Download Chapter 4 Astronomical Telescopes and Instruments: Extending

Michael Seeds
Dana Backman

This chapter’s
discussion of
astronomical
research
concentrates
on large
telescopes and
the special
instruments and
techniques
used to
analyze light.

Modern astronomers analyze light using
sophisticated instruments and
techniques to investigate the
compositions, motions, internal
processes, and evolution of celestial
objects.
› To understand this, you must learn about the
nature of light.
If you have noticed the colors in a soap
bubble, then you have seen one effect of
light behaving as a wave.
 When that same light enters the light meter
on a camera, it behaves as a particle.

› How light behaves depends on how you treat it.
› Light has both wavelike and particlelike properties.
 Light
is composed of a
combination of electric and
magnetic waves that can travel
through empty space.
› Unlike sound, light waves do not require a
medium and thus can travel through a vacuum.
 As
light is made up of both
electric and magnetic fields, it
is referred to as
electromagnetic radiation.
› Visible light is only one form of
electromagnetic radiation.

The changing electric and magnetic
fields of electromagnetic waves travel
through space at about 300,000
kilometers per second (186,000 miles per
second).
› That is commonly referred to as the speed of
light.
› It is, however, the speed of all electromagnetic
radiation.
 The
electromagnetic spectrum is
simply the types of
electromagnetic radiation
arranged in order of increasing
wavelength.
› Rainbows are spectra of visible light.
 The
colors of visible light have
different wavelengths.
› Red has the longest wavelength.
› Violet has the shortest.
The
average wavelength of
visible light is about 0.0005
mm.
› 50 light waves would fit end-to-end across the
thickness of a sheet of paper.
 It
is too awkward to measure such
short distances in millimeters.
 So, physicists and astronomers
describe the wavelength of light
using either of two units:
› Nanometer (nm), one billionth of a meter (10-9 m)
› Ångstrom (Å), named after the Swedish
astronomer Anders Ångström, equal to 10-10 m or
0.1 nm
 The
wavelength of visible light
ranges between about 400 nm
and 700 nm, or, equivalently, 4,000
Å and 7,000 Å.
› Infrared astronomers often refer to wavelengths
using units of microns (10-6 m).
› Radio astronomers use millimeters, centimeters, or
meters.
 Beyond
the red end of the visible
range lies infrared (IR) radiation—
with wavelengths ranging from 700
nm to about 1 mm.
 Radio
waves have even longer
wavelengths than IR radiation.
› The radio radiation used for AM radio
transmissions has wavelengths of a few hundred
meters.
› FM, television, and also military, governmental,
and amateur radio transmissions have
wavelengths from a few tens of centimeters to a
few tens of meters.

Microwave transmissions, used for radar
and long-distance telephone
communications, have wavelengths
from about 1 millimeter to a few
centimeters.
 Electromagnetic
waves with
wavelengths shorter than violet
light are called ultraviolet (UV).
 Shorter-wavelength
electromagnetic waves than UV
are called X rays.
 The shortest are gamma rays.
 Although
light behaves as a
wave, under certain conditions, it
also behaves as a particle.
› A particle of light is called a photon.
› You can think of a photon as a minimum-sized
bundle of electromagnetic waves.
 The
amount of energy a photon
carries depends on its
wavelength.
› Shorter-wavelength photons carry more energy.
› Longer-wavelength photons carry less energy.
› A photon of visible light carries a small amount of
energy.
› An X-ray photon carries much more energy.
› A radio photon carries much less.
 Astronomers
are interested in
electromagnetic radiation
because it carries almost all
available clues to the nature of
planets, stars, and other celestial
objects.

Only visible light, some short-wavelength
infrared radiation, and some radio waves
reach Earth’s surface—through what are
called atmospheric windows.

To study the sky from Earth’s surface, you
must look out through one of these
‘windows’ in the electromagnetic
spectrum.
 Astronomers
build optical
telescopes to gather light and
focus it into sharp images.
› This requires careful optical and mechanical
designs.
› It leads astronomers to build very large
telescopes.
› To understand that, you need to learn the
terminology of telescopes—starting with the
different types of telescopes and why some are
better than others.
 Astronomical
telescopes
focus light into an image in
one of two ways.
› A lens bends (refracts) the light as it passes
through the glass and brings it to a focus to
form an image.
› A mirror—a curved piece of glass with a
reflective surface—forms an image by
bouncing light.
 Thus,
there are two types of
astronomical telescopes.
› Refracting telescopes use
a lens to gather and focus
the light.
› Reflecting telescopes use
a mirror.
The main lens in a refracting telescope is
called the primary lens.
 The main mirror in
a reflecting telescope
is called the primary
mirror.

Both kinds of telescopes form a small,
inverted image that is difficult to observe
directly.
 So, a lens called
the eyepiece is used
to magnify the image
and make it convenient
to view.


The focal length is the distance from a lens
or mirror to the image it forms of a distant
light source such as
a star.
 Creating
the proper optical shape
to produce a good focus is an
expensive process.
› The surfaces of lenses and mirrors must be
shaped and polished to have no irregularities
larger than the wavelength of light.
› Creating the optics for a large telescope can
take months or years; involve huge, precision
machinery; and employ several expert optical
engineers and scientists.
Refracting telescopes have serious
disadvantages.
 Most importantly, they suffer from an
optical distortion that limits their usefulness.

› When light is refracted through glass, shorter
wavelengths bend more than longer wavelengths.
› As a result, you see a color blur around every
image.
› This color separation is called chromatic aberration
and it can be only partially corrected.
 Astronomers
also build radio
telescopes to gather radio
radiation.
› Radio waves from celestial objects—like visible light
waves—penetrate Earth’s atmosphere and reach
the ground.

You can see how the dish reflector of a
typical radio telescope focuses the radio
waves so their intensity can be measured.
› As radio wavelengths are so long, the disk
reflector does not have to be as perfectly smooth
as the mirror of a reflecting optical
telescope.
Astronomers struggle to build large
telescopes because a telescope can
help human eyes in three important
ways.
 These are called the three powers of a
telescope.

› The two most important of these three powers
depend on the diameter of the telescope.
 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.

The light-gathering power is proportional to
the area of the primary mirror—that is,
proportional to the square of the primary’s
diameter.
› A telescope with a diameter of 2 meters has four
times (4X) the light-gathering power of a 1-meter
telescope.
› That is why astronomers use large telescopes and
why telescopes are ranked by their diameters.

One reason radio astronomers build big
radio dishes is to collect enough radio
photons—which have low energies—and
concentrate them for measurement.
 Resolving
power refers to the
ability of the telescope to
reveal fine detail.

One consequence of the wavelike
nature of light is that there is an
inevitable small blurring called a
diffraction fringe around every point of
light in the image.
› You cannot see any detail smaller than the
fringe.
 Astronomers
can’t eliminate
diffraction fringes.
 However, the fringes are smaller in
larger telescopes.
› That means they have better resolving power
and can reveal finer detail.
› For example, a 2-meter telescope has diffraction
fringes ½ as large, and thus 2X better resolving
power, than a 1-meter telescope.
 One
way to improve resolving
power is to connect two or more
telescopes in an interferometer.
› This has a resolving power
equal to that of a telescope
as large as the maximum
separation between
the individual telescopes.
The first interferometers were built by
radio astronomers connecting radio
dishes kilometers apart.
 Modern technology has allowed
astronomers to connect optical
telescopes to form interferometers with
very high resolution.

 Aside
from diffraction fringes,
two other factors limit resolving
power:
› Optical quality
› Atmospheric conditions

Also, when you look through a telescope,
you look through miles of turbulence in
Earth’s atmosphere, which makes images
dance and blur—a condition
astronomers call seeing.
A
related phenomenon is the
twinkling of a star.
› The twinkles are caused by turbulence in Earth’s
atmosphere.
› A star near the horizon—where you look through
more air—will twinkle more than a star overhead.
› On a night when the atmosphere is unsteady, the
stars twinkle, the images are blurred, and the
seeing is bad.
A
telescope performs best on a
high mountaintop—where the air is
thin and steady.
However, even at good sites,
atmospheric turbulence spreads star
images into blobs 0.5 to 1 arc seconds in
diameter.
 That situation can be improved by a
difficult and expensive technique called
adaptive optics.

› By this technique, rapid computer calculations
adjust the telescope optics and partly
compensate for seeing distortions.

A telescope’s primary function is to
gather light and thus make faint things
appear brighter,
› so the light-gathering power is the most
important power and the diameter of the
telescope is its most important characteristic.

Light-gathering power and resolving
power are fundamental properties of a
telescope that cannot be altered,
› whereas magnifying power can be
changed simply by changing the eyepiece.
Observatories on Earth—Optical and
Radio

Optical astronomers avoid cities because
light pollution—the brightening of the night
sky by light scattered from artificial outdoor
lighting—can make it impossible to see
faint objects.
› In fact, many residents of cities are unfamiliar with
the beauty of the night sky because they can see
only the brightest stars.
Observatories on Earth—Optical and
Radio
 Radio
astronomers face a problem
of radio interference analogous to
light pollution.
› Weak radio signals from the cosmos are easily
drowned out by human radio interference—
everything from automobiles with faulty ignition
systems to poorly designed transmitters in
communication.
Observatories on Earth—Optical and
Radio
 To
avoid that, radio astronomers
locate their telescopes as far from
civilization as possible.
› Hidden deep in mountain valleys, they are able to
listen to the sky protected from human-made radio
noise.
Observatories on Earth—Optical and
Radio
 There
are two important points to
notice about modern
astronomical telescopes.
Observatories on Earth—Optical and
Radio

One, research telescopes must focus their
light to positions at which cameras and
other instruments can be placed.
Observatories on Earth—Optical and
Radio
 Two,
small telescopes can use
other focal arrangements that
would be inconvenient in larger
telescopes.
Observatories on Earth—Optical and
Radio

Telescopes located on the surface of
Earth, whether optical or radio, must move
continuously to stay pointed at a celestial
object as Earth turns on its axis.
› This is called sidereal tracking (‘sidereal’ refers to
the stars).
Observatories on Earth—Optical and
Radio
High-speed computers have allowed
astronomers to build new, giant telescopes
with unique designs.
 The European Southern Observatory has built
the Very Large Telescope (VLT) high in the
remote Andes Mountains of northern Chile.

Observatories on Earth—Optical and
Radio
The VLT actually consists of four telescopes,
each with a computer-controlled mirror 8.2 m
in diameter and only 17.5 cm (6.9 in.) thick.
 The four telescopes can work singly or can
combine their light to work as one large
telescope.

Observatories on Earth—Optical and
Radio

Italian and American astronomers have built
the Large Binocular Telescope, which carries
a pair of 8.4-m mirrors on a single mounting.
Observatories on Earth—Optical and
Radio

The Gran Telescopio Canarias, located atop
a volcanic peak in the Canary Islands, carries
a segmented mirror 10.4 meters in diameter.
› It holds, for the moment, the record as the largest
single telescope in the world.

Other giant telescopes are being planned
with innovative designs.
Observatories on Earth—Optical and
Radio
The largest fully steerable radio telescope
in the world is at the National Radio
Astronomy Observatory in West Virginia.
 The telescope has a reflecting surface 100
meters in diameter made of 2,004
computer-controlled panels that adjust to
maintain the shape of the reflecting
surface.

Observatories on Earth—Optical and
Radio
The largest radio dish in the world is 300 m
(1,000 ft) in diameter, and is built into a
mountain valley in Arecibo, Puerto Rico.
 The antenna hangs on cables above the dish,
and, by moving the antenna, astronomers
can point the telescope at any object that
passes within 20 degrees of the zenith as Earth
rotates.

Observatories on Earth—Optical and
Radio
The Very Large Array (VLA) consists of 27
dishes spread in a Y-pattern across the New
Mexico desert.
 Operated as an interferometer, the VLA has
the resolving power of a radio telescope up
to 36 km (22 mi) in diameter.

Observatories on Earth—Optical and
Radio

Such arrays are very powerful, and radio
astronomers are now planning the Square
Kilometer Array
› It will consist of radio dishes spanning 6,000 km
(almost 4,000 mi) and having a total collecting
area of one square kilometer.
Imaging Systems and Photometers
 The
photographic plate was the
first image-recording device used
with telescopes.
› Brightness of objects imaged on a photographic
plate can be measured with a lot of hard work—
yielding only moderate precision.
Imaging Systems and Photometers
 Astronomers
also build
photometers.
› These are sensitive light meters that can be
used to measure the brightness of individual
objects very precisely.
Imaging Systems and Photometers
 Most
modern astronomers use
charge-coupled devices (CCDs) as
both image-recording devices and
photometers.
› A CCD is a specialized computer chip containing
as many as a million or more microscopic light
detectors arranged in an array about the size of a
postage stamp.
› These array detectors can be used like a small
photographic plate.
Imaging Systems and Photometers
 CCDs
have dramatic advantages
over both photometers and
photographic plates.
› They can detect both bright and faint objects in
a single exposure and are much more sensitive
than a photographic plate.
› CCD images are digitized—converted to
numerical data—and can be read directly into a
computer memory for later analysis.
Imaging Systems and Photometers
Although CCDs for astronomy are
extremely sensitive and thus expensive,
less sophisticated CCDs are now used in
commercial video and digital cameras.
 Infrared astronomers use array detectors
similar in operation to optical CCDs.

› At other wavelengths, photometers are still used
for measuring brightness of celestial objects.
Imaging Systems and Photometers

For example,
astronomical
images are often
reproduced as
negatives—with
the sky white and
the stars dark.
› This makes the faint
parts of the image
easier to see.
Imaging Systems and Photometers
 Astronomers
also manipulate
images to produce false-color
images.
› The colors represent different levels of intensity
and are not related to the true colors of the
object.
Imaging Systems and Photometers

One way is to measure the strength of the
radio signal at various places in the sky
and draw a map in which contours mark
areas of uniform radio intensity.
Imaging Systems and Photometers
 Contour
maps are very common
in radio astronomy and are often
reproduced using false colors.
Spectrographs
 To
analyze light in detail, you
need to spread the light out
according to wavelength into a
spectrum—a task performed by a
spectrograph.
› You can understand how this works by
reproducing an experiment performed by
Isaac Newton in 1666.
Spectrographs
Newton didn’t think in terms of
wavelength.
 You, however, can use that modern
concept to see that the light passing
through the prism is bent at an angle that
depends on
the wavelength.

› Violet (short-
wavelength) light
bends most,
and red (long
wavelength) light
least.
Spectrographs
 Thus,
the white light entering
the prism is spread into what is
called a spectrum.
Spectrographs
 Most
modern spectrographs
use a grating in place of a
prism.
› A grating is a piece of glass with thousands of
microscopic parallel lines scribed onto its
surface.
› Different wavelengths of light reflect from the
grating at slightly different angles.
› So, white light is spread into a spectrum and can
be recorded—often by a CCD camera.
Airborne and Space Observatories

Most of the rest of the spectrum—infrared,
ultraviolet, X-ray, and gamma-ray
radiation—never reaches Earth’s surface.
› To observe at these wavelengths, telescopes must
fly above the atmosphere in high-flying aircraft,
rockets, balloons, and satellites.
The Ends of the Visual Spectrum
› For example, a number of important
infrared telescopes are located on the
summit of Mauna Kea in Hawaii—at an
altitude of 4,200 m (13,800 ft).
The Ends of the Visual Spectrum

For many years, the NASA Kuiper Airborne
Observatory (KAO) carried a 91-cm
infrared telescope and crews of
astronomers to altitudes of over 12 km
(40,000 ft).
› This was in order to get above 99 percent or more
of the water vapor in Earth’s atmosphere.
The Ends of the Visual Spectrum
 Now
retired from service, the KAO
will soon be replaced by the
Stratospheric Observatory for
Infrared Astronomy (SOFIA).
› This is a Boeing 747-P aircraft that will carry a 2.5-
m (100-in.) telescope to the fringes of the
atmosphere.
The Ends of the Visual Spectrum
 No
mountain is that high, and no
balloon or airplane can fly that
high.
 So, astronomers cannot observe
far-UV, X-ray, and gamma-ray
radiation—without going into
space.
Spitzer
Hubble
Telescopes in Space
The largest X-ray telescope to date, the
Chandra X-ray Observatory, was
launched in 1999 and orbits a third of the
way to the moon.
 Chandra is named for the late IndianAmerican Nobel Laureate
Subrahmanyan Chandrasekhar, who was
a pioneer in many branches of
theoretical astronomy.

Telescopes in Space

The telescope has made important
discoveries about everything from star
formation to monster black holes in
distant galaxies.
Telescopes in Space

The European INTEGRAL satellite was
launched in 2002 and has been very
productive in the study of violent
eruptions of stars and black holes.
Telescopes in Space

The GLAST (Gamma-Ray Large Area
Space Telescope), launched in 2008, is
capable of mapping large areas of the
sky to high sensitivity.
Telescopes in Space
Modern astronomy has come to
depend on observations that
cover the entire electromagnetic
spectrum.
 More orbiting space telescopes
are planned that will be more
versatile and more sensitive.

Combinations