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Module P7 L4

May 30, 2018

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Page 1: Module P7 L4

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Before 1609, all

astronomy was done

with the ³naked eye´ ±that is to say, without

telescopes.

The picture shows thefamous astronomer 

Tycho Brahe making

observations with his

³mural quadrant´.

(The person ³inside´

the quadrant is just a

picture.)

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Galileo¶s Telescope

In 1609, Galileo Galilei was one of the first people to turn the newly invented

telescopes to the sky.He discovered many things including:

the moons of Jupiter, named the ³Galilean Moons´ in his honour 

mountains on the Moon ± before this, people thought it was a perfectly smooth sphere

sunspots ± although he damaged his eyes by staring directly at the Sun through

his telescope.

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The Polish

astronomer Hevelius

built this 20 metrelong telescope in

1673 ± the

magnification was

very large but it wasvery difficult to use.

Light pollution from towns

and cities was not aproblem at that time!

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The Irish astronomer Lord

Rosse drew this picture of the

M51 galaxy in 1825.

The photograph of the same

galaxy was taken by the Hubble

Space Telescope in 2005.

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Telescopes have become bigger over 

the years as astronomers try to

gather more light from distant objects.

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The Hubble

Space Telescope

has a smaller aperture than

many Earth

based

telescopes.

However, it is not

affected byweather, air 

pollution or light

pollution.

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How to Build a Telescope

1. You need a lens with a long focal lengthto be the objective lens which gathers

the light.

2. You need a lens with a short focal lengthto be the eyepiece lens which helps to

produce a real, magnified and inverted

image.

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Objective Eyepiece

Focal length of 

objective lens

Focal length of 

eyepiece lens

1º 9º

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Magnification

lenseyepieceof lengthfocal

lensobjectiveof lengthfocal ionmagnificat !

lenseyepieceolengthocal

lensob ectiveolengthocal ionmagni icat !

Example: The objective lens of a telescope has a focal length of 1.2 m and its

eyepiece has a focal length of 0.10 m.a) Calculate the magnification.

b) A galaxy has an angular size of 0.2º. What is its angular size when viewed

through this telescope?

120.10

1.2 ionmagni icat !!

a)

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Magnification

lenseyepieceolengthocal

lensob ectiveolengthocal ionmagni icat !

Example: The objective lens of a telescope has a focal length of 1.2 m and its

eyepiece has a focal length of 0.10 m.a) Calculate the magnification.

b) A galaxy has an angular size of 0.2º. What is its angular size when viewed

through this telescope?

 Angular size = 12 x 0.2 = 2.4 ºb)

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Magnification

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Magnification

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