Transcript 4b Telescopes

By Mehdi Moussali and Élie Chamai
École La Dauversière, Montreal, June 2000
Content validation and linguistic revision: Stéphane Lamarche
Science
animée, 2000
Translated from French by Nigel Ward
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Introduction to
the telescope
Functioning of
the telescope
Giant telescopes
Concave mirrors
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:
an Italian astronomer en 1616
Niccolo Zucchi
Then, a Frenchman in 1630
Marin Mersenne
Then, in 1663 an Englishman
James Gregory
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Birth:
Death:
Nationality: English
year of the construction of his
telescope :
After the other inventors, Isaac Newton was the
first to construct a telescope with a spherical
metal mirror .
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concave mirror
Convergence of the light
rays towards the focus.
focus
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secondary mirror
(plane)
primary mirror
(concave)
L
i
g
h
t
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An alternative to the Newtonian telescope (left)
is the Cassegrain telescope (right)
light
Secondary
mirror
light
Secondary
mirror
focus
primary mirror
primary mirror
focus
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.
The tube
The ‘spider’ supports the
secondary mirror.
the eyepiece holder
The interior of the
tube is covered by
mat black paint which
strongly limits the
unwanted reflection
of light.
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/f
where M is the magnification,
F the focal length of the primary mirror,
and f is the focal length of the eyepiece
Mechanics
The fork
supports the axis
of declination.
The axis of
declination
makes the
tube pivot.
The axis of
right ascension
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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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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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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’.
Graphic
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).
Graphic
Limiting magnitude as a function of the diameter of the mirror.
Mirror diameter (mm)
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Books
Gagnon, Roger. Construction d’un télescope amateur, Montréal,
Conseil de la jeunesse, 1977, 60 pages.
Gagnon, Roger. La fabrication d’un miroir de télescope,
Montréal, Conseil de la jeunesse, 1977, 67 pages.
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