Download unit a activity 4d - telescopes - student readings-klh.ss

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READING 1:
It takes more than one kind of telescope to see the light
Why we need different types of telescopes to look at outer space
Feature Story April 20, 1999: NASA Science News presents "Feature Stories", where you can sit back, relax, and enjoy an
in-depth look at ongoing research (or sometimes a story that's just plain fun).
1. By studying the electromagnetic emissions of objects such as stars, galaxies, and black holes, astronomers hope to
come to a better understanding of the universe. Although many astronomical puzzles can only be solved by comparing
images of different wavelengths, telescopes are only designed to detect a particular portion of the electromagnetic
spectrum. Astronomers therefore often use images from several different telescopes to study celestial phenomena.
Shown below is the Milky Way Galaxy as seen by radio, infrared, optical, X-ray and gamma-ray telescopes.
The Multi-Wave Milky Way Galaxy
radio
infrared
visual
X-ray
gamma ray
2. Different types of telescopes usually don't take simultaneous readings. Space is a dynamic system, so an image taken
at one time is not necessarily the precise equivalent of an image of the same phenomena taken at a later time. And
often, there is barely enough time for one kind of telescope to observe extremely short-lived phenomena like gammaray bursts. By the time other telescopes point to the object, it has grown too faint to be detected.
So why haven't scientists created a telescope designed to look at everything at once?
3. "Nature has determined the design of our telescopes," says Dr. Martin Weisskopf, an astrophysicist at NASA's
Marshall Space Flight Center. The differing wavelengths among the various energies create different instrumental needs.
This results in dissimilar, incompatible detecting devices.
Right: The electromagnetic spectrum.
Radio has long wavelengths and low energies,
while gamma rays have very short wavelengths and high
energies.
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4. Telescopes rely on the interaction between energy and matter. The atomic matter that forms the telescope has to
somehow interpret the energy emitted from astronomical objects. This energy is in the form of electromagnetic waves.
Although the first telescope was created 400 years ago, we didn't have a complete picture of the electromagnetic
spectrum until the early part of this century. As our knowledge of physics improves, scientists are able to develop
increasingly superior telescopes. But as the technology advances and becomes more specialized, differences among
telescope designs become more pronounced
The Development of Telescopes
5. Most of the universe is invisible to us because we only see the visible light portion of the electromagnetic spectrum.
When most people think of telescopes they think of visible light, or optical,
telescopes.
6. When the first optical telescope appeared in the 1570s, the design was
simple - one concave and one convex lens fitted inside a tube. The tube acted
as a receiver, or 'light bucket'. The lenses bent, or refracted, the light as it
passed through the glass and thus made the scene appear 3 to 4 times larger.
Galileo improved upon the design and by 1609 had developed a 20-power
refracting telescope. Galileo made the telescope famous when he discovered the valleys and
mountains of the moon and spotted four of Jupiter's satellites.
Left: Galileo Galilei (1564-1642), Italian astronomer, mathematician and physicist.
7. The glass lenses in the Galileo telescope weren't very clear, however - they were full of little
bubbles and had a greenish tinge due to the iron content of the glass. Also, the shape of the glass
lenses gave the field of view very fuzzy edges.
8. The magnification of Galileo's telescope could only be improved by focusing the light farther
behind the primary lens, which resulted in longer and longer telescopes. But once telescopes
reached 140 feet in length they became almost useless for observation. It was impossible to keep the lenses properly
aligned at such long lengths. Longer telescopes also required larger lenses, and after a lens reached 1 meter (3.28 ft.) in
diameter it would deform, sagging under its own weight.
Right: Johannes Hevelius' 150-ft. telescope (Machina Coelestis, 1673). Reprinted
with permission by the Royal Astronomical Society, London.
9. Isaac Newton invented the first reflecting telescope in 1671. By using a curved
mirror to reflect and focus the light inside the tube, he was able to reduce the
length of the telescope dramatically. The reflecting telescope solved another
problem inherent in the refracting telescope: chromatic aberration.
10. In 1672, Newton described how white light is actually a
mixture of colored light. Each color has its own degree of
refraction, so curved lenses split white light into the colors of
the spectrum. This chromatic aberration caused central images in refracting telescopes to be
surrounded by rings of different colors. Planets seen through a refracting telescope would appear
to be encircled by a rainbow.
Left: Sir Isaac Newton (1642-1727), English mathematician and physicist.
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11. By 1730, Newton's reflecting telescope had caught on with the
scientific community. Even today, large optical telescopes are based
upon Newton's basic design. Yet another bonus of Newton's
reflecting telescope is that it can also be used to study ultraviolet
and infrared light. The Hubble Space Telescope, famous for its
stunning optical images of the universe, also works in the ultraviolet
and infrared parts of the spectrum.
12. But it wasn't until the 1930s that astronomers even began
looking for other parts of the electromagnetic spectrum. Karl Jansky inadvertently discovered galactic emissions of radio
waves in 1933. Working at Bell Telephone Laboratories, Jansky was trying to find what caused short-wave radio
interference in Trans-Atlantic communications. By building a rotating radio telescope to look at the horizon, he
eventually discovered that most of the static resulted from engine ignition noise
and distant lightning storms. But Jansky also discovered that some radio noise was
coming from the center of the Milky Way Galaxy.
Left: The "Jansky Antenna" doesn't look much like modern aerials, i.e., TV
antennas or satellite dishes, because it was designed to receive shortwave
signals coming over the horizon.
13. Like optical telescopes, radio telescopes have reflectors and
receivers. Most radio telescopes need to be large in order to
accommodate radio's longer wavelengths and lower energies.
Resolution is also a factor: low-frequency radio waves would be
unfocused and fuzzy in smaller telescopes. Radio telescopes also need
to be large in order to overcome the radio noise, or "snow," that
naturally occurs in radio receivers. We generate a large amount of
noise on Earth as well, so smaller telescopes would lose some
astronomical radio signals amid our daily production of rock music,
television broadcasts and cellular phone calls. An example of a modern
radio telescope is The Very Large Array in New Mexico (right),
composed of 27 antennas electronically combined to give the
resolution of an antenna 36 kilometers (22 miles) across.
14. Radio and optical telescopes can be used on Earth, but some resolution is lost due to Earth's atmosphere. By viewing
from the other side of the sky, the Hubble Space Telescope allows astronomers to see the universe without the
distortion and filtering that occurs as light passes through the Earth's atmosphere.
15. Infrared and ultraviolet light are affected more dramatically by the Earth's
atmosphere. Their telescopes must therefore always be positioned high above
the ground or in space. Infrared telescopes are placed on mountaintops, far
above the low-lying water vapor that interferes with infrared light.
Left: The NASA Infrared Telescope Facility 3.0 meter telescope at the summit
of Mauna Kea, Hawaii. Photo courtesy of Ernie Mastroianni.
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16. Ultraviolet telescopes have to be placed even higher than infrared telescopes. The Earth's stratospheric ozone layer,
located 20 to 40 kilometers above the Earth's surface, blocks out UV wavelengths shorter than 300 nanometers. By the
1940s, scientists were launching rockets with rudimentary UV detectors onboard.
The Earth's atmosphere scatters or absorbs high-energy radiation, protecting us from the damaging effects of UV, X-rays
and gamma rays. The atmosphere does such a good job that telescopes designed to detect these portions of the
electromagnetic spectrum have to be positioned outside the atmosphere.
17. Studies of astronomical objects in high energy X-rays and gamma rays began in the early 1960s. Although high
altitude balloons and rockets can provide X-ray and gamma ray data, the best results come from satellites orbiting
completely outside the Earth's atmosphere. NASA's first X-ray telescope was launched from Kenya on Dec. 12, 1970.
Because this date was the 7th anniversary of Kenya's independence, the satellite was named Uhuru (Swahili for
"freedom").
Left: X-rays can be reflected using a combination of paraboloidal
mirrors and hyperboloidal mirrors. This is the arrangement of
mirrors in the Chandra X-ray Observatory, scheduled for launch in
July, 1999.
X-ray telescope mirrors are coated with gold or other metals;
Uhuru's mirrors were coated with beryllium, for instance. The
mirrors have shallow angles of reflection because X-rays are so
short they only reflect at angles almost parallel to the rays
themselves. At steeper mirror angles the rays won't reflect instead they will
penetrate the mirror like a bullet
embedding itself in a wall.
18. Because gamma rays are even shorter than X-rays, there is no way to prevent them from passing right through a
detection device. Since mirrors can't be used to focus gamma rays, a method
had to be developed for detecting gamma rays indirectly.
Right: Illustration of a crystal gamma-ray detector. The electrons expelled by
gamma-rays act like a trigger on an alarm, letting the detector know when
gamma rays are passing through.
19. The American physicist Arthur Holly Compton discovered that gamma rays
would expel electrons as they moved through a detector. Modern gamma-ray
detectors use crystals or liquids that are triggered by these expelled gammaray electrons to record the passing gamma rays as flashes of light. The first
gamma-ray satellite, Explorer XI, was launched in 1961, a year before
Compton's death. The Compton Gamma Ray Observatory, launched in 1991 and still orbiting the Earth today, was
named in his honor.
Left: Arthur Holly Compton (1892-1962) won the 1927 Nobel Prize in physics for his work with
gamma rays.
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Physics 101
A. Most objects give off several frequencies of energy simultaneously. Your body, for instance, glows in thermal infrared
down to radio. But in order to get astronomical data about different wavelengths, scientists have to use several different
types of telescopes. There is no such thing as an 'all-wave' telescope. The problem with having one telescope able to
detect the entire electromagnetic spectrum lies in the differences in detection techniques.
B. "Telescopes are designed with one goal in mind: to build a device that interacts with radiation coming from the
cosmos," says Dr. Tony Phillips, a radio astronomer now consulting for
the Science@NASA web site.
C. Different energy wavelengths interact with matter in different ways.
Radio waves will reflect from a metal that X-rays pass right through.
These differences in the interaction between matter and energy have
resulted in telescopes designed to only accommodate very specific
wavelengths.
Right: A mosaic of different astronomical phenomena at various
wavelengths.
D. Phillips says that telescopes designed for different parts of the
electromagnetic spectrum often look dramatically unlike one another.
"Low-frequency radio telescopes look wildly different from microwave
telescopes, even though both study the radio portion of the spectrum,"
he states. "And low-frequency radio telescopes don't bear the remotest
resemblance to X-ray telescopes."
E. With present technology, it is not possible to build one telescope able
to efficiently survey the entire electromagnetic spectrum. Scientists
follow established laws of physics in building telescopes, and an allwave telescope would have to break those laws.
F. "That's the wall that keeps us from building one device for everything," says Phillips.
Because it is not currently possible to create an all-wave telescope, the next choice is to create a device that uses many
telescopes at once.
G. "What we want is a Christmas tree," says Weisskopf. "What we want is a system that can look at all of the emissions
simultaneously." Matched telescopes could be aligned to look at the same thing at the same time. A device containing
all the different types of telescopes would necessarily have to be a satellite so that X-rays and gamma rays could be
detected.
H. Several multi-wavelength observatories have already flown - Skylab, the Solar Maximum Mission, and the Solar and
Heliospheric Observatory (SOHO). The Skylab space station in particular is hailed as a good model for conducting multiwavelength studies in space. Launched in 1973, Skylab had eight coordinated telescopes located on its Apollo Telescope
Mount (ATM). The eight telescopes studied the Sun's spectrum from X-ray almost
down to infrared, all with very high quality resolution. Skylab was coordinated with
ground-based astronomers as well. Whenever ground observers detected active
solar prominences, flares, or mass ejections, they would notify the astronauts, who
would then point their telescopes to record the event.
Right: Skylab space station orbiting Earth in 1973.
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I. The problem in developing this type of technology today, says Weisskopf, is two-fold. Money is the most immediate
impediment. It would cost several billion dollars just to create a high quality combined optical and X-ray telescope.
J. More difficult to tackle is the social mind-set of scientists. Scientists are often trained to specialize; to study only one
segment of the electromagnetic spectrum. Hence we have many X-ray astronomers, radio astronomers, and so on, with
fewer scientists following a multi-wavelength approach. Facilities and instruments are built to study only portions of the
spectrum, rather than phenomena as a whole. There are no instruments designed to just study globular clusters, for
instance.
K. Phillips agrees that wavelength specialization is prevalent in the scientific community, but he believes this attitude is
changing. Up until the last thirty years, many astronomers built their own telescopes, thereby focusing all their attention
on one portion of the spectrum. Today, engineers build the telescopes based on what the astronomers want to study.
Because astronomers no longer build the telescopes, they are more willing to look at other wavelengths.
L. "Astronomers are now becoming more multi-wave literate in order to solve
astrophysical puzzles," says Phillips.
M. Because specialized telescopes are so well developed and are still strongly
supported by scientists, the most logical approach would be to coordinate the
telescopes already in existence. This happened recently due to an accident with
satellite equipment. The Compton Gamma Ray Observatory (left) once stored data
on its satellite tape recorders and dumped data about gamma-ray bursts down to
ground stations several times each day. However, in 1992 the tape recorders failed.
Since there was no way to save the data, it had to be transmitted instantly. The
bright side of this accident was that this instant transmission of information made it possible for other types of
telescopes to have immediate, 'real-time' burst alerts. On Jan. 23, 1999, gamma-ray and visible light robotic telescopes
coordinated an observation of a gamma-ray burst. When the gamma-ray burst was detected, the GRO quickly sent the
information out through the Internet. The Robotic Optical Transient Search Experiment (ROTSE), a visible light telescope,
used this information to lock on to the burst 22 seconds after it began. This allowed scientists to see for the first time a
gamma-ray burst explosion in the visible light portion of the spectrum.
Right: The Robotic and Optical Search Experiment (ROTSE), operated by Carl Akerloff at the University of Michigan.
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READING 2:
Electromagnetic Spectrum
Measuring the electromagnetic spectrum
You actually know more about it than you may think! The electromagnetic (EM) spectrum is just a name that
scientists give a bunch of types of radiation when they want to talk about them as a group. Radiation is energy
that travels and spreads out as it goes-- visible light that comes from a lamp in your house and radio waves that
come from a radio station are two types of electromagnetic radiation. Other examples of EM radiation are
microwaves, infrared and ultraviolet light, X-rays and gamma-rays. Hotter, more energetic objects and events
create higher energy radiation than cool objects. Only extremely hot objects or particles moving at very high
velocities can create high-energy radiation like X-rays and gamma-rays.
Here are the different types of radiation in the EM spectrum, in order from lowest energy to highest:
Radio: Yes, this is the same kind of energy that radio stations emit
into the air for your boom box to capture and turn into your favorite
Mozart, Madonna, or Justin Timberlake tunes. But radio waves are
also emitted by other things ... such as stars and gases in space. You
may not be able to dance to what these objects emit, but you can use it
to learn what they are made of.
Microwaves: They will cook your popcorn in just a few minutes!
Microwaves in space are used by astronomers to learn about the
structure of nearby galaxies, and our own Milky Way!
Infrared: Our skin emits infrared light, which is why we can be seen in
the dark by someone using night vision goggles. In space, IR light
maps the dust between stars.
Visible: Yes, this is the part that our eyes see. Visible radiation is
emitted by everything from fireflies to light bulbs to stars ... also by
fast-moving particles hitting other particles.
Ultraviolet: We know that the Sun is a source of ultraviolet (or UV)
radiation, because it is the UV rays that cause our skin to burn! Stars
and other "hot" objects in space emit UV radiation.
X-rays: Your doctor uses them to look at your bones and your dentist
to look at your teeth. Hot gases in the Universe also emit X-rays .
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Gamma-rays: Radioactive materials (some natural and others made by
man in things like nuclear power plants) can emit gamma-rays. Big
particle accelerators that scientists use to help them understand what
matter is made of can sometimes generate gamma-rays. But the
biggest gamma-ray generator of all is the Universe! It makes gamma
radiation in all kinds of ways.
A Radio Wave is not a Gamma-Ray, a Microwave is not an X-ray ... or is it?
We may think that radio waves are completely different physical objects or events than gamma-rays. They are
produced in very different ways, and we detect them in different ways. But are they really different things? The
answer is 'no'. Radio waves, visible light, X-rays, and all the other parts of the electromagnetic spectrum are
fundamentally the same thing. They are all electromagnetic radiation.
Radio waves, visible light, X-rays, and all the
other parts of the electromagnetic spectrum are
fundamentally the same thing, electromagnetic
radiation.
Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each
traveling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount (or
bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the
various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have
photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then
visible, ultraviolet, X-rays, and ... the most energetic of all ... gamma-rays.
Actually, the electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency. Each
way of thinking about the EM spectrum is related to the others in a precise mathematical way. So why do we
have three ways of describing things, each with a different set of physical units? After all, frequency is
measured in cycles per second (which is called a Hertz), wavelength is measured in meters, and energy is
measured in electron volts.
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The electromagnetic spectrum can be expressed in
terms of energy, wavelength, or frequency.
The answer is that scientists don't like to use big numbers when they don't have to. It is much easier to say or
write "two kilometers or 2 km" than "two thousand meters or 2,000 m". So generally, scientists use whatever
units are easiest for whatever they are working with. In radio astronomy, astronomers tend to use wavelengths
or frequencies. This is because most of the radio part of the EM spectrum falls in the range from about 1 cm to 1
km (30 gigahertz (GHz) to 100 kilohertz (kHz)). The radio is a very broad part of the EM spectrum. Infrared
astronomers also use wavelength to describe their part of the EM spectrum. They tend to use microns (or
millionths of meters) for wavelengths, so that they can say their part of the EM spectrum falls in the range 1 to
100 microns. Optical astronomers use wavelengths as well. Scientists use both angstroms (0.00000001 cm, or
10 -8 cm in scientific notation) and nanometers (0.0000001, or 10-7, cm). In the newer "SI" version of the metric
system, we think of visible light in units of nanometers or 0.000000001 meters (10-9 m). In this system, the
violet, blue, green, yellow, orange, and red light we know so well has wavelengths between 400 and 700
nanometers. This range is only a small part of the entire EM spectrum, so you can tell that the light we see is
just a little fraction of all the EM radiation around us! By the time you get to the ultraviolet, X-ray, and gammaray regions of the EM spectrum, lengths have become too tiny to think about any more. So scientists usually
refer to these photons by their energies, which are measured in electron volts. Ultraviolet radiation falls in the
range from a few electron volts (eV) to about 100 eV. X-ray photons have energies in the range 100 eV to
100,000 eV (or 100 keV). Gamma-rays then are all the photons with energies greater than 100 keV.
Why Do We Have to Go to Space to See All of the Electromagnetic Spectrum?
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Electromagnetic radiation from space is unable to reach the surface of the Earth except at a very few
wavelengths, such as the visible spectrum, radio frequencies, and some ultraviolet wavelengths. Astronomers
can get above enough of the Earth's atmosphere to observe at some infrared wavelengths from mountain tops or
by flying their telescopes in an aircraft. Experiments can also be taken up to altitudes as high as 35 km by
balloons which can operate for months. Rocket flights can take instruments all the way above the Earth's
atmosphere for just a few minutes before they fall back to Earth, but a great many important first results in
astronomy and astrophysics came from just those few minutes of observations. For long-term observations,
however, it is best to have your detector on an orbiting satellite ... and get above it all!
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