By Mehdi Moussali and Élie ChamaiBy Mehdi Moussali and Élie Chamai

École La Dauversière, Montreal, June 2000École La Dauversière, Montreal, June 2000

Content validation and linguistic revision: Stéphane Lamarche

Science animée, 2000

Content validation and linguistic revision: Stéphane Lamarche

Science animée, 2000

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Translated from French by Nigel WardTranslated from French by Nigel Ward

Introduction to Introduction to the telescopethe telescope

Optical and Optical and mechanical mechanical

properties of a properties of a telescopetelescope

Functioning of Functioning of the telescopethe telescope

ApparentApparent magnitude of magnitude of stastarsrs

GGianiant t telescopestelescopes The inventors The inventors

References References Concave Concave mirrorsmirrors

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Since its invention by top scientists, the telescope has changed our way of looking at space.

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The invention of the telescope was done by several people including:The invention of the telescope was done by several people including:

an Italian astronomer en 1616 Niccolo Zucchi

Then, a Frenchman in 1630Marin Mersenne

Then, in 1663 an Englishman

James Gregory

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inventors

Birth:

Nationality:

year of the construction of his telescope :

Death:

English

After the other inventors, Isaac Newton was the first to construct a telescope with a spherical metal mirror .

After the other inventors, Isaac Newton was the first to construct a telescope with a spherical metal mirror . Menu

newton

concave mirror

focus

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Convergence of the light rays towards the focus.

Concave mirrors

primary mirror (concave)

secondary mirror (plane)

Light

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newtonian

Secondary mirror

primary mirror

Secondary mirror

primary mirrorfocus

focus

An alternative to the Newtonian telescope (left) is the Cassegrain telescope (right)

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cassegrain

A Newtonian optical tube is composed of the following elements:

mirrors the barillet ???This support makes it

possible to support the principal mirror

securely at the bottom of the tube and to

orient it.

Optical tube 1

The ‘spider’ supports the secondary mirror.

The tubeThe interior of thetube is covered by

mat black paint which strongly limits

the unwanted reflection of light.

the eyepiece holder

Optical tube 2

The eyepiece

The amount of magnification depends on the eyepiece.

To know the magnification which an eyepiece will give, you must know the focal length of the primary mirror.

The magnification can then be calculated for each eyepiece using the following formula:

M= F/fwhere M is the magnification,F the focal length of the primary mirror, and f is the focal length of the eyepiece

Mechanics

Optical tube 3

The axis of declination

The fork

The axis of right ascension

makes the tube pivot.supports the

axis of declination.

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mechanics

ESO (European Southern Observatory) has constructed 4 giant telescopes in the north of Chile.

This image represents one of the telescopes. The mirror of the instrument weighs 23 tonnes, so imagine how large it must be!

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Giant telescopes

Here is an image made by one of these telescopes. They are among the most precise and clear telescopes in the world.

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woaw

Scientifically, the brightness of a star, as seen from earth, is called its ‘apparent magnitude’. We use a system developed by the ancient Greeks in which the brightest stars are said to be of first magnitude (magnitude 1) and the dimmest stars visible to the naked eye in ideal conditions are said to be of the sixth magnitude (magnitude 6). This means that the bigger the magnitude number, the dimmer the star. There are about 2500 stars with magnitudes 1 to 6 so that is the number of stars you can see in ideal conditions.

If you live in a city then there is likely to be a lot of light pollution and you may be able to see only stars of magnitude 4 or brighter – that means only about 250 stars. The dimmest magnitude you can see in real conditions is called the ‘limiting magnitude’.

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Seeing stars

Telescopes make stars appear brighter since they gather a lot more light than the tiny pupil of our eye is able to do without help. For example, a telescope with a mirror of diameter 250 mm increases the brightness by 8 magnitudes so that at a location where the limiting magnitude is 6 (meaning that magnitude 6 stars are just visible to the naked eye) then the 250 mm telescope would allow us to see stars of magnitude 14 (6+8 = 14). In other words the limiting magnitude without the telescope is 6 and with the telescope is 14.

Here is the formula for the magnitude gain of the telescope in terms of the mirror diameter Dm and the diameter of the pupil of your eye Dp (typically about 6.5mm): magnitude gain = 5 * Log10 (Dm/Dp).

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Magnitude gain

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Limiting magnitude as a function of the diameter of the mirror.

Mirror diameter (mm)

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BooksBooksGagnon, Roger. Gagnon, Roger. Construction d’un télescope amateurConstruction d’un télescope amateur, ,

Montréal, Conseil de la jeunesse, 1977, 60 pages.Montréal, Conseil de la jeunesse, 1977, 60 pages.

Gagnon, Roger. Gagnon, Roger. La fabrication d’un miroir de télescope,La fabrication d’un miroir de télescope, Montréal, Conseil de la jeunesse, 1977, 67 pages.Montréal, Conseil de la jeunesse, 1977, 67 pages.

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references