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RADIATION, SPECTROSCOPY ANDTELESCOPES
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We learn about the stars by studying the
electromagnetic radiation that they
emit.
Visible light is one particular type ofelectromagnetic radiation.
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In astronomy, we cannot perform
experiments with our objects (stars,galaxies, ).
The only way to investigate them, is by
analyzing the light (and other radiation)which we observe from them through theelectromagnetic spectrum
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Types of Electromagnetic
Radiation radio
infrared
visible light ultraviolet
X rays
Gamma rays
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The frequency of a wave is related
to its velocity and wavelengththrough the formula
wavelength x frequency = velocity
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wavelength
is measured in units of length meters
millimeter (mm) = 0.001 m = 10-3 m
micrometer (&mu m) = 0.000001 m = 10
-6
m nanometer (nm) = 0.000000001 m = 10-9 m
frequency
is expressed in units of inverse time (1/sec) called Hertz (1 Hz = 1/sec)
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The electromagnetic spectrum ranges
from radio waves at one extreme to
gamma rays at the other extreme.
http://physics.uoregon.edu/~jimbrau/BrauImNew/Chap03/FG03_09.jpghttp://physics.uoregon.edu/~jimbrau/BrauImNew/Chap03/FG03_09.jpg7/31/2019 2 a Intro Astronomy Lintag
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In addition,
Different
colors of
visible light
correspond todifferent
wavelengths.
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Light as a Wave
Light waves are characterized by a wavelength land a frequency f.
f = c/l
c = 300,000 km/s =3*108 m/s
f and l are related through
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The Importance of Spectroscopy
Spectroscopy allows scientists to infer thenature of matter at great distances ;
the chemical composition of distance starscan be revealed
also, important information on the origin,evolution, and destiny of stars in the universehas been discovered
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Optical Telescopes
Astronomers use
telescopes to gathermore light from
astronomical objects.
The larger thetelescope, the more
light it gathers.
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Refracting/Reflecting Telescopes
RefractingTelescope:Lens focuseslight onto thefocal plane
ReflectingTelescope:
Concave Mirrorfocuses lightonto the focal
plane
Almost all modern telescopes are reflecting telescopes.
Focal length
Focal length
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Secondary Optics
In reflectingtelescopes:Secondary
mirror, to re-direct light pathtowards back or
side of
incoming lightpath.
Eyepiece: Toview and
enlarge thesmall imageproduced in
the focalplane of the
primary
optics.
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Disadvantages of Refracting
Telescopes
Chromaticaberration: Differentwavelengths are focused at differentfocal lengths (prism effect).
Can becorrected, butnot eliminatedby second lensout of differentmaterial.
Difficult and expensive toproduce: All surfaces must beperfectly shaped; glass mustbe flawless; lens can only be
supported at the edges
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The Powers of a Telescope:
Size Does Matter
1. Light-gatheringpower: Depends
on the surface areaA of the primarylens / mirror,proportional to
diameter squared:
A = p (D/2)2
D
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The Powers of a Telescope
2. Resolving power: Wave nature of light=> The telescope aperture producesfringe rings that set a limit to theresolution of the telescope.
amin = 1.22 (l/D)
Resolving power = minimum angulardistance amin between two objects that canbe separated.
For optical wavelengths, this gives
amin = 11.6 arcsec / D[cm]
amin
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The Powers of a Telescope (3)
3. Magnifying Power = ability of the telescope tomake the image appear bigger.
The magnification depends on the ratio of focal lengths
of the primary mirror/lens (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve theresolving power of the telescope!
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The Best Location for a
Telescope
Far away from civilization to avoid light pollution
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The Best Location for a
Telescope (2)
On high mountain-tops to avoid atmosphericturbulence ( seeing) and other weather effects
Paranal Observatory (ESO), Chile
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Traditional Telescopes (1)
Traditional primary mirror:sturdy, heavy to avoid
distortions.
Secondary mirror
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Traditional Telescopes (2)
The 4-mMayall
Telescope atKitt PeakNational
Observatory(Arizona)
d i d l i
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Advances in Modern Telescope Design
2. Simpler, stronger mountings (Alt-azimuth mountings)
to be controlled by computers
1. Lighter mirrors with lighter support structures,to be controlled dynamically by computers
Floppy mirror Segmented mirror
Modern computer technology has made
possible significant advances in telescopedesign:
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Examples of Modern Telescope
Design (1)
Design of the LargeBinocular Telescope
(LBT)
The Keck I telescope mirror
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Examples of Modern Telescope
Design (2)
8.1-m mirror of the Gemini Telescopes
The Very Large Telescope (VLT)
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The Hubble Space Telescope
Avoidsturbulence inthe Earths
atmosphere
Extendsimaging andspectroscopyto (invisible)infrared andultraviolet
Launched in 1990;maintained andupgraded by severalspace shuttle servicemissions throughout
the 1990s and early2000s
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Sources :
http://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter3_4.html
http://faculty.tcc.edu/KBroun/PowerPoint%20Slides%20Contents.htm
http://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter3_4.htmlhttp://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter3_4.htmlhttp://faculty.tcc.edu/KBroun/PowerPoint%20Slides%20Contents.htmhttp://faculty.tcc.edu/KBroun/PowerPoint%20Slides%20Contents.htmhttp://faculty.tcc.edu/KBroun/PowerPoint%20Slides%20Contents.htmhttp://faculty.tcc.edu/KBroun/PowerPoint%20Slides%20Contents.htmhttp://faculty.tcc.edu/KBroun/PowerPoint%20Slides%20Contents.htmhttp://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter3_4.htmlhttp://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter3_4.htmlhttp://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter3_4.html