In this chapter, we will change directions slightly--leaving a discussion of motion as in Chapters 2 and 3--to discuss the theory of light. In his Optics published in 1637, Rene' Descartes asserted that the propagation of light is accomplished by the movement from one place to another of a disturbance through some all pervasive medium. He had in mind a very mechanical model, which later proved to be artifical and awkward. Descartes strenuously rejected the notion that, "something material passes from the objects to our eyes to make us see colors and light." Newton took Descartes mechanical model in a much more literal sense, and proposed a particle theory of light. Many of Descartes' and Newton's ideas either were at the time or were later shown to be inconsistent with experimental evidence. It was Christian Huygens in his book, Treatise on Light, published in 1690, who was able to answer most of the experimental facts concerning light by asserting that light was a wave phenomenon moving at very high speeds.
By the late nineteenth century, understanding of our complex world through Newton's mechanics seemed to reveal an amazing unity. Newtonian concepts had been pushed to the atomic realm in a kinetic theory of molecular motion, and sound was well understood to be mechanical vibrations in air. Early hopes that light, electricity, and magnetism would also be explained in mechanical terms through Newtonian concepts were, however, never realized. James Clerk Maxwell was able to unify these three fields in a beautifully successful electromagnetic theory of light. But it did not encompass Newtonian mechanics. Maxwell's theory was only the beginning of a revolution in the concepts of physical science brought on by the study of light. This revolution has shown that Newtonian concepts, in spite of their many successes, are only part of the story.
4.1. Waves and the Transport of Energy
4.1.1. Definition of a Wave
Astronomers have learned most of what we know about stars and galaxies by analyzing the electromagnetic radiation coming from them. Electromagnetic radiation is a form of energy, and the light to which our eyes respond is but one part of it. Since it has no material aspects, electromagnetic radiation is energy that can move through the empty reaches of the Universe.
Experience tells us that energy must be capable of being moved
from place to place, that is, energy is transported by some mechanism
from one location to another. Waves are one way of transporting
energy. What is a wave? It is a moving disturbance. How does
this account for the transport of energy? Imagine that two people
several feet apart hold the ends of a rope; when one jiggles the
rope, a wave travels from one end of the rope to the other.
Particles are not being conveyed from one end of the rope to the
other, but what is moving is a disturbance. We know that energy
is transported by the disturbance because when the disturbance
arrives, the receiving hand is jiggled. That is, the wave in
the rope does work on the hand, giving it kinetic energy as it
is set into motion. Another example of a wave is the disturbance
that propagates across the surface of a pond after a stone is
dropped into the water. A wave can thus be defined as
a disturbance that transports energy from one point to another.
4.1.2. Properties of Waves
To understand waves better, consider they are described quantitatively. In Figure 1, the distance between successive crests or troughs is called the wavelength. The number of complete cycles of the disturbance passing a fixed point per second is called the frequency of the wave. The velocity of the wave is the distance it travels per unit of time; this is just the length of each wave (its wavelength) multiplied by the number of waves passing a fixed point per unit of time (its frequency).
The last quantity used to describe a wave is its amplitude. This is the greatest height the crests reach or the greatest depth to which the troughs fall. Let us call the amplitude of the crest positive and the amplitude of the trough as negative, so that the undisturbed position is zero. The energy transported by the wave is proportional to the square of the amplitude. For example, tripling the amplitude increases the energy carried by the wave by a factor of nine, whereas halving the amplitude decreases the energy to one-fourth the original amount.
Returning to our experience of watching waves on the surface
of a pond, one notes that the wave moves out in all 360o of direction
across the surface. The crests of the wave are in the form of
circles, and they in turn are followed by circular troughs; thus
we see alternating concentric crests and troughs. Our eyes tend
to follow the moving crests, which we call wavefronts.
Perpendicular to the wavefront defined by the crests, that is,
in the radial directions of the circular crests, is a ray
that shows the direction that that portion of the wave is moving.
One aspect of waves which is of great importance to us is the
concept of superposition. If we drop two stones into a pond at
different points, we notice that the wavefronts defined by the
crests of the two waves penetrate each other and pass unaffected
through the other wave. We can state this concept as a mathematical
principle of superposition as follows:
A combined wave is called a composite wave, meaning that it is
composed of two or more. We can form composite waves out of individual
waves that have different wavelengths, that have different amplitudes,
that have been shifted slightly relative to each other, such as
crest to trough, or that are going in the same or opposite directions.
Many individuals speaking in a room at one time is a good example
of the importance of superposition. For if the principle of superposition
was not obeyed by sound waves, the multiple sound waves would
distort each other and everyone would hear only incoherent noise,
not speech.
Historically, the concept of electromagnetic radiation being
waves began in 1862, when the Scottish physicist James Clerk Maxwell
(1831-1879) showed that light is energy carried in the form of
a traveling wave composed of electric and magnetic fields. The
electric and magnetic fields vary in intensity and are at right
angles to each other and to the direction in which the wave is
propagating (Figure 2). The electric and magnetic fields continually
interact with each other to form the electromagnetic wave.
While maintaining themselves, these fields continue to propagate
until the energy of the wave is converted into some other form
of energy. At that point, the electromagnetic wave ceases to
exist. Electromagnetic radiation in the natural world occurs
over a wide range of wavelengths or, equivalently, a wide range
of frequencies. The product of the wavelength and frequency gives
the velocity at which the electromagnetic wave travels. The amount
of energy the wave transports is proportional to the square of
the wave's amplitude.
In his Two New Sciences, Galileo suggested that the velocity
of light is finite rather than infinite, but very large compared
with sound velocities. The first definite evidence that light
moves at a finite velocity, however, was found by Danish astronomer
Ole Roemer (1644-1710). It is now-known that the velocity of
light measured in empty space is 299,792 km/s (3 x 105 km/s in
round figures, or 186,300 mi/s). All scientific knowledge gained
thus far indicates that this is the upper limit for the velocity
at which energy can be transported in the Universe. This makes
the speed of light a fundamental constant of nature, which apparently
has the same value throughout the Universe.
Maxwell's proposal that light is an electromagnetic wave, as
we shall see, was not the last word in attempting to infer the
physical nature of light from its observed properties. Visualizing
light as waves spreading out from a radiating source, however,
helps us to understand many aspects of it.
If we arrange the entire sequence of wavelengths for electromagnetic
radiation, such that the shortest is on the left and the longest
is on the right, we produce what is called the electromagnetic
spectrum, as shown in Figure 4. Toward the short-wavelength
end is the very limited portion to which our eyes are sensitive,
called the visible spectrum, or visible light, or
just light. The physiological response of the eye to the
various wavelengths composing the visible spectrum results in
what we perceive as the color spectrum.
Short wavelengths in the visible spectrum are violet, with progressively
longer wavelengths producing the response we identify as the range
of hues from blue, green, yellow, and orange to red in the color
spectrum. Visible light is electromagnetic radiation with wavelengths
between approximately 35 x 10-6 and 70 x 10-6 cm. These wavelengths
correspond to frequencies between 8.5 x 1014 and 4.3 x 1014 hertz
(Hz). (One hertz equals one cycle, or oscillation, of
the wave per second.) The lowest frequencies of visible light
appear red to our eyes, the highest frequencies appear violet,
and between these is the rest of the color spectrum.
All types of electromagnetic radiation display those properties
associated with waves; for example, all propagate in the same
way with the same speed in empty space, and all transport energy.
For convenience, however, we divide the nonvisible portions of
the electromagnetic spectrum into regions according to wavelength,
such as the ultraviolet or the infrared and so on. We label these
different regions not because of any intrinsic difference in the
radiation, but because we have different ways of detecting radiation
depending on its wavelength. (These detection methods are discussed
in later chapters.) Gamma rays, X-rays, and ultraviolet radiation
constitute the regions with wavelengths shorter than visible light,
whereas infrared, microwave, and radio radiation constitute the
regions with wavelengths longer than visible light.
Because of the wide range in the numerical value of wavelengths,
some units of measurement are more convenient than others for
describing a region of the electromagnetic spectrum. For the
visible spectrum, angstroms are convenient. An angstrom
(A) is a hundred-millionth of a centimeter (1 A = 10-8 cm). Visible
radiation lies approximately between 3500 A (the violet end of
the spectrum) and 7000 A (the red end). X-rays are also measured
in angstroms, but infrared wavelengths are generally expressed
in microns (1 µm = 104 A = 10-4 cm). Astronomers use the
hertz as the unit for measuring frequency for all electromagnetic
radiation.
From his theoretical study of the emission of radiation by ideal
radiators (blackbodies) Max Planck (1858-1947), a German physicist,
concluded that they do not emit or absorb radiant energy in a
continuous fashion but only discontinuously in discrete units,
which later were called photons. This means that the energy
transported by an electromagnetic wave is not continuously distributed
over the wavefront defined by the crests; on the contrary, the
energy is located at discrete points, the photons, along the wavefront.
In 1905, Einstein used Planck's idea of a discrete nature for
the emission of light to explain a phenomenon discovered in 1887
known as the photoelectric effect. This effect cannot
be understood if light has only a wave nature. Since that time,
an extensive body of experimental and theoretical evidence has
been collected to verify the photon concept, that is, that light
does indeed exhibit a discrete nature.
What are some of the properties of photons? They move with the
velocity of light, travel in straight lines, are electrically
neutral, are massless, and the energy content in each photon is
inversely proportional to its wavelength. The shorter the wavelength,
the more energetic is the photon; the longer the wavelength, the
less energetic is the photon.
Picture a radiating body as emitting photons of differing discrete
amounts of energy in all directions (Figure 5). The photons
are created inside atoms of the radiating body from which they
receive their energy content. While traveling through space,
their energy content remains constant. When photons encounter
matter, they may be absorbed by its atoms, and in so doing they
lose their identity by transferring their energy to the atom.
The creation and destruction of photons by atoms is a classic
example of the conservation of energy.
The concept of light as being simultaneously discrete photons
and continuous waves seems self-contradictory and totally contrary
to experience. For when we think of discrete entities, such as
marbles or pebbles, applicable concepts, such as size, precise
location, etc, come to mind. But for massless photons, such concepts
have no meaning. As the physicist Max Born said, "The ultimate
origin of the difficulty lies in the fact (or philosophical principle)
that we are compelled to use the words of common language when
we wish to describe a phenomenon, not by logical or mathematical
analysis, but by a picture appealing to the imagination. Common
language has grown by everyday experience and can never surpass
these limits." When we resort to laboratory experiments
to resolve the contradictions, we find that laboratory experiments
are designed to inquire about either light's wave nature or its
corpuscular nature; no experiment will simultaneously yield the
discrete and the wave properties of light. Again quoting Born,
"We can therefore say that the wave and corpuscular descriptions
are only to be regarded as complementary ways of viewing one and
the same objective process, a process which only in definite limiting
cases admits of complete pictorial interpretation...."
Again we must remind ourselves that the human mind tries to picture
the world in terms of mechanistic models from everyday experience
and not mathematical images. And we must continually struggle
not to revert to the position that unless the concept is consistent
with that experience, the concept is meaningless and reveals no
element of reality. Wavelength is a characterization of the wavelike
properties of light, while the energy content of a photon refers
to its discrete nature. The fact that we can link wavelength
and energy content in a mathematical equation (Ephoton = hc/wavelength) is strong argument in favor of the duality--simultaneously wave
and photon--we find in the nature of light.
Light traveling through empty space moves in a straight line.
In everyday experience we encounter light not in empty space
but passing through various media, such as the air of the Earth's
atmosphere, dust or water vapor clouds, pools of water, windows,
or telescopes. Under these circumstances, the velocity of light
may be slowed, and the direction of a light wave may be changed.
These changes are best understood through the wave properties
of light rather than its photon properties.
Several properties illustrate the wave characteristics of light.
One is reflection, which occurs when light strikes the
boundary between two different materials, such as glass and air.
When a light ray moving in air reaches such a boundary, part
of it may be reflected, as shown in Figure 6. The reflected
ray lies in the plane formed by the incident ray and the perpendicular
to the boundary. The ordinary mirror, or looking glass, is an
illustration of reflection.
In addition, part of the incident ray may be transmitted through
the glass rather than being reflected. The transmitted ray does
not, however, continue along the same straight line; it is bent
toward the perpendicular (shown as the dashed line in Figure 6).
This change in direction is called refraction. If the
medium into which the ray moves is more dense than that from which
it comes, the angle of refraction will be less than the angle
of incidence. If its density is less, then the angle of refraction
will be greater. A good example of refraction can be seen by
placing a spoon in a glass of water. The handle looks bent at
the point where the spoon enters the water because part of the
handle is in the same medium (air) as you and part is under water,
so light must pass through the water-air boundary where it is
refracted.
Light shows another wave property, diffraction, which
is the spreading out of light past the edges of an opaque body
(Figure 7). Instead of being propagated strictly in a straight
line, light, like sounds waves, bends around corners. The spread
is greater for longer wavelengths. Because the wavelengths of
visible light are very small, we do not normally observe diffraction
in the everyday world. However, one example is to observe a distant
street light through an ordinary window screen. By adjusting
one's distance from the screen, one can see alternating light
and dark diffraction rings surrounding the street light.
Nearly all natural light sources, such as stars, emit electromagnetic
waves composed of many wavelengths. How do waves of different
wavelengths add to produce a composite wave? If waves of the
same wavelength from two sources are superimposed so that their
crests and troughs coincide, they are said to be in phase
with each other, and their amplitudes add to produce a sum greater
than the amplitudes of the individual waves; the light is said
to "interfere constructively." If the crests of one
set of waves fall on the troughs of the other, they are said to
be out of phase with each other, and their amplitudes cancel
each other; the light is said to "interfere destructively."
Interference is common to all types of waves. In fact, its occurrence
was strong evidence that light is a wave phenomenon. Light waves
of one or many different wavelengths may interfere constructively
or destructively. Such waves are called composite waves,
or white light, since that is the physiological response
they evoke. Stars, for example, are white-light sources, although
the color of the composite light from the stars may be white,
red, yellow, or blue (see Table 15.5). If we can add waves together,
then we must also be able to separate a composite wave into its
constituent wavelengths. Indeed we can, as we shall discuss in
Chapter 5.
The surface area illuminated by an expanding sphere of light
(or a portion of it) increases as the square of the radius of
the sphere, that is, as the square of the distance from the light
source. Since the total amount of energy leaving, say, the Sun
(Figure 8) in all directions in space is the same at all distances,
the amount of radiation passing through each unit of area of the
expanding sphere of light must diminish with the square of the
distance.
For example, suppose that at 1 AU from the Sun the apparent brightness
of the radiation over 1 km2 of surface area is 1 unit. At 2 AU,
each square kilometer will receive 1/4 of a unit of illumination;
at 3 AU, 1/9 of a unit; at 4 AU, 1/16 of a unit; and so on. This relationship between apparent brightness and distance is known as the inverse-square law of light:
This law is applied in many kinds of astronomical work, as we
shall see in Chapter 15 on.
If an observer is moving relative to a source of waves, such
as a source of light waves, or the source is moving relative to
the observer, then the observer will experience or measure a change
in the wavelength of the wave. For example, this familiar effect
can be heard as the rising and falling pitch (frequency) of race
car engines at the Indianapolis 500 as cars approach and then
move away. This phenomenon is known as the Doppler effect,
named for Christian Doppler (1803-1853), the Austrian physicist
who first explained it. For electromagnetic radiation The Doppler
effect can be stated as follows:
To help us understand, suppose a stationary light source, such
as a star (Figure 9), is radiating concentric waves of one wavelength
in all directions. Then observers in any direction, if stationary,
would see successive crests of the wave passing them at the same
rate at which they were emitted by the star. If, however, the
star begins to move at uniform velocity to the right, two observers
O and P along the line of motion would see crests passing them
at rates different from that with which they were emitted. To
see why, assume that wavefront 1 was produced when the star was
at position 1, wavefront 2, when it was at position 2, and so
on. Because of the greater distance the wave travels in reaching
observer O, each successive wave crest passes him or her at a
slower rate (lower frequency) than when the star was stationary.
Because the waves travel a shorter distance to reach P, the successive
crests pass at a faster rate (higher frequency). The wavelength
is shifted toward longer wavelengths (redshifted) as the star
recedes from O and toward shorter wavelengths (blueshifted) as
the star approaches P. Observers Q and R, located at right angles
to the moving star, would detect no change in the rate for crests
passing them. Observers elsewhere would notice some change, the
amount depending on the angle between their radial direction to
the star and the line of motion.
It is immaterial whether the light source is in motion, or the
observer, or both: The size of the Doppler effect found depends
only on the net relative motion along the line of sight between
the source and the observer. The amount of the wavelength shift
due to the Doppler effect is directly proportional to the velocity
of approach (blueshift) or recession (redshift) as long as the
relative velocity is well below the velocity of light. (In Chapter
26, we will discuss the Doppler effect when the relative velocity
is a substantial fraction of the velocity of light.) The constant
of proportionality is the ratio of undisplaced wavelength to the
velocity of light. This means that the wavelength shift is greater
the longer the wavelength of the radiation. Since all bodies
in the Universe are moving, the Doppler effect is an important
tool for detecting and measuring the amount of that motion along
the line of sight.
Having a foundation now in the properties of light, we can procede
to consider the means by which astronomer collect and analyze
light from astronomical bodies. The basic collecting instrument
is the telescope which we will discuss in this and the following
sections. Telescopes, as well as other optical instruments, depend
for their operation on the wave properties of light. On the other
hand, the analysis of light to extract information about the light
source requires some further study of matter and radiation which
we will undertake in the next chapter and in Chapter 14..
In optical astronomy, astronomers work with the image
of the light source formed by the principal image-forming part
of the telescope, which is called the objective; the objective
is either a lens or a mirror (Figure 10). Light rays from the
light source are refracted in passing through a lens and are reflected
from a mirror. The image is formed where light rays converge
to a position known as the focus. The focal length
of the objective is the distance behind the lens to the focus
or the distance in front of the mirror to the focus. The telescopic
image of a star is just a point of light, while that of an extended
object, such as the Moon, is extended but inverted, as shown in
the figure.
In telescopes using either mirrors or lenses, an eyepiece, another
small lens, is used to magnify the image much as a magnifying
glass enlarges small print. Or instead of an eyepiece, a photographic
plate may be inserted into the focal plane, transforming the telescope
into a camera, where the objective serves as the camera lens.
The advantage of photography over observing with the eye is that
time exposures can record fainter objects than those the eye sees,
and in addition, the photograph is available for later study.
The image formed by either a lens or a mirror has certain properties
that depend on the diameter of the objective, or aperture,
and its focal length. One property is the size of the image.
Since the image of a star is a point, size is not an important
consideration. But for an extended object, such as a galaxy,
the image size depends on the angular size of the galaxy on the
sky and on the focal length of the objective.
The brightness of the image is important because it determines
whether the object can be seen and how long it will take to photograph.
The brightness of an image of a star depends on how much
light is intercepted by the objective. Hence its brightness is
proportional to the area of the objective or to the square of
the aperture. Doubling the aperture but leaving the focal length
the same increases the area of the objective or its light-gathering
power four times, concentrating four times as much light into
the same-size image.
When photographing a galaxy, the image brightness depends on
the amount of radiant energy per unit area of the image. The
objective's area (or the square of its aperture) still determines
the total amount of energy collected, but the total energy is
distributed over an extended image. Thus the larger the image's
area, the smaller the energy per unit of area. The image size
of a galaxy increases in proportion to the focal length, so for
a given telescope aperture the surface brightness of the image
decreases as the focal length is made longer.
How well a telescope can discriminate between two objects close
together on the sky or can bring out fine details in an extended
object is called its resolving power. Because of the wave
nature of light, the image of a star is actually a diffraction
pattern (Figure 7); it appears as a bright central spot,
called a diffraction disk, surrounded by progressively
fainter rings. When the diffraction patterns of two stars that
are close together no longer overlap, we can see separate stellar
images, as shown in Figure 11. The larger the telescope's aperture,
the smaller the diffraction disk of each image. A large aperture
therefore improves the resolution of closely adjoining features
by making the diffraction effect of adjacent objects overlap less.
We define resolving power as the smallest angle between
two close objects whose images can just be separated by a telescope.
This critical angle is directly proportional to the wavelength
of the observed radiation and inversely proportional to the aperture
of the objective.
Telescopes that use lenses for the objective are known as refracting
telescopes, whereas those which employ a mirror are called
reflecting telescopes. The objectives of early refracting
telescopes could not form sharp images because of a condition
known as spherical aberration; these single lenses also
failed to bring all colors to a common focus, a condition known
as chromatic aberration. These conditions can now be minimized
by using a compound lens, or two lenses of different types
of glass cemented together, as the objective in refracting telescopes.
Spherical aberration also occurs in reflecting telescopes. If
the surface of the mirror is parabolic rather than spherical,
then spherical aberration is eliminated, although some minor deficiencies
still remain.
Why are the big modern telescopes of the reflecting type? Reflecting
telescopes have many advantages over refractors: The reflecting
telescope is free from chromatic aberration, making it ideal for
all-purpose photography and spectroscopy. Also, since a lens
must be supported by its edges, there is a limit to the size of
a lens that will not break from its own weight. But a mirror,
such as the one shown in Figure 12, can be supported both at
its edges and from the back, and such a means of support allows
much larger mirrors to be built than lenses. The largest refractor
has an aperture slightly over 1 m, but the largest reflector is
6 m in diameter.
There are other advantages to reflectors: The glass for the mirror
in a reflecting telescope need not be so optically pure as that
required for a large lens because the light reflects off the front
surface and does not pass through the mirror, as it does through
a lens. In addition, a mirror has only one surface that must
be painstakingly ground--a compound lens has four. To counter
changes in temperature that would affect the focal length of the
reflector, large mirrors are constructed of fused quartz or of
a zero-expansion pyroceramic material. The mirror's surface is
coated with a thin layer of highly reflecting aluminum that is
replaced many times during the life of the telescope.
Reflecting telescopes can be designed for many kinds of astronomical
work through choice of the focal arrangement (Figure 13) to
suit the type of observation. For photography, photometry, and
spectroscopy of faint objects, the prime focus is best
because its small focal length lessens the exposure time required.
The Newtonian focus, most useful for small telescopes,
is now little used by professional astronomers. In both these
arrangements, the observer works at a considerable distance above
the observatory floor, since both focal positions are near the
entrance of the telescope.
In the Cassegrain focal arrangement, a secondary mirror
at the entrance of the telescope is used to slow the rate at which
light rays converge after reflecting off the objective mirror,
effectively increasing the telescope's focal length. The secondary
mirror reflects the converging rays to the bottom of the telescope
and through a hole in the objective mirror to a focus behind the
objective. This is a much more convenient observing position
since it is near the floor and behind the telescope. Of all the
observations made with the 5-m Hale telescope on Palomar Mountain,
75 percent are from the Cassegrain focus.
One might think that putting the secondary mirror and its supports
or the observer's cage for the prime focus into the path of the
light rays would obscure part of the image, but the only effect
is to cut down the amount of light reaching the objective. The
loss is small, and the quality of the image is not affected.
Equipment that is too heavy and bulky to be attached to the back
of the primary mirror or is sensitive to changes in gravity as
the telescope moves can be placed in a room below the observatory
floor. Through the use of an auxiliary flat mirror, the long
converging beam from the primary mirror can be diverted down the
hollow polar axis around which the telescope rotates and into
the room below. With this coude focal arrangement, the
focal position always remains the same no matter which way the
telescope points.
An optical telescope, in order to follow an object as the Earth's
rotation carries it across the sky, must be free to move. In
order to track stars accurately and to permit a telescope to be
pointed in any direction, an equatorial mounting system
is used for most telescopes (Figure 14). This system has two
axes of rotation: The telescope can be made to rotate in an east-west
sense, called hour-angle, around its polar axis, which
is aligned with the Earth's axis of rotation; the declination
axis, which is perpendicular to the polar axis, is used to rotate
the telescope in a north-south sense. (For a discussion of astronomical
coordinates, see Appendix 2.)
Large telescopes are usually pointed by a computer from an operating
console and guided thereafter with hand controls. Once a large
telescope is properly pointed, the computer operates as a clock
to slowly turn the telescope westward around its polar axis at
the same rate as the Earth turns eastward, thereby keeping the
area of interest always in the telescope's field of view. The
great simplicity in an equatorial mounting is that tracking requires
continuous motion about only one of its two axes. The disadvantage,
which applies only to the largest telescopes now in operation
and planned for the future, is that the polar axis is inclined
in the Earth's gravitational field and must rotate on one edge
of its end. In this position, gravity creates a large mechanical
stress on the polar axis that presents a very difficult engineering
problem.
One means of preventing some of the mechanical stress on the
polar axis is to align it with gravity. Such a mounting is known
as altazimuth mounting; with it a telescope rotates about
a vertical axis and about a horizontal axis. This mounting's
disadvantage is that unlike the equatorial mounting, it must turn
continuously about both axes at the same time in order to track
a star. When the telescope approaches the area of the sky directly
overhead, continuous tracking becomes virtually impossible. Even
with this disadvantage, the altazimuth mounting will be the primary
mounting for very large telescopes to be constructed in the future.
The geographic location of an observatory are important in order
to obtain all the capability built into the telescope and its
mounting. An ideal site for an optical observatory is a mountaintop
where turbulent motions in the atmosphere are minimal, the air
is dry and transparent, and the sky is dark. The southwestern
part of the United States or spots along the West Coast are satisfactory
locations, in addition to having many clear days and nights.
Kitt Peak National Observatory is located in such a site, on a
mountaintop, about 65 miles southwest of Tucson, Arizona, and
is shown in Figure 15.
The principal problems in building very large telescopes on the
Earth's surface today are cost and construction time. A new 5-m
Hale telescope would now cost about $25 million and take 10 years
to build, while a 10-m telescope would cost $200 million and take
20 years to build and a 25-m telescope would cost about $3 billion
and take 50 years to build. Clearly some dramatic changes in
design are needed to lower cost and construction time.
A new telescope design, called the Multiple-Mirror Telescope
(Figure 12), has been installed by the Smithsonian Institution
and the University of Arizona in southern Arizona. It uses a
mosaic of independent mirrors of small size coordinated by laser
beams to collect and focus light in order to simulate the collecting
ability of a large-aperture single mirror. The telescope consists
of a circular array of six identical 1.8-m mirrors on an altazimuth
mounting; the array has light-gathering power equivalent to that
of a 4.5-m single mirror. The six mirrors are not thick solid
ones but are of a new lightweight design, as shown in Figure 12.
They are partially hollow, which requires a smaller mechanical
structure to move them. Thus the cost of the Multiple-Mirror
Telescope was about one-third that of a conventionally designed
telescope, and it required less time to build. This instrument
has been successful in demonstrating the practicality of the multiple-mirror
concept.
The Multiple-Mirror Telescope is not the only new design; others
are displayed in Figure 16, which presents artist conceptions
of new designs for future large telescopes. One new design that
will definitely be built is the University of California's 10-m
segmented-mirror telescope, which will hopefully go into operation
in the early 1990s atop Mauna Kea, Hawaii. Using the multiple-mirror
design, a second very large telescope, known as the 15-m National
New Technology Telescope, has been proposed and will begin construction
in fiscal year 1988 if funded. Although it has been argued that
the success of Space Telescope, a 2.4-m reflecting telescope
that is to be put into orbit in early 1988, will lessen the need
for a mammoth new telescope on the ground, the reverse is probably
true--it will increase the need. Since Space Telescope
will not be limited by light losses produced by the atmosphere,
it will primarily operate at shorter wavelengths than the visible.
Hence it would be desirable to have a very large telescope on
the ground working in conjunction with Space Telescope
to observe in the visible region of the spectrum.
4.1.3. Superposition of Waves
Superposition Principle: When two or more waves move
simultaneously through a particular region of space, each wave
proceeds independently, as if the other were not present. The
resulting amplitude of the combined waves is just the algebraic
sum of the amplitudes of the individual waves.
4.2. Electromagnetic Radiation
4.2.1. Light as Waves
4.2.2. The Electromagnetic Spectrum
4.2.3. Photons: The Discrete Nature of Light
4.3. Wave Properties of Light
4.3.1. Reflection, Refraction, and Diffraction
4.3.3. Brightness of Electromagnetic Waves
Inverse-Square Law of Light: The apparent brightness
b varies inversely as the square of the distance d from the light
source; that is, b  1/d2.
Doppler Principle: Electromagnetic radiation received by an observer
will have a shorter wavelength if source and observer approach
each other and a longer wavelength if they recede from each other;
the amount of change in wavelength is directly proportional to
the velocity along the line between source and observer.
4.4.1. Formation of an Image
4.4.2. Properties of an Image
4.5. Reflecting and Refracting Telescopes
4.5.1. Advantages and Disadvantages
4.5.2. Focal Position for Reflecting Telescopes
4.5.3. Telescope Mountings
4.5.4. Other Approaches to Making Telescopes
Copyright 1995 J. C. Evans
Physics & Astronomy Department, George Mason University
Maintained by J. C. Evans; jevans@gmu.edu