Just who invented the
telescope is not a straightforward question.
Three Dutchmen are credited with trying to apply for a patent for their
own designs of a telescope; Hans Lippershey, Zacharias Jansen and Jacob Metius
all applied for a patent in 1608. Of the
three only Lippershey was successful, the government paying him a handsome fee
for replicating his design. Word of this
“Dutch perspective glass” reached the ears of Galileo who, famously, was one of
the first to turn his telescope skyward using its 30 times magnification to
discover four of Jupiter’s moons, Venus’s phases.
The simplest version of a
telescope uses two lenses slotted at either end of a tube. The first lens squeezes rays of light inwards
so that the eye perceives them as coming from a larger source. The second lens acts as an eyepiece making
the light rays parallel again before they enter the eye so that they can be
focused. The bending of these rays of
light is called refraction. Light
travels more slowly in denser materials, such as glass, compared with air. This explains the mirage of a puddle on a hot road. Rays from the sky bend to skim the roads
surface because light changes speed in the layer of hot air lying just above
the sun baked asphalt. Hot air is less
dense than cooler air, so the light bends away from the vertical and we see the
sky’s reflection on the tarmac, looking like a puddle. The angle by which a ray bends is related to
the relative speeds at which it travels in the two materials.
Refracting telescopes
with two lenses have drawbacks, the image appears upside-down this is because the
light rays cross over before they reach the eyepiece, For astronomy this isn’t
usually an issue; a star looks much the same upside down as it does right way
up, this discrepancy can be resolved by including a third lens to invert the
image once again but, then the telescope can become long and unwieldy. The
second issue is more problematic for astronomers as refracting telescopes
produce blurred colour images. Light of
different wavelengths are refracted by different amounts, blue light waves are
bent more than red light waves so the colours separate out and the final image
loses clarity. There are new types of
lens available today that can minimize this but there size and power are
limited.
Reflecting telescopes. To solve the problems endemic with refracting
telescopes, Newton invented the reflecting telescope. Using a curved mirror rather than a lens to
bend the light he essentially halved the length of the telescope, folding it in
half and making it easier to handle. His
design also avoided the differential blurring because the mirrored surface
reflects all colours of light in the same way.
However, mirror silvering techniques were not advanced in newton’s day
so it took centuries for the design to be perfected.
Today, most professional
astronomical telescopes use a giant mirror rather than a lens to collect
celestial light and bounce it back to the eyepiece. The size of the mirror dictates how much
light can be collected – a big area lets you see very faint objects. The
mirrors in modern optical telescopes can be the size of a room, the largest
currently in use such as those in the twin giant Keck telescopes on Mauna Kea
in Hawaii, are about 10 meters across.
Even bigger ones up to 100 meters in diameter are planned in the coming
decades.
Very large mirrors are
tricky to construct since they become so heavy that their shape distorts when
the telescope tilts to scan the sky.
Clever construction methods are needed to make them as light as
possible. Some are built in many
segments; others are carefully spun so they are very thin yet accurately
sculpted. An alternative solution,
called ‘adaptive optics’ is to constantly correct the mirrors shape using a
network of tiny pistons glued underneath to push up the surface when it sags.
Beyond the telescopes
themselves the clarity of astronomical images is degraded by turbulence in our
atmosphere, the scintillation. On even the clearest night stars twinkle, those near the
horizon twinkle more than those overhead.
They do so because pockets of air moving in front of them. Astronomers call the blurring of the stars by
our atmosphere ‘seeing’. The size of the optical components in the telescope
also gives an absolute limit to the concentration of starlight due to another behaviour
of light, diffraction – the bending of light rays around the edge of a lens,
aperture or mirror.
To get the best images of stars and planets, astronomers carefully select
special locations for their telescopes.
On the surface of the earth they build them on high sites where the air is
thin, like mountains, and where the airflow is smooth, such as near the coast.
The best sites are in the Chilean Andes and Hawaii’s volcanic peaks. The ultimate site is space where there is no
atmosphere. The deepest images ever
taken of the universe have been made by the orbiting Hubble space telescope.
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| The Hubble Space Telescope. |
Telescopes can operate at wavelengths other than the visible light
range. Infrared, or heat, can be
detected with instruments that are like night vision goggles mounted on
telescopes as long as the equipment is kept cool. Because of their very short wavelengths,
x-rays are best pursued in space using satellites with reflective optics. Even
radio waves can be detected with large single dishes such as the one at Arecibo (as seen in Goldeneye)
or arrays of many smaller antennae, such as the Very Large array in New Mexico
(as seen in the film contact). Perhaps
the ultimate telescope is the Earth itself – fundamental particles whiz through
it every day, and physicists have placed traps to try and catch them as they
do
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