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Astro

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History of Radio Telescopes
Describe the development of single dish radio telescopes and advantages of radio interferometers. Discuss major discoveries that were possible after detection of radio waves from space and how they changed our understanding of Universe.
The development of single dish radio telescopes has facilitated a wider range of astronomical discovery through improvements to the instruments by which radio astronomers study celestial objects at radio frequencies. Radio waves are a type of electromagnetic radiation, and differ from ordinary visible light due to a longer wavelength and lower frequency. Engineer Karl Guthe Janksy is associated with the first radio antenna built in 1931 to identify astronomical radio sources. This involved an arrangement of reflectors and diploes, or pairs of equal and oppositely magnetized poles separated by a distance, intended to receive short wave radio signals. Labelled “Janksy’s Merry Go Round” due to its position on a turntable, Janksy monitored his instrument over several months and was able to measure the length of an astronomical sidereal day, which is the time it took for a fixed object located on the celestial sphere to return to the same location in the sky. Most notably, was his serendipitous discovery of the Milky Way in 1933 through comparison of his observations with optical astronomical maps, and eventual conclusion that the radiation source peaked when his antenna was aimed at the densest part of the Milky Way in the constellation of Sagittarius. Furthering Jansky’s work in 1937 was Grote Reber’s first parabolic single dish radio telescope designed to broaden the search and detection for static or noise. The information Janksy and Grote discovered was presented in a mixture of properties, such as frequency, amplitude, and repetitive patterns. Some combinations of this information was able to be mathematically arranged into radio pictures of associated cosmic objects, making leaps and bounds in astronomical discovery.
In decades following the inception of single dish radio telescopes, much larger parabolic reflectors measuring 100 meters across in diameter were being built. This involved the introduction of fixed and steerable radio telescopes, as the weight and cumbersome nature of such scale instruments treaded with deformation in any other capacity. The Effelsberg 100m Radio Telescope in Bad Münstereifel, Germany, was the largest fully steerable telescope for 29 years until 2000. Beginning in 1968, it took 3 years to build before inauguration on August 1, 1972. The resulting 7 854 metre squared aperture attracted widespread noise from sources including the temperature of the telescope itself, its reflector, and its receiver. This is because anything with a temperature above absolute zero emits electromagnetic noise, which must be detected, selected, and rejected by the radio telescope. Tests throughout the proceeding years indicated that the accuracy of the mirror surface of 1 millimetre had not only been met, but exceeded significantly.

In contrast to single dish radio telescopes, interferometry involves the superimposement of waves to extract relevant information and is achieved by combining multiple radio antennas to effectively form a single antenna with greater collecting area than the individual components. The key to understanding interferometry is in the understanding of interference. In physics and astronomy, interference refers to the overlapping of radio waves on top of one another. The energies associated with each wave are combined to produce a third wave, which is a product of the two overlapping waves in a process called superposition. The basic idea behind interferometry involves beam splitting – separating a beam of light, or electromagnetic radiation, into two equal halves through a thin piece of silver-coated glass. From here, the beam known as a reference beam shines onto a mirror and then a camera, or screen. The second beam shines at, or through, the object to be measured before being directed back to the beam splitter and then onto the identical camera on which the first beam is shining. The extra distance covered by the second beam means that it is no longer matching the phase of the first beam, and this creates interference fringes. Figure A below illustrates the process of interferometry…

There are several advantages in using astronomical interferometers for radio astronomy, with the outstanding factor involving angular resolution and aperture. Angular resolution refers to the ability of an instrument to decipher minute details of an object and is correlated with image resolution, whilst aperture is the opening through which light travels in devices such as a telescope. The prominent benefit of a radio interferometer is that the angular resolution is similar to a telescope with the same aperture as a single large device containing all of the individual photon collecting components. In essence, radio interferometry can produce better resolution than optical astronomy. This also means that radio interferometers have a very high vertical sensitivity, typically measured in nanometres. The combining of multiple radio antennae not only improves the imagery and ability of radio astronomers to experiment, but has proven to be more accessible. An array can successfully observe both day and night, penetrating clouds in the process. Only turbulence and substantial rain pose a threat to the use of radio interferometers….

The detection of radio waves from space has facilitated major discoveries and subsequently sparked constant change around our understanding of the Universe. The earliest noteworthy observations involving radio waves were a focus on radio emissions from the sun, using basic interferometers in the late 1940s directed by J Paul Wild, an Australian radio physicist. These observations led to discoveries including the one million degrees temperature estimate of the solar corona, as well as radio bursts associated with flare activity. These early discoveries are still being extended today, which refinement surrounding the explanation of diverse radio activity emanating from the sun. It revealed the active nature of the Sun in a way which typifies the discovery process of modern astronomy. The angular sizes of solar emissions were also discovered later that decade. However, an astronomical milestone facilitated by the detection of radio waves was the single frequency radiation from hydrogen atoms in space. This was influential in mapping the structure of the Milky Way in the early 1950s as radio waves were not as hampered as the light, which was obscured by intervening gas and dust. In this process, Harvard’s Harold Ewen proved Hendrick van de Hulst’s 1944 theoretical proposals, and paved the way for constructing a map of the galactic features within a plane.

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