2 a Intro Astronomy Lintag

7/31/2019 2 a Intro Astronomy Lintag

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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.jpg


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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

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