Introduction to Radio Astronomy and Interferometrey
Radio astronomy uses radio waves to study regions of space.
Radio waves are a form of electromagnetic radiation - like visible light waves only with much longer wavelengths. Visible light waves are oscillating electromagnetic signals with wavelegths of less than one millionth of a metre, whereas radio waves have wavelengths in the range of a few millimetres to several metres. Radio telescopes gather and concentrate the radio waves from an astronomical source. The signal received is then electronically processed so that it can be measured by a computer. When a picture of an area of the sky is to be built up the signals are stored in a computer and displayed as a radio-wave intensity map of the region being observed. The Australia Telescope National Facility is a world leader in the technology associated with radio astronomy and in the astrophysics of sources that emit radio waves.
Single-dish radio telescopes produce blurry images.
When an optical image of an object is formed by reflecting light rays from a mirror onto a focus, quite sharp images of extended objects can be made. A single-dish radio telescope is not nearly as effective at detecting the fine detail in the objects it focuses on, and the radio image of an object from a single-dish radio telescope is very blurry. The blurring stems from a basic principle that affects all types of telescopes. In order to "resolve" images (ie make sharp images), the diameter of a telescope's collecting area must be many times greater than the wavelength of the radiation it detects. Light waves have wavelengths of less than one millionth of a metre, and so collecting mirrors are large enough compared to a wavelegth of light that they can resolve the details of objects observed. However, radio waves have wavelengths of roughly one tenth of a metre and so even large radio telescope dishes produce blurry images. A single radio dish would have to be many kilometres across to achieve a sharp image at radio wavelengths and such telescopes have been too difficult to build. Interferometry is a technique used to overcome the blurring and produce sharper radio images.
Astronomers are able to electronically simulate the effect of a very large dish by using the combined signal from many smaller single-dish radio telescopes. The technique they use is called interferometry. Interferometry exploits one advantage of the much larger wavelength of radio waves, which is that detectors can be used which measure the phase as well as the intensity of the radio waves received. If a pair of single-dish radio telescopes, separated on the ground, observes a single small source, the combined output oscillates with time. This is because, as the Earth rotates, the relative phase of the signals received by the two dishes varies as the distances from the source to each dish vary. This means that sometimes the radio wave from the object received by the two different dishes will add up in phase and produce a large signal and sometimes be out of phase and produced a small signal, depending on the slightly different distances from the object to each dish. Interferometry uses the constructive and destructive addition of the radiation to determine information about the intensity and size of the object being observed.The spacing between the radio telescope dishes in an interferometer determines the size of the objects that can be resolved by the interferometer.
If a source being observed covers an angle in the sky much smaller than the ratio (detected wavelength)/(twice the separation between two radio telescope dishes) then the combined output from the pair of dishes observing the source oscillates strongly with time. The ratio above is called the resolution of the telescope. As the separation between the dishes increases, the angular size of source that the telescope can detect decreases. For sources of angular size much larger than the resolution, the output ceases to vary and less information can be obtained about the sources. To study large-scale structure, astronomers need to use a lower resolution ie a more closely spaced pair of dishes. An interferometer works most efficiently, in the sense of returning the most information about the intensity and size of the source being observed, for a source whose size is comparable with the telescope's resolution.Using many dishes together in an interferometer array allows astronomers to form more complete images of objects.
Most sources have a complicated shape, with both large and small-scale structure. Better images of observed sources can be formed if more than two dishes are used at once, and as more powerful computers have become available signals from more dishes have been able to be combined electronically. The computer must carefully coordinate the movements of all the dishes. Each dish can be combined as a pair with every other dish to maximise the number of spacings between dishes. This maximises the size-scales in the source about which the interferometer can obtain information. The rotation of the Earth under the source then changes the distances from the source to each dish. The interference pattern generated by each pair as their signals are combined is fed to a computer device called a correlator, which electronically merges these multiple patterns.This is how an aperture synthesis telescope such as the Australia Telescope Compact Array works.
By this technique, which also goes under the name Earth-rotation synthesis, observations of a stable radio source by a number of interferometer pairs are combined to form detailed radio images of objects.VLBI (Very Long Baseline Interferometry) enables very small objects to be clearly resolved.
To clearly resolve very fine detail and small objects the radio telescope dishes have to be very far apart. For VLBI the elements of the interferometer may be separated by thousands of kilometres, over several continents, and even on spacecraft. For such large separations it is impractical to physically connect the dishes together. In VLBI, the signals from each element are sepatately recorded along with the timing pulses from an atomic clock. The recordings are then physically brought together at a later date and processed by a computer which uses the time signals to ensure the correct registration of one radio signal with respect to another.Go to Introduction to Radio Astronomy and Interferometry Summary
Last update by Michelle Storey. 14/2/99
Projects
Public