A radio telescope is capable of tuning in to the radio end of the electromagnetic
spectrum. Stars emit all forms of radiation from their surfaces, but our eyes
are only capable of seeing a very narrow band of visible light at 380-720 nm.
A radio telescope can tune in to electromagnetic waves which are much, MUCH
longer than a few hundred nanometers. The radio telescope listens to starlight
at wavelengths of centimeters to kilometers, although the long wavelengths are
seldom useful because of the sheer length between wavecrests. This may seem
surprising, but recall that radio waves are really a form of "light,"
as electromagnetic radiation. These waves move at light speed of 300,000 km/sec,
but their wavelengths are much longer and less energetic. Since radio waves
easily pass through gas and dust, these telescopes can be very useful. First,
a radio telescope can be operated 24 hours a day because sunlight and clouds
do not block their energy. Second, these telescopes can see objects which might
be obscured behind a dust cloud in interstellar or even intergalactic space.
Stuff which the naked eye could not see with a reflecting or refracting telescope
can be exposed by a radio telescope.
is a diagram of an average radio telescope. Just like optical telescopes, energy
from a star (galaxy in the image) can be focused to a narrow beam which is highly
amplified ... sort of a reverse magnification of an image. The result is a highly
enlarged signal from a distant source. The parts of a radio telescope include
the reflecting dish, an antenna, an amplifier, and a recorder. These components
work together to give astronomers a view into the radio emissions of deep space
objects. Photographic film is not used to "see" the image, but computers
can interpret the incoming signal and generate an intensity image of the radio
wave source. The result is a computer-generated image whose colors differentiate
the features of the radio source. Check out the NRAO site that will connect
you directly to the National
Radio Astronomy Observatory website and their page of images.
students have probably seen the movie "Contact." The first location
where Ellie is working is the giant fixed radio telescope at Aricebo, Puerto
Rico. This telescope is embedded in a natural mountain bowl and it used by astronomers
to hunt for pulsars ... fast rotating neutron stars. Later, Ellie moved to New
Mexico to the Very Large Array of radio telescopes. This unusual collection
of telescopes can be moved by railway to various configurations and then wired
together to act as one very large reception dish. This linkup of many radio
telescopes is called interferometry and with this technique the VLA acts as
one radio dish whose diameter is the distance between the farthest dishes. Thus
the "size" of the VLA "dish" is 40 km. The Very Long Baseline
Array (VLBA) consists of dishes spread from Hawaii to the Virgin Islands, and
gives the effect of an 8000 km diameter dish. HALCA, the Highly Advanced Laboratory
for Communications and Astronomy, consists of an 8 meter radio dish in orbit
around the earth. Used with ground-based radio telescopes, this acts as a dish
with a diameter of 32,000 km, thus greatly increasing the resolving power when
looking at objects billions of light years distant. While the movie was fanciful
in their extraterrestrial assumptions, to date no signal from intelligent life
in space has been received.
this picture of a pair of radio dishes atop Mauna Kea. They are testing two
different designs, one that is open to the air, and the other that has the "dish"
covered. There are 36 mounting surfaces, one of which you can see in the foreground
of the picture. The astronomers who will run this equipment will construct 8
radio telescopes of identical size and can move them in a great variety of patterns
among the 36 mounting sites. More scopes could have been constructed, but the
native Hawaiians have sacred ground near this particular location, and to minimize
the affect of such construction interference, they number of radio scopes was
kept to 8 mobile units.
So you have seen how we can listen to electromagnetic radiation with these
giant telescopes, but remember that most of the wavelengths of the em spectrum
are blocked by the earth's atmosphere. To really learn the details about stars,
we need to send telescopes above the atmosphere aboard rocket-launched satellites.
To learn about what these space telescopes have learned,
click on Space Telescopes, or
return to the Introduction to Light
and Telescopes, the Syllabus,
or the Home page.
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