In the exam you are expected to know about:
Single dish radio telescopes, general principles and resolving power
Similarities with optical telescopes: objective, mirror, detector, tracking of source;
Differences from optical telescopes: resolving power, limit of resolution, need for scanning to build up image.
Objective diameter, precision of about λ/20 needed in shape of dish.
Use of wire mesh.
The Radio Telescope
The radio telescope was first devised during the 1930's. During the Second World War, considerable advances were made with all sorts of techniques used for radio for communication, detection, and navigation. Sophisticated apparatus was made for eavesdropping on enemy radio traffic to gain intelligence. (The Germans were excellent soldiers but never appreciated the need for good intelligence; the Allied Powers were very good at intelligence and deception).
The initial driving force for RADAR (Radio detection and ranging) was originally to make a radio death-ray to make bombers fall out of the sky in flames! The initial driving force for radio telescopy was to listen out for radio signals from other extra-terrestrial civilisations (little green men).
In Britain, the first work to be done with Radio Telescopes was carried out by Professor Sir Bernard Lovell and a team from the University of Manchester. They set up war-surplus radio equipment bought from the army in a field in the Middle of Cheshire. The work grew and in the middle nineteen fifties undertook the construction of a massive radio telescope, which is shown below.
This massive instrument at Jodrell Bank is 75 m across. Its story is major work in itself, but it eventually started work in the late nineteen fifties, almost exactly at the same time as electric trains powered by 25 000 V overhead power lines started running on the main line that passes 200 m from the instrument.
The dish is parabolic, reflecting radio waves onto an antenna at the principal focus. The radio waves are very weak, and the focusing by the reflector makes them much more intense.
The diameter of the dish is sometimes called the objective diameter.
The antenna detects the intensified radio waves.
The receiver has to be tuned in, just like any other radio set.
The signal is passed down to very high quality amplifiers, and the signals are analysed by a computer.
Like a light telescope, the instrument has to be tracked, otherwise the object will be lost.
The power of the telescope is proportional to the square of its diameter.
The largest radio telescope in the world is in Puerto Rico. It is built between some small hills that had a roughly parabolic valley. It is 300 m across.
Question 1 How much more powerful is this instrument than the one at Jodrell Bank? ANSWER
The Puerto Rico Instrument has the valley floor paved in metal sheeting to act as the mirror.
Question 2 What disadvantage does the Puerto Rico instrument have over the one at Jodrell Bank? ANSWER
Resolving Power
The resolving power of a radio telescope is governed by the same kind of factors as a light microscope. It is given by Rayleigh's Criterion:
Question 3 What is the angular resolution of 10 metre wavelength radio waves by the Jodrell Bank Telescope whose diameter is 75 m? ANSWER
From your answer below you can see that the resolution is not very good. To resolve between radio sources, the telescope has to scan across to detect the precise origin of each source.
The dish does not have to be as perfect as mirror for a light telescope. As long as the surface is within about 1/20 wavelength, then the focusing will be unaffected by imperfections. Also the reflector does not have to solid. Fine wire mesh will do, since radio waves will not pass through a gap less than one wavelength.
Radio Sources in the Universe
Radio astronomy has:
revealed the existence of radio sources, such as quasars and pulsars;
been used to analyse chemical elements in objects;
tracked the movement of planets using the Doppler effect;
looked at microwave radiation which gives evidence for the big bang.
The picture below shows a radio source called M82
And the next picture shows the same patch of sky visible to all sorts of different wavelengths.
Radio waves can penetrate dust, so we can look at the centre of our galaxy. However radio waves of wavelength less than about 1 cm are blocked out by carbon dioxide and water. Radio waves of wavelength 20 m and above are absorbed by the atmosphere. Also radio signals from Earth can cause interference, just like light pollution for light telescopes. Passing satellites can also obscure the field of view.
Many radio telescopes have been set up well away from cities. Satellites with radio telescopes have been used to investigate the microwave radiation that points to the Big Bang.
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Summary
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