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Chapter 5
Telescopes
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How do humans
observe and explore
space?
Observing the Universe
• How do scientists use the
electromagnetic spectrum to study
the universe?
• What types of telescopes and
technology are used to explore
space?
Observing the Sky
Telescopes enable astronomers to observe
many more stars than they could with their
eyes alone.
telescope
from Greek tele, means “far”; and
Greek skopos, means “seeing”
Learning Objectives
Upon completing this chapter you should be able to:
1. Classify the common types of telescope designs.
2. Compare the advantages and disadvantages of reflectors and refractors.
3. Describe what causes refraction and how lenses focus light.
4. Identify the important aspects for determining a telescope's sensitivity.
5. Compare the light gathering power of different telescopes.
6. Describe the factors affecting telescope resolution, and calculate the diffraction
limit for a telescope.
7. Describe the idea of interferometry and how astronomers use it to improve
resolution.
8. Describe the methods used for detecting visible light and other wavelengths of
electromagnetic radiation.
9. Discuss the problems caused by observing through the Earth's atmosphere, and
describe the methods astronomers use to overcome these problems.
10. Identify the wavelength ranges in which telescopes cannot operate from the
ground and the reasons for this.
11. Describe the causes and remedies for light pollution.
Ancient peoples often gazed up in wonder at the many
points of light in the night sky. But they could see few
details with their eyes alone.
It was not until the invention of the telescope in
1608 that people could observe objects in the sky more
closely.
Recall that a telescope is a device that makes distant
objects appear to be closer. The telescope
revolutionized astronomy.
Scientists now had a tool that allowed them to see many
objects in space for the first time.
A telescope enables the astronomer to observe
things not visible to the naked eye.
Although our eyes are superb detectors, they
cannot see extremely faint objects or fine details
on distant sources.
A telescope overcomes these difficulties—first, by
collecting more light than the eye can collect
“light-gathering power”, and second, by increasing
the detail discernible “resolving power.”
Electromagnetic Radiation
To understand how telescopes work, it's useful to
understand the nature of electromagnetic
radiation.
Light is a form of electromagnetic radiation (ih lek
troh mag NET ik), or energy ‘ that can travel
through space in the form of waves. You can see
stars when the light that they produce reaches your
eyes.
Forms of Radiation Scientists call the light you can see visible light.
Visible light is just one of many types of electro-magnetic radiation.
Many objects give off radiation that you can't see.
Objects in space give off all types of electromagnetic radiation.
The electromagnetic spectrum includes the entire
range of radio waves, infrared radiation, visible light,
ultraviolet radiation, X-rays, and gamma rays.
Telescopes are instruments that collect and focus light and
other forms of electromagnetic radiation.
Observing the Sky (cont.)
Michael Matisse/Getty Images
Astronomers use
many kinds of
telescopes to study
the light energy
emitted by stars and
other objects in
space.
Electromagnetic Waves
• Stars radiate energy into space. This energy
travels as electromagnetic waves.
• The entire range of radiant energy carried by
electromagnetic waves is the
electromagnetic spectrum.
Most wavelengths of
the electromagnetic
spectrum are not
visible to the human
eye.
Electromagnetic Waves (cont.)
• The type of radiant energy a star emits
depends on the star’s temperature.
• Some stars are so far away that it takes
billions of years for their radiant energy to
reach Earth.
Telescopes make distant objects appear larger
and brighter.
A telescope that uses lenses or mirrors to collect
and focus visible light is called an optical
telescope.
Modern astronomy is based on the detection of many
forms of electromagnetic radiation besides visible light.
Non-optical telescopes collect and focus different types of
electromagnetic radiation, just as optical telescopes collect
visible light.
Telescopes
There are many kinds of telescopes, but they all have the
same primary purpose--to collect light.
By light, we mean any part of the electromagnetic
spectrum, not only visible light.
Thus, we have radio telescopes, and X-ray telescopes, and
even gamma-ray telescopes.
As we will find, the shape, size, and internal workings of
telescopes depends a great deal on what part of the
electromagnetic spectrum it is designed to work for.
We begin the discussion with the most familiar type of
telescope, the optical telescope.
The two main types of optical telescope:
• Reflecting Telescope -- uses a curved mirror to
focus the light (like a shaving mirror).
• Refracting Telescope -- uses a lens to focus the
light (like eye-glasses). Refraction means lightbending. The light bends (changes direction)
because it slows down on passing through the
glass. Red light slows more than blue.
•
Classify the common types of telescope designs
Advantages of reflecting telescope over a refracting telescope:
A refracting telescope uses a lens, which
bends red light more than blue light, so
the image has color halos. This is
called chromatic aberration.
A reflecting telescope reflects all
wavelengths of light at the same angle, so
there are no color halos.
A lens has two surfaces to be figured,
which is more difficult to control its
shape.
A mirror has only one surface to be
figured, so it is easier to control the
shape.
A lens passes light through, so the glass
has to be very transparent and pure, and
some wavelengths (infrared and
ultraviolet) are absorbed by glass.
A mirror reflects the light, so the material
that it is made from does not have to be
transparent, and infrared and ultraviolet
light reflects equally well.
A lens can only be mounted by its edges,
so a large lens can sag under its own
weight.
A mirror can be supported from the back,
so it is less subject to sagging. Also, a
mirror can be hollow, to reduce weight.
Compare the advantages and disadvantages of reflectors and refractors
Earth-Based Telescopes
• Optical telescopes
gather visible light.
• Refracting telescopes
and reflecting
telescopes are the two
types of optical
telescopes.
A telescope that uses a curved mirror to
concentrate light from a distant object is a
reflecting telescope.
A telescope that uses a convex lens to
concentrate light from a distant object is a
refracting telescope.
Tools of the Trade: Telescopes
• Stars and other celestial objects are too far away to
test directly
– Astronomers passively collect radiation emitted from
distant objects
– Extremely faint objects make collection of radiation
difficult
• Specialized Instruments Required
– Need to measure brightness, spectra, and positions with
high precision
– Astronomers use mirrored telescopes and observatories
• Modern Astronomers are rarely at the eyepiece,
more often they are at a computer terminal!
The Powers of a Telescope
• Collecting Power
– Bigger telescope, more
light collected!
• Focusing Power
– Use mirrors or lenses to
bend the path of light
rays to create images
• Resolving Power
– Picking out the details
in an image
To double the resolving power of a telescope, you must increase the diameter by
a factor of two
The two most important properties of any telescope are the light gathering power and the resolving power.
• Light collected proportional to
“collector” area
– Pupil for the eye
– Mirror or lens for a telescope
• Telescope “funnels” light to our eyes
for a brighter image
• Small changes in “collector” radius
give large change in number of
photons caught
• How bright that object appears to us
depends on the number of its photons
that enter our eye per second, a number
limited by the size of our eyes.
Astronomers overcome that limit by
“collecting” photons with a telescope,
which then “funnels” the photons to
our eyes. The bigger the telescope's
collecting area, be it a lens or mirror,
the more photons it collects.
Light Gathering Power
• Telescopes described
by lens or mirror
diameter (inches)
• Refraction
– Light moving at an angle
from one material to
another will bend due to a
process called refraction
The lens of a refractor focuses the light by
bending the rays
– Refraction occurs because
the speed of light is
different in different
materials
Once light has been gathered, it
must be focused to form an
image or to concentrate it on a
detector.
Telescopes in which light is
gathered and focused by a lens
are called “refracting
telescopes,” or refractors for
short.
A larger-diameter mirror or lens
gives a telescope a greater lightgathering power. The result is a
brighter image, which allows us
to see dim objects that are
invisible in telescopes with
smaller gathering areas
Compare the light gathering power of
different telescopes
• The light gathering power of telescopes
depends on how big the collecting area is on
the telescope. So the telescope with the greater
collecting area has a greater light gathering
power.
Due to the refraction caused by atmospheric irregularities stars twinkle.
Describe what causes refraction and how lenses focus light
Refraction
The Sun looks flattened near the horizon because the larger refraction near the horizon
lifts the lower edge of the Sun more than the upper edge and makes the Sun look
flattened.
Disadvantages to Refractors
• Lenses have many disadvantages in large
telescopes!
– Large lenses are extremely expensive to fabricate
– A large lens will sag in the center since it can only
be supported on the edges
– Dispersion causes images to have colored fringes
– Many lens materials absorb short-wavelength light
Reflecting Telescopes
• Reflectors
– Used almost
exclusively by
astronomers today
– Twin Keck telescopes,
located on the 14,000
foot volcanic peak
Mauna Kea in Hawaii,
have 10-meter collector
mirrors!
– Light is focused in front
of the mirror
• A secondary mirror may
be used to deflect the
light to the side or
through a hole in the
primary mirror
• Multi-mirror instruments
and extremely thin
mirrors are two modern
approaches to dealing
with large pieces of glass
in a telescope system
Reflecting Telescopes
Styles of Refractors
Infrared telescope
Infrared telescope, instrument designed to detect and resolve
infrared radiation from sources outside Earth’s atmosphere such
as nebulae, young stars, and gas and dust in other galaxies.
Infrared telescopes do not differ
significantly from reflecting telescopes
designed to observe in the visible
region of the electromagnetic spectrum.
The main difference between the two
types is in the physical location of the
infrared telescope…
The resolving power of a telescope is affected by the property of light called diffraction.
Resolving Power
• A telescope’s ability to
• Waves are diffracted as they
discern detail is referred to as
pass through narrow openings
its resolving power
• A diffracted point source of
• Resolving power is limited by
light appears as a point
the wave nature of light through
surrounded by rings of light
a phenomenon called
diffraction
If you mark two black dots close together on a piece of paper and look at them from the other
side of the room, your eye may see them as a single dark mark, not as separate spots. Likewise,
stars that lie close together or markings on planets may not be distinguishable. A telescope's
ability to discern such detail depends on its resolving power.
Identify the important aspects for determining a telescopes
sensitivity
•
•
•
•
The greater the light gathering power
Quality of mirror or lens
Build quality
A telescope of larger primary mirror or lens is more
sensitive than one with a smaller one.
• The light-gathering power of a telescope depends on its
collecting area, which is proportional to the square of the
mirror diameter. To study the faintest sources of radiation,
astronomers must use large telescopes. Another important
aspect of a telescope is its angular resolution, the ability to
distinguish between light sources lying close together on
the sky.
Resolving Power and Aperture
Diffraction seriously limits the detail visible through a telescope.
In fact, diffraction theory shows that if two points of light that are
separated by an angle α (measured in arc seconds*) are observed at a
wavelength λ through a telescope whose diameter is D, the points
cannot be seen as separate sources unless D is greater than (>)
• Two points of light separated by an angle a
(in arcsec) can be seen at a wavelength l (in nm) only if the
telescope diameter D (in cm) satisfies:
D > 0.02 l/a
Describe the factors affecting telescope resolution, and
calculate the diffraction limit for a telescope
• The Atmosphere, weather, light pollution
• D>2.5x10^5λ/a is how to calculate the
diffraction limit
Describe the idea of interferometry and how astronomers use
it to improve resolution
• Interferometry is when multiple telescopes are used to
interfere the wave with each other, and when that
crests of two waves arrive together, they create a
bright region.
• Where the crest of one wave arrives simultaneously
with the trough of another, they cancel and create a dark
patch. The result is a complex pattern of altering light
and dark regions, which can be analyzed bay a
computer to create an image of the object observed.
– For a given wavelength, resolution is
increased for a larger telescope
diameter
– An interferometer accomplishes this
by simultaneously combining
observations from two or more widelyspaced telescopes
Increasing Resolving Power: Interferometers
– Diffraction effects can never
be totally eliminated, but they
can be reduced by enlarging
the opening through which the
light passes, so that its waves
do not mix as severely.
Astronomers sometimes
accomplish this with a device
called an interferometer.
Using interferometry, scientists can use a few smaller telescopes to
take images with the same resolution as a much larger telescope.
Interferometers
• The resolution is determined by the
individual telescope separations and not
the individual diameters of the telescopes
themselves
• Key to the process is the wave nature of
interference and the electronic processing
of the waves from the various telescopes
The interferometer is so-named because when it mixes the
separate beams, the light waves of one “interfere” with the
waves from the other.
Where the crests of two waves arrive together, they create a bright
region. Where the crest of one wave arrives simultaneously with the
trough of another, they cancel and create a dark patch. The result is a
complex pattern of alternating light and dark regions, which can be
analyzed by a computer to create an image of the object observed.
Once light has been collected, it must be detected
and recorded.
In olden days, the detector was the eye of an
astronomer who sat at the telescope eyepiece and
wrote down data or made sketches of the object
being observed.
The human eye, marvel that it is, has difficulty
seeing very faint light.
Many astronomical bodies are too distant or too
dim for their few photons to create a sensible
effect on the eye.
From the late 1800s until the 1980s, astronomers
generally used photographic film to record the light from
the bodies they were studying.
Film forms an image by absorbing photons that cause a
chemical change, making the film dark where light has
fallen on it.
This process, however, is very inefficient: fewer than 4%
of the photons striking the film produce a useful image.
The result of such low efficiency is that many hours
are needed to accumulate enough light to create an
image of faint objects. Moreover, the film must be
developed, thereby delaying the observing process
even further.
• Astronomers today use many kinds of
electronic detectors.
• One of the main types is the CCD (chargecoupled device).
• Modern CCDs can make pictures with a
sensitivity to faint light approximately 20
times greater than photographic film.
The Moon appears bigger near the horizon due to an optical illusion.
Detecting the Light
• The Human Eye
– Once used with a telescope to record observations or make
sketches
– Not good at detecting faint light, even with the 10-meter
Keck telescopes
• Photographic Film
– Chemically stores data to increase sensitivity to dim light
– Very inefficient: Only 4% of striking photons recorded on
film
• Electronic Detectors
– Incoming photons strike an array of semiconductor pixels that
are coupled to a computer
– Efficiencies of 75% possible
– CCD (Charged-coupled Device) for pictures
Nonvisible Wavelengths
• Many astronomical
objects radiate in
wavelengths other than
visible
– Cold gas clouds radiate in
the radio
– Dust clouds radiate in the
infrared
– Hot gases around black holes
emit x-rays
Visible light, which we can see because its wavelengths are detectable
by our eyes, is just one of many wave bands of electromagnetic
radiation, as discussed in chapter 4. Many astronomical objects,
however, radiate at wavelengths that our eyes cannot see, and so
astronomers have devised ways to observe such objects.
Earth-Based Telescopes (cont.)
Radio telescopes collect
invisible radio waves and
some microwaves. They
look like TV satellite
dishes.
Steve Allen/Brand X Pictures
Describe the methods used for detecting visible light and other
wavelengths of electromagnetic radiation
• CCD, or chargecoupled device and its
20 times more sensitive
than photographic film
• Radio telescopes that
pick up on the radio
energy emitting form
cold clouds of gas in
interstellar space
Telescopes in space collect energy of all
wavelengths, including those absorbed by Earth’s
atmosphere.
Radio Observations
(A) A false-color
picture of a radio galaxy. We can't see radio waves, so colors are used to
represent their brightness—red brightest, blue dimmest. (B) A false-color X-ray picture of Cas
A, an exploding star. In this case colors represent different wavelengths of X-ray photons (bluer
colors corresponding to more energetic photons)
• False color images are typically used to depict
wavelength distributions in non-visible observations
• In a false color image, colors can represent photon
energies or the intensity of electromagnetic radiation.
A galaxy with almost no starlight but plenty of cool clouds of
hydrogen gas would be best observed with a radio telescope.
Radio telescopes vary widely, but they all have two
basic components:
(1)a large radio antenna and
(2) a sensitive radiometer, or radio receiver.
The sensitivity of a radio telescope—the ability to
measure weak sources of radio emission—depends
both on the area and efficiency of the antenna and
on the sensitivity of the radio receiver used to
amplify and to detect the signals.
For broadband continuum emission over a range of
wavelengths, the sensitivity also depends on the
bandwidth of the receiver.
Discuss the problems caused by observing through the earths
atmosphere, and describe the methods astronomers use to overcome
these problems
• When astronomers try to look into space from the ground,
they have to look through the atmosphere, which can distort
and blur images through the telescope.
Astronomers have turned to a method called adaptive optics.
Sophisticated, deformable mirrors controlled by computers can
correct in real-time for the distortion caused by the turbulence
of the Earth's atmosphere, making the images obtained almost
as sharp as those taken in space.
Adaptive optics allows the corrected optical system to observe
finer details of much fainter astronomical objects than is
otherwise possible from the ground.
Identify the wavelength ranges in which telescopes cannot operate
from the ground and the reasons for this
• The only wavelengths that can get through our atmosphere
that are useful to astronomers are visible light, and radio
waves. Ultraviolet rays can get through but most gets
absorbed by water vapor and carbon dioxide.
• The other wavelengths are filtered out by our atmosphere
because they are harmful to earth, so we launch telescopes
into space so they can receive those wavelengths.
The 305-metre (1,000-foot) radio telescope at the Arecibo Observatory, Puerto Rico.
Arecibo Observatory, Puerto Rico.
Designing a telescope for observing X rays presents different
challenges.
X rays entering a normal telescope would strike the mirror
surface and be absorbed, making the telescope no more effective
for observing than a slab of concrete.
Astronomers have found, however, that X rays can be reflected if
they strike a smooth surface at a very shallow angle, somewhat as a
rock can skip over the surface of water if thrown nearly horizontally.
An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in
the X-ray spectrum. In order to get above the Earth's atmosphere, which is opaque to
X-rays, X-ray telescopes must be mounted on high altitude rockets, balloons or
artificial satellites.
Major Space Observatories
• Why put them in space?
Artificial Satellites
Any small object that orbits a larger object is a
satellite.
satellite
from Latin satellitem, means
“attendant” or “bodyguard”
Artificial Satellites (cont.)
• Rockets place
satellites into orbit
around Earth or other
objects in space.
• Satellites send
information back
to Earth.
CORBIS
Artificial Satellites (cont.)
• Today, Earth-orbiting satellites are used to
transmit television and telephone signals and
to monitor weather and climate.
• An array of satellites called the Global
Positioning System (GPS) is used for
navigation in cars, boats, airplanes, and even
for hiking.
Space vs. Ground-Based Observatories
• Space-Based Advantages
– Freedom from atmospheric blurring
– Freedom of atmospheric absorption
• Ground-Based Advantages
– Larger collecting power
– Equipment easily fixed
• Ground-Based Considerations
– Weather, humidity, and haze
– Light pollution
The best site for placing a ground-based optical telescope is a mountain top.
Observatories
• The immense telescopes and their associated
equipment require observatories to facilitate their
use and protection from the elements
• Thousands of observatories are scattered throughout the
world and are on every continent including Antarctica
• Some observatories:
– Twin 10-meter Keck telescopes are largest in U.S.
– The Hobby-Eberly Telescope uses 91 1-meter mirrors set in
an 11-meter disk
– Largest optical telescope, VLT (Very Large Telescope) in
Chile, is an array of four 8-meter mirrors
Computers and Astronomy
• For many astronomers, operating a computer
and being able to program are more important
than knowing how to use a telescope
• Computers accomplish several tasks:
– Solve equations
– Move telescopes and
feed information to detectors
– Convert data into useful form
– Networks for communication
and data exchange
• Refraction is also
responsible for
seeing
– Twinkling of
stars
– AKA
Scintillation
• Temperature and
density differences
in pockets of air shift
the image of the star
Scintillation
– Twinkling of stars in sky,
called scintillation, is
caused by moving
atmospheric irregularities
refracting star light into a
blend of paths to the eye
Atmospheric Blurring
– The condition of the sky
for viewing is referred to
as the seeing
– Distorted seeing can be
improved by adaptive
optics, which employs a
powerful laser and
correcting mirrors to offset
scintillation
The distortion of an image
due to an atmospheric
turbulence is “seeing”.
Describe the causes and remedies for light pollution
One of the biggest
problems for ground
based astronomy
today is light pollution
makes it difficult to
observe faint objects.
Artificial light pollutes the
view of 83% of Earth’s
population according to
recent data.
Figure 1. The effect of light pollution on the night sky. This split image shows how artificial light
washes out most of the faint detail in the constellation Orion.
Light Pollution
Unwanted light can travel directly into your eyes ruining
the dark adaption they need to observe faint celestial
objects.
It can also invade telescopes causing washed out images
and unwanted glare. This form of light pollution involves
light traveling directly from an unwanted light source
(such as a street lamp) to your eye/telescope.
Light pollution is excessive,
misdirected or inappropriate outdoor
lighting. Too much of light pollution
washes out view of the Universe, result
in increase in the energy consumption,
interferes with astronomical research…
Describe the causes and remedies for light pollution
• Urban Sky glow- artificial lights pointing
upward, making the sky glow and blocks
the view of stars.
• It can be minimized if cities would dim, or
turn lights off at night.
Gamma-ray telescope
Gamma-ray telescope, instrument designed to detect and
resolve gamma rays from sources outside Earth’s
atmosphere.
Gamma rays are the shortest waves (about 0.1 angstrom or less) and
therefore have the highest energy in the electromagnetic spectrum.
Since gamma rays have so much energy, they pass right through the
mirror of a standard optical telescope. Instead, gamma rays are
detected by the optical flashes they produce when interacting with
the material in a specially designed instrument such as a scintillation
detector. Earth’s atmosphere blocks most gamma rays, so most
gamma-ray telescopes are carried on satellites and balloons.
However, some ground-based telescopes can observe the Cherenkov
radiation produced when a gamma ray strikes Earth’s upper
atmosphere.
The Compton Gamma Ray Observatory as seen through the space shuttle
window during deployment in …
NASA
X-ray telescope, instrument designed to detect and
resolve X-rays from sources outside Earth’s
atmosphere.
Because of atmospheric absorption, X-ray
telescopes must be carried to high altitudes by
rockets or balloons or placed in orbit outside the
atmosphere. Balloon-borne telescopes can detect
the more penetrating (harder) X-rays, whereas those
carried aloft by rockets or in satellites are used to
detect softer radiation.
X-rays are blocked by ozone and
oxygen present in the Earth's
atmosphere.
Ultraviolet telescope, telescope used to examine the ultraviolet
portion of the electromagnetic spectrum, between the portion seen as
visible light and the portion occupied by X-rays.
Ultraviolet radiation has wavelengths of about 400 nanometres (nm)
on the visible-light side and about 10 nm on the X-ray side. Earth’s
stratospheric ozone layer blocks all wavelengths shorter than 300 nm
from reaching ground-based telescopes. As this ozone layer lies at an
altitude of 20–40 km (12–25 miles), astronomers have to resort to
rockets and satellites to make observations from above it.
Telescopes
Review
Using ___________, scientists can use a few
smaller telescopes to take images with the
same resolution as a much larger telescope.
A. Satellite telescopes
B. Charge-coupled devices (CCDs)
C. Interferometry
D. Adaptive optics
Using ___________, scientists can use a few
smaller telescopes to take images with the
same resolution as a much larger telescope.
A. Satellite telescopes
B. Charge-coupled devices (CCDs)
C. Interferometry
D. Adaptive optics
A warm dust cloud in which stars are forming
would be best observed with a(n) _________
telescope.
A. Radio
B. Gamma-Ray
C. Infrared
D. Optical (visible light)
A warm dust cloud in which stars are forming
would be best observed with a(n) _________
telescope.
A. Radio
B. Gamma-Ray
C. Infrared
D. Optical (visible light)
In a false color image,
A. Only one color is used.
B. Color information from optical telescopes is
combined with information from a non-visible
wavelength telescope to make the image.
C. Stars appear black and the background
appears white.
D. Colors can represent photon energies or the
intensity of electromagnetic radiation.
In a false color image,
A. Only one color is used.
B. Color information from optical telescopes is
combined with information from a non-visible
wavelength telescope to make the image.
C. Stars appear black and the background
appears white.
D. Colors can represent photon energies or the
intensity of electromagnetic radiation.
If your pupils have a diameter of about 5mm,
about how many times more light gathering
power does a telescope with a diameter of about
20 cm (8 inches) have than your pupils?
A. 4 times
B. 16 times
C. 40 times
D. 160 times
E. 1600 times
If your pupils have a diameter of about 5mm,
about how many times more light gathering
power does a telescope with a diameter of
about 20 cm (8 inches) have than your
pupils?
A. 4 times
B. 16 times
C. 40 times
D. 160 times
E. 1600 times
One of the biggest problems for ground based
astronomy today is
A. All the best mountains already have
telescopes on them.
B. Atmospheric pollution has significantly
worsened the seeing.
C. Space-based observatories are making most of
the observations.
D. Light pollution makes it difficult to observe
faint objects.
One of the biggest problems for ground based
astronomy today is
A. All the best mountains already have telescopes
on them.
B. Atmospheric pollution has significantly
worsened the seeing.
C. Space-based observatories are making most of
the observations.
D. Light pollution makes it difficult to observe faint
objects.
. A pencil inside a water glass appears to be
bent because of
A. Reflection of light.
B. Refraction of light.
C. Diffraction of light.
D. None of the above.
A pencil inside a water glass appears to be
bent because of
A. Reflection of light.
B. Refraction of light.
C. Diffraction of light.
D. None of the above.
Why does the Moon appear bigger near the
horizon?
A. Due to an optical illusion.
B. Due to refraction.
C. Due to reflection.
D. Due to the compression produced by the
Earth's atmosphere.
Why does the Moon appear bigger near the
horizon?
A. Due to an optical illusion.
B. Due to refraction.
C. Due to reflection.
D. Due to the compression produced by the
Earth's atmosphere.
The ability of a telescope to show two very
close objects separately is called its
________.
A. Light gathering power
B. Resolving power
C. Magnification
D. None of the above
The ability of a telescope to show two very
close objects separately is called its
________.
A. Light gathering power
B. Resolving power
C. Magnification
D. None of the above
Tom has a 4-inch refracting telescope and Steve
has a 3-inch reflecting telescope. Whose
telescope has a higher resolving power?
A. Tom's, because lenses are more efficient in
showing the objects separately.
B. Steve's, because mirrors are more efficient in
showing the objects separately.
C. Tom's, because the larger the diameter, the
better the resolution.
D. Steve's, because the smaller the diameter, the
better the resolution.
Tom has a 4-inch refracting telescope and Steve
has a 3-inch reflecting telescope. Whose
telescope has a higher resolving power?
A. Tom's, because lenses are more efficient in
showing the objects separately.
B. Steve's, because mirrors are more efficient in
showing the objects separately.
C. Tom's, because the larger the diameter, the
better the resolution.
D. Steve's, because the smaller the diameter, the
better the resolution.
The best site for placing an X-ray telescope is
_________.
A. A mountain top
B. A valley
C. Near an ocean
D. A desert
E. Above the Earth's atmosphere
The best site for placing an X-ray telescope is
_________.
A. A mountain top
B. A valley
C. Near an ocean
D. A desert
E. Above the Earth's atmosphere
An interstellar gas is emitting 10-centimeter
wavelength radiation and a nearby star is
emitting 10-micrometer wavelength radiation.
Which of these can you observe through an
Earth-based telescope?
A. The interstellar gas
B. The star
C. Both of them
D. Neither of them
An interstellar gas is emitting 10-centimeter
wavelength radiation and a nearby star is
emitting 10-micrometer wavelength radiation.
Which of these can you observe through an
Earth-based telescope?
A. The interstellar gas
B. The star
C. Both of them
D. Neither of them
Why do stars twinkle?
A. Due to rapid changes occurring on their
surfaces.
B. Due to their movement across the sky.
C. Due to the refraction caused by
atmospheric irregularities.
D. Due to imperfections in the human eye.
Why do stars twinkle?
A. Due to rapid changes occurring on their
surfaces.
B. Due to their movement across the sky.
C. Due to the refraction caused by atmospheric
irregularities.
D. Due to imperfections in the human eye.
Which space telescope is observing X-rays?
A. The Hubble Space Telescope (HST)
B. The Chandra Observatory
C. The Spitzer Space Telescope
D. The Extreme Ultraviolet Explorer (EUVE)
Which space telescope is observing X-rays?
A. The Hubble Space Telescope (HST)
B. The Chandra Observatory
C. The Spitzer Space Telescope
D. The Extreme Ultraviolet Explorer (EUVE)
Which of the following telescopes is most
suitable for observing cool gas clouds?
A. X-ray telescope.
B. Radio telescope.
C. Visible light telescope.
D. All of the above.
E. Cool gas clouds cannot be observed by
telescopes.
Which of the following telescopes is most
suitable for observing cool gas clouds?
A. X-ray telescope.
B. Radio telescope.
C. Visible light telescope.
D. All of the above.
E. Cool gas clouds cannot be observed by
telescopes.
If a 3 meter diameter telescope is doubled in
size, then its new light collecting power would
A. Not change.
B. Double.
C. Increase by a factor of four.
D. Reduce by half.
If a 3 meter diameter telescope is doubled in
size, then its new light collecting power would
A. Not change.
B. Double.
C. Increase by a factor of four.
D. Reduce by half.
______ is the most important quality of an
astronomical telescope.
A. Magnification
B. Resolving power
C. Ability to see at night
D. Rigidity
______ is the most important quality of an
astronomical telescope.
A. Magnification
B. Resolving power
C. Ability to see at night
D. Rigidity
Which of the following is an inherent
disadvantage of radio telescopes?
A. Radio telescopes cannot detect visible light.
B. Radio telescopes have low magnification.
C. Radio signals are very weak, and their photons
do not penetrate the atmosphere easily.
D. The long wavelength of radio waves results in
lower resolving power, compared to other
telescopes of the same size.
E. They only work at night.
Which of the following is an inherent disadvantage
of radio telescopes?
A. Radio telescopes cannot detect visible light.
B. Radio telescopes have low magnification.
C. Radio signals are very weak, and their photons
do not penetrate the atmosphere easily.
D. The long wavelength of radio waves results in
lower resolving power, compared to other
telescopes of the same size.
E. They only work at night.
Which is the most efficient means of
recording light?
A. Photographic films
B. Photomultiplier tubes
C. Charge-coupled device (CCD)
D. The human eye
Which is the most efficient means of recording
light?
A. Photographic films
B. Photomultiplier tubes
C. Charge-coupled device (CCD)
D. The human eye
X-rays are blocked by ________ and
_________ present in the Earth's atmosphere.
A. Water molecules; carbon dioxide
B. Ozone; oxygen
C. Nitrogen; helium
D. Electric charges; clouds
X-rays are blocked by ________ and
_________ present in the Earth's atmosphere.
A. Water molecules; carbon dioxide
B. Ozone; oxygen
C. Nitrogen; helium
D. Electric charges; clouds
The best site for placing a ground-based
optical telescope is _________.
A. A mountain top
B. A valley
C. In an urban setting
D. in a location that has few sunny days
throughout the year.
The best site for placing a ground-based
optical telescope is _________.
A. A mountain top
B. A valley
C. In an urban setting
D. in a location that has few sunny days
throughout the year.
What is "seeing"?
A. The ability of a telescope to show two very
close objects separately.
B. The capacity of a telescope to gather more
light.
C. The ability of a telescope to see in the
night.
D. The distortion of an image due to an
atmospheric turbulence.
What is "seeing"?
A. The ability of a telescope to show two very
close objects separately.
B. The capacity of a telescope to gather more
light.
C. The ability of a telescope to see in the night.
D. The distortion of an image due to an
atmospheric turbulence.
Carl has a 5-inch refracting telescope and Jim
has a 3-inch reflecting telescope. Whose
telescope has a higher light-gathering power?
A. Carl's, because lenses gather more light.
B. Jim's, because mirrors gather more light.
C. Carl's, because the larger the diameter, the
more light to be collected.
D. Jim's, because the smaller the diameter, the
more light to be collected.
Carl has a 5-inch refracting telescope and Jim
has a 3-inch reflecting telescope. Whose
telescope has a higher light-gathering power?
A. Carl's, because lenses gather more light.
B. Jim's, because mirrors gather more light.
C. Carl's, because the larger the diameter, the
more light to be collected.
D. Jim's, because the smaller the diameter, the
more light to be collected.
What is a disadvantage of using a single, large lens in
a telescope?
A. Large lenses are expensive to fabricate.
B. A lens has to be supported only at its edges, so the
lens can sag in the middle.
C. Different colors of white light on passing through a
lens focus at different points and result in a blurred
image.
D. Some lens materials completely absorb short
wavelengths.
E. All of the above.
What is a disadvantage of using a single, large
lens in a telescope?
A. Large lenses are expensive to fabricate.
B. A lens has to be supported only at its edges, so
the lens can sag in the middle.
C. Different colors of white light on passing
through a lens focus at different points and result
in a blurred image.
D. Some lens materials completely absorb short
wavelengths.
E. All of the above.
Why does the Sun look flattened near the
horizon?
A. The Sun's light is reflected off the horizon,
making the Sun appear compressed.
B. The Earth's dense atmosphere compresses the
gaseous Sun.
C. The larger refraction near the horizon lifts the
lower edge of the Sun more than the upper edge
and makes the Sun look flattened.
D. The Sun is cooler on the horizon, so it looks
flattened.
Why does the Sun look flattened near the
horizon?
A. The Sun's light is reflected off the horizon,
making the Sun appear compressed.
B. The Earth's dense atmosphere compresses the
gaseous Sun.
C. The larger refraction near the horizon lifts the
lower edge of the Sun more than the upper edge
and makes the Sun look flattened.
D. The Sun is cooler on the horizon, so it looks
flattened.
The two most important properties of any
telescope are
A. The light gathering power and the length of
the tube.
B. The length of the tube and the magnification.
C. The magnification and the light gathering
power.
D. The resolving power and the magnification.
E. The light gathering power and the resolving
power.
The two most important properties of any
telescope are
A. The light gathering power and the length of
the tube.
B. The length of the tube and the
magnification.
C. The magnification and the light gathering
power.
D. The resolving power and the
magnification.
E. The light gathering power and the
The resolving power of a telescope is affected
by the property of light called
A. Refraction.
B. Diffraction.
C. Reflection.
D. Seeing.
E. Attenuation.
The resolving power of a telescope is affected
by the property of light called
A. Refraction.
B. Diffraction.
C. Reflection.
D. Seeing.
E. Attenuation.
Ignoring the effects of the atmosphere, what is
the theoretical resolution of an 8 inch
telescope (about 20 cm) if you are looking at
visible light with a wavelength of about 500
nm?
A. About half an arcsecond.
B. About half a degree.
C. About 1/100 of an arcsecond.
D. About 25 arcseconds.
Ignoring the effects of the atmosphere, what is
the theoretical resolution of an 8 inch telescope
(about 20 cm) if you are looking at visible light
with a wavelength of about 500 nm?
A. About half an arcsecond.
B. About half a degree.
C. About 1/100 of an arcsecond.
D. About 25 arcseconds.
A galaxy with almost no starlight but plenty
of cool clouds of hydrogen gas would be best
observed with a(n) _________ telescope.
A. X-ray
B. Radio
C. Infrared
D. Optical (visible light)
A galaxy with almost no starlight but plenty
of cool clouds of hydrogen gas would be best
observed with a(n) _________ telescope.
A. X-ray
B. Radio
C. Infrared
D. Optical (visible light)
To double the resolving power of a telescope,
you must _________.
A. Increase the diameter by a factor of two
B. Increase the collecting area by a factor of
two
C. Decrease the diameter by half
D. Decrease the collecting area by half
To double the resolving power of a telescope,
you must _________.
A. Increase the diameter by a factor of two
B. Increase the collecting area by a factor of two
C. Decrease the diameter by half
D. Decrease the collecting area by half
T or F
An optical telescope is able to resolve blue
objects better than red objects.
T or F
An optical telescope is able to resolve blue
objects better than red objects.
T or F
Gamma ray telescopes, such as the Fermi
Gamma Ray Space Telescope, are launched
into space because they are dangerous to
operate on Earth.
T or F
Gamma ray telescopes, such as the Fermi
Gamma Ray Space Telescope, are launched
into space because they are dangerous to
operate on Earth.
T or F
To double the light gathering power of a
telescope, we need to double the diameter.
T or F
To double the light gathering power of a
telescope, we need to double the diameter.
T or F
Most of the modern large optical telescopes
are refractors.
T or F
Most of the modern large optical telescopes
are refractors.
T or F
In a reflecting telescope, the secondary mirror
causes a hole in the center of the image.
T or F
In a reflecting telescope, the secondary mirror
causes a hole in the center of the image.
How do astronomers calculate a telescope's light
collecting area (A)? (where r = radius of the
telescopes mirror or lens, and B is "pi" or
3.14159)
A) A = 2Br
B) A = Br2
C) A = 4Br2
D) A = 4/3 Br3
How do astronomers calculate a telescope's light
collecting area (A)? (where r = radius of the
telescopes mirror or lens, and B is "pi" or
3.14159)
A) A = 2Br
B) A = Br2
C) A = 4Br2
D) A = 4/3 Br3
Telescope A has a mirror twice the diameter
of telescope B's mirror. How does A's lightgathering power compare to B's?
A) A gathers 1/2 the light that B does.
B) A gathers twice the light that B
does.
C) A gathers 4 times the light that B
does.
D) A gathers 1/4th as much light as B
does.
Telescope A has a mirror twice the diameter
of telescope B's mirror. How does A's lightgathering power compare to B's?
A) A gathers 1/2 the light that B does.
B) A gathers twice the light that B
does.
C) A gathers 4 times the light that B
does.
D) A gathers 1/4th as much light as B
does.
A telescope's resolving power measures its
ability to see
A) fainter sources.
B) more distant sources.
C) finer details in sources.
D) larger sources.
A telescope's resolving power measures its
ability to see
A) fainter sources.
B) more distant sources.
C) finer details in sources.
D) larger sources.
One way to increase the resolving power of a
telescope is to
A) make its mirror bigger.
B) make its mirror smaller.
C) replace its mirror with a lens of the
same diameter.
D) observe objects using longer
wavelengths.
One way to increase the resolving power of a
telescope is to
A) make its mirror bigger.
B) make its mirror smaller.
C) replace its mirror with a lens of the
same diameter.
D) observe objects using longer
wavelengths.
What causes the image of a star to "twinkle" or
scintillate?
A) irregularities in the density of layers
of
Earth's atmosphere, rapidly refracting
the light.
B) the flickering fusion process in the
stars.
C) a purely physiological reaction in the
eye.
D) none of these answers are correct.
What causes the image of a star to "twinkle" or
scintillate?
A) irregularities in the density of layers
of
Earth's atmosphere, rapidly refracting
the light.
B) the flickering fusion process in the
stars.
C) a purely physiological reaction in the
eye.
D) none of these answers are correct.
What process limits a telescope's resolving
power?
A) Reflection
B) Refraction
C) Diffraction
D) Distraction
What process limits a telescope's resolving
power?
A) Reflection
B) Refraction
C) Diffraction
D) Distraction
What is the difference between a reflecting and a
refracting telescope?
A)
A reflecting telescope uses a lens to
focus light; a refracting telescope uses a
mirror.
B)
A reflecting telescope uses a mirror to
focus
light; a refracting telescope uses a lens.
C)
Both use a mirror to focus light, but a
reflecting telescope uses mirrors
elsewhere
to bend light.
D)
There is no difference. "Refracting" is an
outmoded way of saying "reflecting."
What is the difference between a reflecting and a
refracting telescope?
A)
A reflecting telescope uses a lens to
focus light; a refracting telescope uses a
mirror.
B)
A reflecting telescope uses a mirror to
focus
light; a refracting telescope uses a lens.
C)
Both use a mirror to focus light, but a
reflecting telescope uses mirrors
elsewhere
to bend light.
D)
There is no difference. "Refracting" is an
outmoded way of saying "reflecting."
Light travels __ in water than in air
A) faster
B) slower
C) at the same speed
D) perpendicular to
Light travels __ in water than in air
A) faster
B) slower
C) at the same speed
D) perpendicular to
Which of the these is a reason for using mirrors rather
than lenses in telescopes?
A)
Lenses are more expensive to make than
mirrors.
B)
Most transparent materials focus light of
different colors to different spots.
C)
Lenses can only be supported by their
edges, making them sag in the middle
(mirrors can be supported from behind).
D)
All of the above.
Which of the these is a reason for using mirrors rather
than lenses in telescopes?
A)
Lenses are more expensive to make than
mirrors.
B)
Most transparent materials focus light of
different colors to different spots.
C)
Lenses can only be supported by their
edges, making them sag in the middle
(mirrors can be supported from behind).
D)
All of the above.
Why use an interferometer?
A)
Its two widely-spaced mirrors act like
one giant telescope with increased collecting
area.
B)
Its two widely-spaced mirrors act like
one
giant telescope with increased resolving
power.
C)
By putting one mirror above the other
you
can make the instrument much smaller.
D)
It can detect light at wavelengths not
available to single telescopes.
Why use an interferometer?
A)
Its two widely-spaced mirrors act like
one giant telescope with increased collecting
area.
B)
Its two widely-spaced mirrors act like
one
giant telescope with increased resolving
power.
C)
By putting one mirror above the other
you
can make the instrument much smaller.
D)
It can detect light at wavelengths not
available to single telescopes.
Which of the following astronomical objects
emit radiation mostly at non-visible
wavelengths?
A) dust clouds in space
B) hot gas surrounding black holes
C) cold interstellar gas clouds
D) all of the above
Which of the following astronomical objects
emit radiation mostly at non-visible
wavelengths?
A) dust clouds in space
B) hot gas surrounding black holes
C) cold interstellar gas clouds
D) all of the above
Do astronomers use ground-based X-ray telescopes?
A)
Yes, because they can penetrate gas
clouds.
B)
No, because no astronomical objects emit
xrays.
C)
No, because x-rays cannot get through
the
Earth's atmosphere.
D)
No, because astronomers have not yet
devised detectors for x-rays.
Do astronomers use ground-based X-ray telescopes?
A)
Yes, because they can penetrate gas
clouds.
B)
No, because no astronomical objects emit
xrays.
C)
No, because x-rays cannot get through
the
Earth's atmosphere.
D)
No, because astronomers have not yet
devised detectors for x-rays.
Why does the useful resolving power of a
ground-based telescope not match it's theoretical
value?
A) Mirrors can't be built accurately
enough.
B) The atmosphere blurs the image,
decreasing the resolving power.
C) Mirrors cannot collect enough light
to
reach their theoretical expectations.
D) The theoretical value can only be
reached when there is a Full Moon.
Why does the useful resolving power of a
ground-based telescope not match it's theoretical
value?
A) Mirrors can't be built accurately
enough.
B) The atmosphere blurs the image,
decreasing the resolving power.
C) Mirrors cannot collect enough light
to
reach their theoretical expectations.
D) The theoretical value can only be
reached when there is a Full Moon.
On some telescopes, actuators on the mirror
change its shape to match distortions in the
atmosphere. What is this technique called?
A) Actuary observing
B) Interferometry.
C) Refraction
D) Adaptive Optics
On some telescopes, actuators on the mirror
change its shape to match distortions in the
atmosphere. What is this technique called?
A) Actuary observing
B) Interferometry.
C) Refraction
D) Adaptive Optics
Which of the following is a reason to build an
observatory in space?
A) They are much less expensive than
ground-based observatories.
B) They can last forever.
C) To avoid atmospheric blurring.
D) There is no good reason to build a
space-based observatory.
Which of the following is a reason to build an
observatory in space?
A) They are much less expensive than
ground-based observatories.
B) They can last forever.
C) To avoid atmospheric blurring.
D) There is no good reason to build a
space-based observatory.