral lines in the spectrum of a star, galaxy, or planet.Imaging Systems
Th e original imaging device in astronomy was the photographic
plate. It could record images of faint objects in long time exposures
and could be stored for later analysis. But photographic plates have
been almost entirely replaced by electronic imaging systems.
Most modern astronomers use charge-coupled devices
(CCDs) to record images. A CCD is a specialized computer chip
containing millions of microscopic light detectors arranged in an
array about the size of a postage stamp. Although CCDs for
astronomy are extremely sensitive and therefore expensive, less
sophisticated CCDs are used in video and digital cameras. Not
only can CCD chips replace photographic plates, but they have
some dramatic advantages. Th ey can detect both bright and faint
objects in a single exposure, are much more sensitive than pho-
tographic plates, and can be read directly into computer memory
for later analysis.
You can easily sharpen and enhance images from your digi-
tal camera because the image from a CCD is stored as numbers
in computer memory. Astronomers can also manipulate images
to bring out otherwise invisible details. Astronomers can pro-
duce false-color images in which diff erent colors represent dif-
ferent levels of intensity and are not related to the true colors of
the object. Or they can use false colors to represent diff erent
wavelengths not otherwise visible to the human eye, as in
■ Figure 6-16. False-color images are so useful they are com-
monly used in many other fi elds, such as medicine and
meteorology.
In the past, measurements of intensity and color were made
using specialized light meters attached to a telescope or on pho-
tographic plates. Today, nearly all such measurements are made
more easily and more accurately with CCD images.
The Spectrograph
To analyze light in detail, astronomers need to spread the light out
according to wavelength to form a spectrum, a task performed by
a spectrograph. You can understand how this instrument works
if you imagine reproducing an experiment performed by Isaac
Newton in 1666. Newton bored a small hole in the window shut-
ter of his bedroom to admit a thin beam of sunlight. When he
placed a prism in the beam, it spread the light into a beautiful
spectrum that splashed across his bedroom wall. From this and
related experiments Newton concluded that white light is made
of a mixture of all the colors.
Light passing through a prism is bent at an angle that
depends on its wavelength. Violet (short wavelength) bends
most, red (long wavelength) least, so the white light is spread
into a spectrum (■ Figure 6-17). You could build a simple spec-
trograph using a prism to spread the light and a lens to guide the
light into a camera.
Nearly all modern spectrographs use a grating in place of a
prism. A grating is a piece of glass with thousands of microscopic
■ Figure 6-16
(a) The Sombrero Galaxy is 29 million light-years from Earth and contains
many billions of stars. This visual wavelength image shows the clouds of dust
in its disk. (NASA, Hubble Heritage Team; STScI/AURA) (b) This infrared image
is reproduced in false color to show different wavelengths of infrared emis-
sion. Red represents the longest wavelengths and blue the shortest. (NASA/
JPL-Caltech, Hubble Heritage Team; STScI/AURA)