Transcript Light and Telescopes

Light and Telescopes
Why light?
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With only a very few exceptions, all the
information about stars and galaxies
reaches the Astronomers in the form of
light
So what is light?
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Light is an electromagnetic wave
It can also be thought of as a particle called a
photon: g
This apparent contradiction is called the
particle/wave duality
The apparent contradiction is due to our inability
to describe something that is neither entirely a
wave nor a particle
At times it’s convenient to speak of light as a
wave; at other times convenience begs for a
particle
Early Studies 1
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Euclid, circa 300BC,
noted that light travels
in straight lines and
derived the law of
reflection
Early Studies 2
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Roger Bacon 12141294 was a pioneer in
the study of optics
On the right is a page
from his treatise Opus
Majus describing the
path of light
Early Studies 3
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Isaac Newton 16421727 showed that
white light is made up
of many colors
He used a prism to
break light into its
constituent colors
The Speed of Light
The speed of light, denoted ‘c’, is a fixed
speed (we’ve “c’een this before!)
 We use c because the speed of a wave is
constant in a particular medium
 The ultimate speed limit is 300,000
km/sec, about 186,000 miles a
second~670 million miles an hour.
 Light can travel more slowly than this, for
instance in glass or water, but never faster
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Frequency and Wavelength
Frequency f is how many waves occur
each second, and wavelength l is,
obviously, the length of the wave
 Frequency is measured in Hz and
wavelength is measured in meters
 The product of those two terms gives you
the speed of the wave
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c = fl
The above equation is for demonstration
purposes—don’t panic!
 It demonstrates that, if c can’t change but f
does, then l must change reciprocally
 Remember this, because when we talk
about Red Shift this relationship will be
cited
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The Full Spectrum
Sources
Animation
Other Sources
Electric Charges running along a wire—
long waves like radio
 Hot ionized atoms—short waves like Xrays
 Splitting of the atomic nucleus—very short
waves like gamma rays
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Changing Directions 1
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Reflection: bouncing
light so that it focuses
to a point.
There are other
mirrors of course, but
less useful for
Astronomy
Changing Directions 2
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Refraction: the
bending of light as it
goes from one
medium to another.
Changing Directions 3
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Diffraction: the bending
of a wave as it goes
around corners
Alternative to a prism
Qualities that make a good
telescope:
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Light-gathering ability (aperture): the bigger the
lens, mirror, or dish, the more light it can gather
Resolution: the ability to distinguish to distant
objects that are close to each other
Tracking: the ability to follow an object in the sky
for long exposures
Adaptive Optics
Magnification
Location, Location, Location
PDF
Don’t forget the mount!
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What you place the telescope on is
vital!
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Any stray vibrations distort the image
Also, the sky moves (as seen from
Earth)
You need a mount that is both
stable and tracks the sky
Enter Joseph Fraunhoffer
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We’ll meet him again later concerning
the solar spectrum
Alt/Azimuth Mounts
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Altitude (up and down
angle)
Azimuth (around
motion)
If you take the A105L
lab you’ll use
Dobsonians like this
Good and cheap, but
you do all the work
slewing the ‘scope
How the sky moves
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The picture show a v-e-r-y long exposure centered on Polaris
It demonstrates that the farther from the pole star you observe, the
more the sky move in a given time
Alt/azimuth, even if motorized, won’t get the job done
German Equatorial Mount
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Invented by Fraunhoffer, the GEM
uses a series of gears that adjust
for the motion of the sky in the
direction of observation
Early models used falling weights
to drive the system
Astronomer F.G. Wilhelm Struve
used this instrument to measure
over 3,000 double stars with
precision of less than one arc
second.
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“...undetermined which to admire
most, the beauty and elegance of the
workmanship in its most minute parts,
the propriety of its construction, the
ingenious method of moving it, or the
incomparable optical power of the
telescope, and the precision with which
objects are defined.”
An Early Optical Instruments
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Tycho Brahe’s astrolabe
was used before the
telescope to locate
objects in the sky to
within 1 minute of arc
Leonard Diggs, a16th C.
English mathematician,
invented a device called a
‘perspective glass’ to aid
navigation
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Early reflector
Kept secret
Early Telescopes
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Galileo didn’t invent
the telescope, but he
improved on the
Flemish design
Circa 1600
A refractor
Diagram: Galilean
Largest
Refractor:
Yerkes 40”
Diagram: Newtonian
Friedrich Wilhelm Herschel
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Late 18th C.
20 ft
A reflector
Diagram: Schmitt-Cassegrain
Mt. Wilson 100” Hooker reflector
Mt. Palomar 200” Hale telescope
Mauna Kea Keck (ex) interferometer
Large Binocular Telescope, Mt. Graham
Space Telescopes: Hubble
Kepler Space Telescope
James Webb Space Telescope
2018 launch
Radio Telescopes
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Recall the spectrum
slide: light is not
confined to the visible
range
The sky is transparent
to both visible light
and radio waves
The First Radio Telescope
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Karl Jansky, in 1932,
discovered radio
waves impinging the
Earth from outer
space
Modern Telescopes: Owens Valley
Modern Telescopes: Arecibo
Modern Telescopes: VLA
Hess II Cherenkov
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Gamma rays don’t reach the earth’s surface
But (!) they do produce superluminal particles in the
air
These give off Cherenkov radiation which is
detectable on the ground