Tuesday, 27 March 2012

HISTORY OF RADIO ASTRONOMY


Dominion Radio Astrophysical Observatory
The Dominion Radio Astrophysical Observatory is a research facility
founded in 1960 and located south-west of Okanagan Falls, British Columbia, Canada.
The site houses three instruments – an interferometric radio telescope,
 a 26-m single-dish antenna, and a solar flux monitor –
and supports engineering laboratories.
The DRAO is operated by the Herzberg Institute of Astrophysics
of the National Research Council of the Canadian government.


Radio astronomy
From Wikipedia, the free encyclopedia

Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The
initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky
observed radiation coming from the Milky Way. Subsequent observations have identified a number of
different sources of radio emission. These include stars and galaxies, as well as entirely new classes of
objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the cosmic microwave
background radiation, which provided compelling evidence for the Big Bang, was made through radio
astronomy.

Radio astronomy is conducted using large radio antennas referred to as radio telescopes, that are
either used singularly, or with multiple linked telescopes utilizing the techniques of radio
interferometry and aperture synthesis. The use of interferometry allows radio astronomy to achieve
high angular resolution, as the resolving power of an interferometer is set by the distance between its
components, rather than the size of its components.
History
Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be
observed from astronomical sources. In the 1860s, James Clerk Maxwell's equations had shown that
electromagnetic radiation is associated with electricity and magnetism, and could exist at any
wavelength. Several attempts were made to detect radio emission from the Sun by experimenters such
as Nikola Tesla and Oliver Lodge, but those attempts were unable to detect any emission due to
technical limitations of their instruments.

Karl Jansky made the discovery of the first astronomical radio source serendipitously in the early
1930s. As an engineer with Bell Telephone Laboratories, he was investigating static that interfered
with short wave transatlantic voice transmissions. Using a large directional antenna, Jansky noticed
that his analog pen-and-paper recording system kept recording a repeating signal of unknown origin.
Since the signal peaked about every 24 hours, Jansky originally suspected the source of the
interference was the Sun crossing the view of his directional antenna. Continued analysis showed that
the source was not following the 24 hour daily cycle of the Sun exactly, but instead repeating on a
cycle of 23 hours and 56 minutes. Jansky discussed the puzzling phenomena with his friend,
astrophysicist and teacher Albert Melvin Skellett, who pointed out that the signal seemed to be typical
of an astronomical source "fixed" in relationship to the stars on the celestial sphere and rotating in
sync with sidereal time. By comparing his observations with optical astronomical maps, Jansky
eventually concluded that the radiation was coming from the Milky Way, and that it was strongest in
the direction of the center of the galaxy, in the constellation of Sagittarius. He also concluded that
since he was unable to detect radio noise from the Sun, the strange radio interference may be
generated by interstellar gas and dust in the galaxy.] He announced his discovery in 1933. Jansky
wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs re-assigned
him to another project, so he did no further work in the field of astronomy. However, his pioneering
efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of
flux density, the jansky (Jy), after him.

Grote Reber was inspired by Jansky's work, and built a parabolic radio telescope 9m in diameter in
his own backyard in 1937. He began by repeating Jansky's observations, and went on to conduct the
first sky survey in the radio frequencies. On February 27, 1942, J.S. Hey, a British Army research
officer, made the first detection of radio waves emitted by the Sun. By the early 1950s, Martin Ryle
and Antony Hewish at Cambridge University had used the Cambridge Interferometer to map the radio
sky, producing the famous 2C and 3C surveys of radio sources.

Techniques
Radio astronomers use different techniques to observe objects in the radio spectrum. Instruments may
simply be pointed at an energetic radio source to analyze its emission. To “image” a region of the sky
in more detail, multiple overlapping scans can be recorded and pieced together in a mosaic image.
The type of instrument used depends on the strength of the signal and the amount of detail needed.

Observations from the Earth's surface are limited to wavelengths that can pass through the
atmosphere. At low frequencies, or long wavelengths, transmission is limited by the ionosphere, which
reflects waves with frequencies less than its characteristic plasma frequency. Water vapor interferes
with radio astronomy at higher frequencies, which has led to building radio observatories that conduct
observations at millimeter wavelengths at very high and dry sites, in order to minimize the water
vapor content in the line of sight.

Radio Telescopes
An optical image of the galaxy M87 (HST), a radio image of same galaxy using Interferometry (Very
Large Array-VLA), and an image of the center section (VLBA) using a Very Long Baseline Array
(Global VLBI) consisting of antennas in the US, Germany, Italy, Finland, Sweden and Spain. The jet
of particles is suspected to be powered by a black hole in the center of the galaxy.
Radio telescopes may need to be extremely large in order to receive signals with low signal-to-noise
ratio. Also since angular resolution is a function of the diameter of the "objective" in proportion to the
wavelength of the electromagnetic radiation being observed, radio telescopes have to be much larger
in comparison to their optical counterparts. For example a 1 meter diameter optical telescope is two
million times bigger than the wavelength of light observed giving it a resolution of roughly 0.3 arc
seconds, whereas a radio telescope "dish" many times that size may, depending on the wavelength
observed, only be able to resolve an object the size of the full moon (30 minutes of arc).

Radio Interferometry
The difficulty in achieving high resolutions with single radio telescopes led to radio interferometry,
developed by British radio astronomer Martin Ryle and Australian-born engineer, radiophysicist, and
radio astronomer Joseph Lade Pawsey and Ruby Payne-Scott in 1946. Surprisingly the first use of a
radio interferometer for an astronomical observation was carried out by Payne-Scott, Pawsey and
Lindsay McCready on 26 January 1946 using a SINGLE converted radar antenna (broadside array)
at 200 MHz near Sydney, Australia. This group used the principle of a sea-cliff interferometer in
which the antenna (formerly a WWII radar) observed the sun at sunrise with interference arising from
the direct radiation from the sun and the reflected radiation from the sea. With this baseline of almost
200 meters, the authors determined that the solar radiation during the burst phase was much smaller
than the solar disk and arose from a region associated with a large sunspot group. The Australia
group laid out the principles of aperture synthesis in their ground-breaking paper submitted in mid
1946 and published in 1947. The use of a sea-cliff interferometer had been demonstrated by numerous
groups in Australia, Iran and the UK during World War II, who had observed interference fringes (the
direct radar return radiation and the reflected signal from the sea) from incoming aircraft.

The Cambridge group of Ryle and Vonberg observed the sun at 175 MHz for the first time in mid July
1946 with a Michelson interferometer consisting of two radio antennas with spacings of some tens of
meters up to 240 meters. They showed that the radio radiation was smaller than 10 arc min in size
and also detected circular polarization in the Type I bursts. Two other groups had also detected
circular polarization at about the same time (David Martyn in Australia and Edward Appleton with J.
Stanley Hey in the UK).

Modern Radio interferometers consist of widely separated radio telescopes observing the same object
that are connected together using coaxial cable, waveguide, optical fiber, or other type of transmission
line. This not only increases the total signal collected, it can also be used in a process called Aperture
synthesis to vastly increase resolution. This technique works by superposing (interfering) the signal
waves from the different telescopes on the principle that waves that coincide with the same phase will
add to each other while two waves that have opposite phases will cancel each other out. This creates a
combined telescope that is the size of the antennas furthest apart in the array. In order to produce a
high quality image, a large number of different separations between different telescopes are required
(the projected separation between any two telescopes as seen from the radio source is called a
baseline) - as many different baselines as possible are required in order to get a good quality image.
For example the Very Large Array has 27 telescopes giving 351 independent baselines at once.

Very Long Baseline Interferometry
Beginning in the 1970s, improvements in the stability of radio telescope receivers permitted telescopes
from all over the world (and even in Earth orbit) to be combined to perform Very Long Baseline
Interferometry. Instead of physically connecting the antennas, data received at each antenna is paired
with timing information, usually from a local atomic clock, and then stored for later analysis on
magnetic tape or hard disk. At that later time, the data is correlated with data from other antennas
similarly recorded, to produce the resulting image. Using this method it is possible to synthesise an
antenna that is effectively the size of the Earth. The large distances between the telescopes enable
very high angular resolutions to be achieved, much greater in fact than in any other field of
astronomy. At the highest frequencies, synthesised beams less than 1 milliarcsecond are possible.

The pre-eminent VLBI arrays operating today are the Very Long Baseline Array (with telescopes
located across North America) and the European VLBI Network (telescopes in Europe, China, South
Africa and Puerto Rico). Each array usually operates separately, but occasional projects are observed
together producing increased sensitivity. This is referred to as Global VLBI. There is also a VLBI
network, the Long Baseline Array, operating in Australia.

Since its inception, recording data onto hard media has been the only way to bring the data recorded
at each telescope together for later correlation. However, the availability today of worldwide, high-
bandwidth optical fibre networks makes it possible to do VLBI in real time. This technique (referred
to as e-VLBI) was pioneered by the EVN (European VLBI Network) who now perform an increasing
number of scientific e-VLBI projects per year.



Canada's Largest Radio Telescope
The 46 m (150 ft) antenna at Algonquin Radio Observatory is Canada's largest antenna. Commissioned in 1965, the telescope is a fine example of a monster machine. The giant dish is fully steerable and can track with arc second precision the faintest object in the sky. Powerful motors turn the giant antenna in azimuth and elevation to point at any location in the sky. The moving part of the antenna rests 1000 tonnes on the pedestal base. 




Radio JOVE students and amateur scientists observe and analyze natural radio emissions of Jupiter, the Sun, and our galaxy.
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Society of Amateur Radio Astronomers
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The Lake Trail Guesthouse, Vancouver Island BC Canada
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