| Radio Astronomy:
The Early Days.
Historically, radio astronomy was the very first of the wavelengths to be studied outside the visible. The story starts with a man named Jansky, who worked for the Bell Labs and was asked by them in 1931 to work out why they were getting "noise" or electrical interference on their telephone lines, which produced a sort of crackling or hiss. Using an elaborate apparatus of antennae on a rotating turntable meters across, Jansky was able to identify three sources:
infrequent noise from nearby lightning strikes (Remember that lightning is a great flow of electricity from ground to air or vice-versa, and the flow of charged particles can give off electromagnetic radiation.);
a general background from more distant thunderstorms and lightning strikes; and
a general background coming from outside the atmosphere- indeed we now recognize that Jansky discovered radio radiation from the Milky Way galaxy.
Bell Labs thanked Jansky for his efforts - and then, remarkably, set him to work on some completely different project. (His name is now commemorated in a unit of "radio flux" or brightness.) The whole subject more-or-less languished, except for the efforts of an amateur named Grote Reber, who built a radio telescope a couple of meters in diameter in his back yard in 1936. He was the only active radio astronomer in the world until about 1944. However, technological developments were advancing - as so often, they were driven by military considerations.
The Second World War led to lots of research into radar, and to the development of dishes and detectors which were well-engineered and quite sensitive. Indeed, radar stations in the war used to pick up spurious signals which they realized came from the sun and the Galaxy. Thus it was that after the war there were experts and equipment ready to leap into this new science. The British in particular were ready for this, and claimed some German-made radar dishes, the best in the world, in the setting up of the first real radio astronomy observatories (like the famous Jodrell Bank, near Manchester).
Modern Considerations.In some senses, radio telescopes are nothing more than scaled-up versions of optical telescopes used at the prime focus. It is worth reminding you that they are made very big simply so that they can resolve details better -- the long wavelength of radio radiation makes this a serious limitation unless the dishes are enormous. To look at different parts of the sky, it is helpful to have "fully steerable" radio telescopes - that is, telescopes that can be pointed to wherever you like. The largest such telescope, near Bonn, Germany, is about 100 metres in diameter. But it is not the largest radio telescope of all -- not by a long bargin. That honour goes to Arecibo.
Arecibo.The Arecibo radio telescope, located in Puerto Rico, is the largest single telescope in the world: it is 1000 feet across. (It features in the movie 'Contact.') A dish this size cannot be steered, but merely sits unmoving in a valley, pointing straight up, as shown on page 189 of your text. But as the Earth turns, the sky passes overhead, so that a whole bunch of astronomical sources can be seen in turn. Moreover, by moving the sensitive detectors around somewhat at the prime focus, it is possible to study a fairly large patch of sky. Arecibo is used for something else too: radar signals are sent out from it towards the sun and other planets. The time taken for the signals to return tells us the distance of those objects (which is how we know the size of the solar system). Moreover, distortions and changes in the signal as it bounces back off various parts of Venus, for instance, allow us to map the surface of Venus: we know about the big mountainous features on Venus - which appears as a tiny speck of light to your eye! - because of the radar reflections we receive across 100 million kilometers of space. (Venus has also been studied, in considerably finer detail, by space probes sent to it and put into orbit around it. But Arecibo was the first big contributor to our understanding of that planet.)
The Very Large Array.Perhaps the best-known radio telescope of all is the so-called Very Large Array: there is a photograph of it in the text, on page 189. (In the movie 'Contact,' the extraterrestrial messages were first detected at the VLA.) It consists of 27 dishes, each 25 metres in diameter, arranged in the shape of a 'Y' spread out along railway tracks (so that the individual antennas can be moved). The signal received from this telescope adds together all the radiation from these dishes, which gives it a large total collecting area; but there is something more to the process than just this aspect. To understand why we use a host of separate moveable dishes, we need to consider the technique of interferometry.
Interferometry.Remember that light is a wave phenomenon, and that you can visualise the light from a distant source arriving like 'wavefronts' onto a long beach. Now imagine two widely-spaced radio telescopes pointing at a star directly above the spot which lies midway between them. The wave will reach both telescopes at the same time, and if you add together the signal received (by sending it through some sort of electrical connection) you will get a big signal. And of course one wavefront is followed by another, as crest after crest of the radiated light 'rolls in.' But the Earth is continuously rotating, and a star which is overhead now will appear to move off to the side as time passes. A few minutes from now, therefore, we will find that when a particular wavefront hits the right-hand antenna, the left-hand antenna is between waves. The added signals are no longer enhanced. Indeed, if the left-hand antenna is at a trough in the signal, the two cancel out, giving no net signal. This is an of the kind we studied before when considering the evidence that and its use is called interferometry. If you had only point-like sources of radiation, and only a pair of telescopes, this would be rather uninformative. As the Earth turned and the source passed overhead, you would see a steady up-and-down in the combined signal. But there are sources of radiation, like clouds of gas or whole galaxies, which are spread out. You can visualize these as a whole bunch of point-like sources all gathered together in some complicated arrangment, each with a different brightnesses, so the combined signal is a complex mixture of all of them at once. How can we unscramble the effects? Well, we can be clever too! We can use more than two antennas, and separate them by different distances -- different baselines -- so that the interference effects are also complex but designed to let us unscramble what the geometry of the source looks like. To do this thoroughly, it may be necessary to have the radio telescopes in a whole variety of different arrangements: that is why the VLA antennas are on railway tracks, so that they can be rearranged from time to time. Needless to say, using the complex radio signal detected to deduce the structure of the source is a difficult mathematical exercise, and very hard to describe succinctly. Suffice it to say that a radio telescope used interferometrically only grudgingly gives up anything like a 'picture of the sky!'
Making Our Interferometers Bigger.One fundamental limit remains: the very best angular resolution you can get is still determined by the total "diameter" of the telescope - that is, by the most widely-separated pair of antennas in the whole arrangement. In the VLA, the two most widely-spaced antennas might be fifty kilometers apart. Can we improve on this, and get even better resolution (which will allow us to see even finer detail)? Until recently, the very best we could do was limited by the size of the Earth itself, in a technique known as very-long-baseline interferometry (or VLBI) - a technique first perfected by Canadian radio astronomers. In such an application, telescopes in two vastly different locations (say, Algonquin Radio Observatory, in Canada's Algonquin Park, and the Parkes Radio Telescope in Australia) look at the same source at the same time. The incoming signals are typically recorded on magnetic tape first, and then later 'mixed together,' with the interference pattern telling us about the detailed structure of the source. Intricate detail can be detected in this way. More recently, developments in space technology allow us to use even longer baselines through the simple (but expensive!) expedient of launching one of our radio telescopes -- although obviously one of moderate size only -- into space. In this way we will be able to resolve even finer detail than is permitted by the VLBI work.
Why Bother? An Important Reminder.It is very important that you keep in mind why we bother to work at a whole variety of different wavelengths. It is not to study the same old objects in slightly different ways (although that can be done to a limited extent). It is principally to study new classes of objects. For instance, clouds of cool interstellar gas are thought to be the regions within which stars are forming. But they are so cool that they do not give off visible light; moreover, the dust (little grains of material) they contain means that any visible light they might produce deep within them cannot get out anyway - just as though they are wrapped in a deep fog. However, radio and millimeter-wave radiation can get out, and allows us to study the composition and nature of these important astrophysical objects. As we will see in the next section, are important in astronomical research. Previous chapter:Next chapter
0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Tuesday, 20 October, 2020.)