Telescopes
Reading: Chapter 4.2-4.5
Telescopes
A telescope is a type of light collecting device
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Telescopes
Telescopes come in various sizes and locations…
Arecibo, Puerto Rico
VLA, New Mexico
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Telescopes
Technology helps make better telescopes…
Newton’s telescope
Hubble Space Telescope
Galileo’s telescope
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Reflection and Refraction
Interfaces and light propagation
•
Light rays can be reflected (bounced off) or refracted (bent) at
an interface between two materials:
Thin Lenses
What is a lens?
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Thin Lenses
What is a lens?
A Lens has usually two refracting
surfaces.
If the surfaces are close, we have a thin
lens.
Properties:
F1)
Parallel rays converge at the focus, F2 (or
Convex lenses
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The eye
• Refraction can cause parallel light rays to converge to a
focus
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The eye
• The focal plane is where light from different directions comes
into focus.
• The image behind a single (convex) lens is actually upsidedown! Your brain flips the image.
The eye
• If you have perfect vision, then the focal plane is on your retina.
• If you have weak eyesight, lenses in your spectacles further
refract light to adjust the focal plane to lie on the retina.
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The eye
Corrective lenses are required since eye changes with age:
(farsighted)
Diopter =
1
Focal Length (in m)
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Camera
Digital
cameras
detect light
with chargecoupled
devices
(CCDs)
• A camera focuses light like an eye and captures the image
with a detector (film or CCDs)
• The CCD detectors in digital cameras are similar to those
used in modern telescopes
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Camera
High quality camera lens array:
Multiple lenses partially correct for various aberrations
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Pictures courtesy of LEICA
Camera
• What are the advantages of a camera over the eye?
– Image can be reliably stored for later analysis.
– Image has more details
– Exposure time (amount of light hitting detector) can be
controlled. Faint objects can be observed with long
exposure times).
• What are the advantages of CCDs over film ?
– More sensitive to light.
– Broader dynamic range: bright and faint objects can be
recorded at the same time.
– Image stored as digital data that can be processed on a
computer.
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Basic designs of telescopes
• Refracting telescope:
Focuses light with lenses
• Reflecting telescope:
Focuses light with mirrors
Refracting Telescope
f2
f1
Angular magnification M is large when f1 is much
greater than f2
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Refracting Telescope
• Refracting telescopes need to be very long, to
maximize the distance between the lenses
• They have large, heavy lenses for good light collection
Reflecting Telescope
• Reflecting telescopes can have much greater
diameters
• Modern telescopes (built after 1900) are
reflectors
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Designs for Reflecting Telescopes
Small telescopes can use other focal
arrangements that would be inconvenient
in larger telescopes
Reflecting Telescope
Example: McMath-Pierce Solar Telescope
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Reflecting Telescope
Example: McMath-Pierce Solar Telescope
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Reflecting Telescope
• What are the advantages of a reflecting telescope over a
refracting telescope ?
– Only the reflecting surface of mirrors in a reflecting
telescope have to be perfectly shaped. In a lens the
entire shape of the lens and both surfaces are
important.
– Objective lenses are heavy and difficult to stabilize at
the top of the telescope. Heavy mirrors at the bottom
of the telescope are less problematic
– Lenses have chromatic aberrations that must be
corrected.
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What are the two most important
properties of a telescope?
1. Light-collecting area: Telescopes with
a larger collecting area can gather a
greater amount of light in a shorter time.
2. Angular resolution: Telescopes that are
larger are capable of taking images with
greater detail.
Light Collecting Area
• A telescope’s diameter tells us its lightcollecting area: Area = !(diameter/2)2
• The largest telescopes currently in use have
a diameter of about 10 meters
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Thought Question
How does the collecting area of a 10-meter
telescope compare with that of a 2-meter
telescope?
a) It’s 5 times greater.
b) It’s 10 times greater.
c) It’s 25 times greater.
Thought Question
How does the collecting area of a 10-meter
telescope compare with that of a 2-meter
telescope?
a) It’s 5 times greater.
b) It’s 10 times greater.
c) It’s 25 times greater.
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Angular Resolution
• The minimum angular
separation that the
telescope can
distinguish.
• Also called resolving
power
Recall that Angular separation= actual separation x 360 degrees/(2! x distance)
Angular Resolution
• Ultimate limit to
resolution comes from
interference of light
waves within a
telescope.
• there is blurring called
a diffraction fringe
around every point of
light in the image.
• You cannot see any
detail smaller than the
fringe.
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Angular Resolution
• This limit on
angular resolution
is known as the
diffraction limit
• Larger telescopes
are capable of
greater resolution
because there’s
less interference
Angular Resolution
• Diffraction limit depends on the wavelength
of light and diameter of the telescope
2
wavelength
! 25 $
Diffraction limit (degrees) = # & '
" 3%
diameter
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Imaging
• Astronomical
detectors
generally
record only
one colour of
light at a time
• Several images
must be
combined to
make fullcolour pictures
Imaging
• The furthest objects in this
picture are 11 billion lightyears from Earth
• This picture is a true
colour image made from
separate exposures taken
in blue, green, and far-red
light. It required 48 orbits
around the Earth (more
than one day of exposure
time) to make the
observation.
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Imaging
HST photograph of Mars during closest approach in 2005
Imaging
• Astronomical detectors can record forms of light our eyes
can’t see
• False-colour or colour coded images use colour to represent
- different energies of nonvisible light
- different atoms in the object
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Imaging
• Columns of cool interstellar
hydrogen gas and dust that are
also incubators for new stars.
• The color image is constructed
from three separate images: Red
shows emission from sulfur ions,
green shows emission from
hydrogen, blue shows emission
from oxygen ions.
Eagle nebula: Pillars of creation
Imaging
Orion Nebula: Composite
from Spitzer and Hubble
Hubble Space Telescope:
Greatest hits
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Imaging
Horsehead Nebula
Whirlpool Galaxy
Websites of the different observatories have many more
images. Check out for example
- www.spitzer.caltech.edu/Media/mediaimages/index.shtml
- www.cfht.hawaii.edu/HawaiianStarlight/
- http://hubblesite.org/gallery/
Spectroscopy
• A spectrograph separates
the different wavelengths
of light before they hit the
detector
• Since the light is separated
out, more total light
(longer exposure times) is
required for the same
telescope to make a
spectrum than to make an
image.
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Spectroscopy
• Spectroscopy gives
information about
- composition of stars and
nebulae
- temperature of stars
- motion of stars and
galaxies (Doppler shift)
How does Earth’s atmosphere
affect ground-based observations?
• The best ground-based sites for
astronomical observing are
–
–
–
–
Calm (not too windy)
High (less atmosphere to see through)
Dark (far from city lights: light pollution)
Dry (few cloudy nights)
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Light Pollution
• Scattering of human-made light
in the atmosphere is a growing
problem for astronomy
Twinkling and Turbulence
Star viewed with groundbased telescope
Same star viewed with
Hubble Space
Telescope
Turbulent air flow in Earth’s atmosphere distorts
our view, causing stars to appear to twinkle
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Calm, High, Dark, Dry
• The best
observing
sites are atop
remote
mountains
Summit of Mauna Kea, Hawaii
Transmission in Atmosphere
• Only radio and visible light pass easily through Earth’s
atmosphere
• We need telescopes in space to observe other forms
• Space telescopes also avoid the problems of light
pollution and atmospheric turbulence
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How can we observe nonvisible
light?
• A standard satellite
dish is essentially a
telescope for
observing radio
waves
Primary mirror
Secondary mirror
Radio Telescopes
• A radio telescope
is like a giant
mirror that reflects
radio waves to a
focus
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IR & UV Telescopes
SOFIA
Spitzer
• Infrared and ultraviolet-light telescopes operate
like visible-light telescopes but need to be above
atmosphere to see all IR and UV wavelengths
X-Ray Telescopes
Chandra
• X-ray telescopes
also need to be
above the
atmosphere
• Focusing of X-rays
requires special
mirrors
• Mirrors are arranged
to focus X-ray
photons through
grazing bounces off
the surface
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X-Ray Telescopes
• Chandra images reveal that the rings of Saturn sparkle in Xrays (blue dots in this X-ray/optical composite).
• The likely source for this radiation is the fluorescence
caused by solar X-rays striking oxygen atoms in the water
molecules that comprise most of the icy rings.
Gamma Ray Telescopes
• Gamma ray
telescopes also need
to be in space
• Focusing gamma
rays is extremely
difficult
Compton Observatory
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How can multiple telescopes
work together?
Interferometry
• Interferometery is a
technique for linking
two or more telescopes
so that they have the
angular resolution of a
single large one
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Interferometry
• First long distance
radio interferometry
(3 km) was
demonstrated in
Canada in 1967
• Current radio
telescope network
spans globe
• Now becoming
possible with
Very Large Array (VLA), New Mexico
infrared and visiblelight telescopes
Interferometry
• VLA, and Arecibo were
featured in the
Hollywood movie
Contact based on the
SETI (Search for
Extraterrestrial
Intelligence) program
• First major Hollywood
movie with a female
physicist as the main
character !
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Summary
• How does light propagate through different materials ?
– Light rays travel in straight lines that can be reflected
or refracted (bent) at an interface between two
materials
• How does your eye form an image?
– It uses refraction to bend parallel light rays so that they
form an image.
– The image is in focus if the focal plane is at the retina.
• How do we record images?
– Cameras focus light like your eye and record the image
with a detector.
– The detectors (CCDs) in digital cameras are like those
used on modern telescopes
Summary
• How does a telescope work?
– Refracting telescopes focus light with lenses
– Reflecting telescopes focus light with mirrors
– The vast majority of professional telescopes
are reflectors
– Collecting area determines how much light a
telescope can gather
– Angular resolution is the minimum angular
separation a telescope can distinguish
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Summary
• What are telescopes used for?
– Imaging
– Spectroscopy
Summary
• How does Earth’s atmosphere affect groundbased observations?
– Telescope sites are chosen to minimize the
problems of light pollution, atmospheric
turbulence, and bad weather.
• Why do we put telescopes into space?
– Forms of light other than radio and visible do
not pass through Earth’s atmosphere.
– Also, much sharper images are possible
because there is no turbulence.
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