Everything about Radio Astronomy totally explained
Radio astronomy is a subfield of
astronomy that studies
celestial objects in the
radio frequency portion of the
electromagnetic spectrum. Radio astronomy techniques are similar to
optical techniques but
radio telescopes have to be much larger due to the longer wavelengths being observed. The field originated from the discovery that most astronomical objects emit radiation in the radio wavelengths as well as optical ones.
History
The idea that celestial bodies may be emitting radio waves had been suspected some time before its discovery. In the 1860's
James Clerk Maxwell's
equations had shown that electromagnetic radiation from stellar sources could exist with any wavelength, not just optical. Several notable scientists and experimenters such as
Thomas Edison,
Oliver Lodge, and
Max Planck predicted that the sun should be emitting radio waves. Lodge tried to observe solar signals but was unable to detect them due to technical limitations of his apparatus.
The first identified astronomical radio source was one discovered
serendipitously in the early 1930s when
Karl Guthe Jansky, an engineer with
Bell Telephone Laboratories, 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 once a day, Jansky originally suspected the source of the interference was the sun. Continued analysis showed that the source wasn't following the 24 hour cycle for the rising and setting of the sun but instead repeating on a cycle of 23 hours and 56 minutes, typical of an astronomical source "fixed" on the
celestial sphere rotating in sync with
sidereal time. By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the
Milky Way and was strongest in the direction of the center of the galaxy, in the
constellation of
Sagittarius . 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 Jansky to another project, so he did no further work in the field of astronomy.
Grote Reber helped pioneer radio astronomy when he built a large parabolic "dish" radio telescope (9m in diameter) in 1937. He was instrumental in repeating Karl Guthe Jansky's pioneering but somewhat simple work, 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, helped progress radio astronomy further, when he discovered that the sun emitted radio waves . 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 types of techniques to observe objects in the radio spectrum. Instruments may simply be pointed at an energetic radio source to analyze what type of emissions it makes. To “image” a region of the sky in more detail, multiple overlapping scans can be recorded and piece together in an image ('
mosaicing'). The types of instruments being used depends on the weakness of the signal and the amount of detail needed.
Radio telescopes
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 a few
arc seconds, whereas a radio telescope "dish" many times that size may, depending on the wavelength observed, may 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 in
1946.
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's 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
Since the 1970s telescopes from all over the world (and even in Earth orbit) have been combined to perform
Very Long Baseline Interferometry. 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's possible to create an antenna that's effectively the size of the Earth.
Using these techniques, radio telescopes are able to achieve much higher angular resolution and image quality than instruments working in other wavelength bands.
Astronomical sources
Radio astronomy has led to substantial increases in astronomical knowledge, particularly with the discovery of several classes of new objects, including
pulsars,
quasars and
radio galaxies. This is because radio astronomy allows us to see things that are not detectable in optical astronomy. Such objects represent some of the most extreme and energetic physical processes in the universe.
Radio astronomy is also partly responsible for the idea that
dark matter is an important component of our universe; radio measurements of the rotation of
galaxies suggest that there's much more mass in galaxies than has been directly observed. The
cosmic microwave background radiation was also first detected using radio telescopes. However, radio telescopes have also been used to investigate objects much closer to home, including observations of the
Sun and solar activity, and radar mapping of the
planets.
Other sources include:
Further Information
Get more info on 'Radio Astronomy'.
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