03 Telescopes - Physics and Astronomy

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Chapter 3
Telescopes
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Telescopes
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Optical Telescopes
Images can be formed through reflection or
refraction.
Reflecting mirror
Collect a
large amount
of light
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Optical Telescopes
Refracting lens
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Optical Telescopes
Reflecting and refracting telescopes
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Optical Telescopes
Modern telescopes are all reflectors:
• Light traveling through lens is refracted
differently depending on wavelength.
• Some light traveling through lens is absorbed.
• Large lens can be very heavy, and can only be
supported at edge.
• Lens needs two optically acceptable surfaces,
mirror only needs one.
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Chromatic aberration
Question 1
Modern telescopes
use mirrors rather
than lenses for all
of these reasons
EXCEPT
a) light passing through lenses can be
absorbed or scattered.
b) large lenses can be very heavy.
c) large lenses are more difficult to make.
d) mirrors can be computer controlled to
improve resolution.
e) reflecting telescopes aren’t affected by
the atmosphere as much.
Question 1
Modern telescopes
use mirrors rather
than lenses for all
of these reasons
EXCEPT
a) light passing through lenses can be
absorbed or scattered.
b) large lenses can be very heavy.
c) large lenses are more difficult to make.
d) mirrors can be computer controlled to
improve resolution.
e) reflecting telescopes aren’t affected by
the atmosphere as much.
Reflecting instruments like
the KECK telescopes can
be made larger, and more
capable, than refractors.
Optical Telescopes
Image acquisition: Chargecoupled devices (CCDs) are
electronic devices, can be
quickly read out and reset.
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CCD Camera Sensor
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Question 2
An advantage of
CCDs over
photographic
film is
a) they don’t require chemical development.
b) digital data is easily stored & transmitted.
c) CCDs are more light sensitive than film.
d) CCD images can be developed faster.
e) All of the above are true.
Question 2
An advantage of
CCDs over
photographic
film is
a) they don’t require chemical development.
b) digital data is easily stored & transmitted.
c) CCDs are more light sensitive than film.
d) CCD images can be developed faster.
e) All of the above are true.
Optical Telescopes
Image processing by computers can sharpen
images.
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Ground Based Telescope
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Early HST
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Computer Enhanced HST
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Later HST
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The Hubble Space Telescope
The Hubble Space Telescope has several
instruments.
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The Hubble Space Telescope
Resolution achievable by the Hubble Space
Telescope limited by optics and not atmosphere
Ground Based – M100
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Telescope Size
Light-gathering power:
Improves detail
Brightness proportional
to square of radius of
mirror
R=1
The figure, part (b) was
taken with a telescope
twice the size of (a)
Area = p
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2
R
R=2
Telescope Size
Multiple telescopes: Mauna Kea
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Telescope Size
The VLT (Very Large Telescope), Atacama, Chile
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Question 3
Diffraction is the
tendency of light
to
a) bend around corners and edges.
b) separate into its component colors.
c) bend through a lens.
d) disperse within a prism.
e) reflect off a mirror.
Question 3
Diffraction is the
tendency of light
to
a) bend around corners and edges.
b) separate into its component colors.
c) bend through a lens.
d) disperse within a prism.
e) reflect off a mirror.
Diffraction affects all
telescopes and limits the
sharpness of all images.
Telescope Size
Resolving power: Ability to distinguish
objects that are close together.
Resolution is proportional to wavelength and
inversely proportional to telescope size.
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Question 4
Resolution is
improved by
using
a) larger telescopes & longer wavelengths.
b) infrared light.
c) larger telescopes & shorter wavelengths.
d) lower frequency light.
e) visible light.
Question 4
Resolution is
improved by
using
10’
a) larger telescopes & longer wavelengths.
b) infrared light.
c) larger telescopes & shorter wavelengths.
d) lower frequency light.
e) visible light.
1”
Diffraction limits resolution; larger telescopes and
shorter-wave light produces sharper images.
Telescope Size
Effect of improving resolution:
(a) 10′; (b) 1′; (c) 5″; (d) 1″
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Question 5
Seeing in
astronomy is a
measurement
of
a) the quality of the telescope’s optics.
b) the transparency of a telescope’s lens.
c) the sharpness of vision of your eyes.
d) the image quality due to air stability.
e) the sky’s clarity & absence of clouds.
Question 5
Seeing in
astronomy is a
measurement
of
a) the quality of the telescope’s optics.
b) the transparency of a telescope’s lens.
c) the sharpness of vision of your eyes.
d) the image quality due to air stability.
e) the sky’s clarity & absence of clouds.
Smeared overall image of star
“Good Seeing” occurs when the
atmosphere is clear and the air is still.
Turbulent air produces “poor seeing,”
and fuzzier images.
Point images of a star
Question 6
Adaptive optics
refers to
a) making telescopes larger or smaller.
b) reducing atmospheric blurring using
computer control.
c) collecting different kinds of light
with one type of telescope.
d) using multiple linked telescopes.
Question 6
Adaptive optics
refers to
a) making telescopes larger or smaller.
b) reducing atmospheric blurring using
computer control.
c) collecting different kinds of light
with one type of telescope.
d) using multiple linked telescopes.
Cluster R136
Shaping a mirror in “real time” can dramatically improve resolution.
High-Resolution Astronomy
Atmospheric blurring due to air movements
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High-Resolution Astronomy
Solutions:
• Put telescopes on mountaintops, especially in
deserts.
• Put telescopes in space.
• Active (adaptive) optics – control mirrors based
on temperature and orientation.
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High-Resolution Astronomy
Solutions:
• Without active (adaptive) optics a 1m
telescope is limited to about 2’
•With active (adaptive) optics the same 1m
telescope could approach 0.1”.
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Radio Astronomy
Radio telescopes:
• Similar to optical reflecting telescopes
• Prime focus
• Less sensitive to imperfections (due to longer
wavelength); can be made very large
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105 m Green Bank, WV
Question 7
Radio
dishes are
large
because
a) radio photons don’t carry much energy.
b) they are painted white.
c) they are cheap to make.
d) they are can operate during the day.
Question 7
Radio
dishes are
large in
order to
a) radio photons don’t carry much energy.
b) they are painted white.
c) they are cheap to make.
d) they are can operate during the day.
Resolution is worse with
long-wave light, so radio
telescopes must be large
to compensate.
Radio Astronomy
Largest radio telescope: 300-m dish at Arecibo
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Question 8
Radio telescopes
are useful because
a) observations can be made day & night.
b) we can see objects that don’t emit
visible light.
c) radio waves are not blocked by
interstellar dust.
d) they can be linked to form
interferometers.
e) All of the above are true.
Question 8
Radio telescopes
are useful because
a) observations can be made day & night.
b) we can see objects that don’t emit
visible light.
c) radio waves are not blocked by
interstellar dust.
d) they can be linked to form
interferometers.
e) All of the above are true.
The Very Large Array
links separate radio
telescopes to create
much better resolution.
Radio Astronomy
Longer wavelength means poorer angular
resolution.
Advantages of radio astronomy:
• Can observe 24 hours a day.
• Clouds, rain,
and snow don’t
interfere.
• Observations
at an entirely
different
frequency; get
totally different
information.
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Centaurus A
Radio Astronomy
Interferometry:
• Combines information
from several widely
spread radio telescopes
as if it came from a
single dish.
• Resolution will be
that of dish whose
diameter = largest
separation between
dishes.
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Radio Astronomy
Interferometry
requires
preserving the
phase relationship
between waves
over the distance
between individual
telescopes.
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Radio Astronomy
Can get radio images whose
resolution is close to optical.
M51
VLA
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4 m Kitt Peak
Radio Astronomy
Interferometry can also be done with visible light,
but much harder due to shorter wavelengths.
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CHARA 1 m at Mount Wilson, CA
Other Astronomies
Infrared radiation can image where visible
radiation is blocked; generally can use optical
telescope mirrors and lenses.
Orion Nebula
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Question 9
Infrared telescopes
are very useful for
observing
a) pulsars & black holes.
b) from locations on the ground.
c) hot stars & intergalactic gas.
d) neutron stars.
e) cool stars & star-forming regions.
Question 9
Infrared telescopes
are very useful for
observing
a) pulsars & black holes.
b) from locations on the ground.
c) hot stars & intergalactic gas.
d) neutron stars.
e) cool stars & star-forming regions.
Infrared images of star-forming
“nurseries” can reveal objects
still shrouded in cocoons of
gas and dust.
Other Astronomies
Infrared telescopes
can also be in space
or flown on balloons.
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Other Astronomies
Ultraviolet images
(a)The Cygnus loop supernova remnant
(b) M81
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Other Astronomies
X rays and gamma rays will not reflect off mirrors
as other wavelengths do; need new techniques.
X rays will reflect at a very shallow angle, and
can therefore be focused.
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Other Astronomies
X-ray image
of supernova
remnant
Cassiopeia A
Cas A
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Other Astronomies
Gamma rays cannot be focused at all; images are
therefore coarse.
Galaxy 3C279
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Other Astronomies
Much can be learned from
observing the same
astronomical object at many
wavelengths. Here, the Milky
Way.
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RADIO
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INFRARED
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VISIBLE
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X-RAY
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GAMMA-RAY
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Question 10
The Hubble Space
Telescope (HST) offers
sharper images than
ground telescopes
primarily because
a) HST is closer to planets & stars.
b) HST uses a larger primary mirror.
c) it gathers X-ray light.
d) HST orbits above the atmosphere.
e) it stays on the nighttime side of
Earth.
Question 10
The Hubble Space
Telescope (HST) offers
sharper images than
ground telescopes
primarily because
a) HST is closer to planets & stars.
b) HST uses a larger primary mirror.
c) it gathers X-ray light.
d) HST orbits above the atmosphere.
e) it stays on the nighttime side of
Earth.
HST orbits less than 400 miles
above Earth – not much closer
to stars & planets!
But it can gather UV, visible,
and IR light, unaffected by
Earth’s atmosphere.
Example Exam Questions
One advantage of the Hubble Space telescope
over ground based ones is that
A. it can better focus x-ray images.
B. in orbit, it can operate close to its diffraction
limit at visible wavelengths.
C. it is larger than any Earth-based scopes.
D. its adaptive optics controls atmospheric
blurring better.
E. it can make better observations of the ozone
layer.
What problem does adaptive optics correct?
A. defects in the optics of the telescope, such as
the original Hubble mirror
B. turbulence in the earth's atmosphere which
creates twinkling
C. the opacity of the earth's atmosphere to some
wavelengths of light
D. chromatic aberration due to use of only a single
lens objective
E. the light pollution of urban areas
An emission line results from an electron falling
from a higher to lower energy orbital around its
atomic nucleus.
A. True
B. False
The angular resolution of an 8 inch diameter
telescope is _______ greater than that of a 2 inch
diameter telescope.
A. 2×
B. 4×
C. 8×
D. 9×
E. 16×
The larger the red shift, the faster the distant
galaxy is rushing toward us.
A. True
B. False
Changing the electric field will have no effect on the
magnetic fields of a body.
A. True
B. False
Doubling the temperature of a black body will
double the total energy it radiates.
A. True
B. False

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