If P is a whole number of wavelengths then the radio waves detected at the two

If p is a whole number of wavelengths then the radio

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If P is a whole number of wavelengths then the radio waves detected at the two telescopes are in phase with each other and the combined signal has a maximum intensity. This is known as constructive interference . If P is a whole number of half wavelengths, then the radio waves are out of phase and cancel each other. The combined signal has a minimum intensity. This is destructive interference . Combining the signals
For a path difference of P, there is a corresponding phase difference between the waves which arrive at the two telescopes. This can be written as = B cos 2 / . As the earth rotates the direction to the source ( ) changes and both P and change with time. For a point source, the intensity of the correlated signal varies sinusoidally between maximum and minimum values. The variations in intensity are known as interference fringes. The separation of the fringes is a measure of the angular resolution of the interferometer. Measuring fringes
For a baseline of length B, the angular resolution is given by: Resolution (R) (arcsec) = 2 x 10 5 x wavelength observed = Projected baseline length Bsin Because B can be made very large, R can be made very small. Most radio arrays have angular resolutions between 0.1 and 10 arcseconds. In Very Long Baseline Interferometry (VLBI), telescopes located in different parts of the world are used to give baselines of 1000s of kilometres and angular resolutions of milli-arcseconds - vastly smaller than for optical telescopes! Angular Resolution of an Interferometer
In the correlator the signals are multiplied together and are averaged over a sampling time of typically 20 to 30 seconds. The correlator is a powerful piece of electronics and software which is at the heart of any radio interferometer or radio array. The correlated signal is called the source visibility - this has two parts: The visibility amplitude is a measure of the detected flux density from the source. The visibility phase provides information on the source position. The visibility amplitudes and phases are written as data onto computer disks where they are stored for later analysis. The source visibility
Single-baseline interferometers can be used to measure the positions of unresolved sources, to an accuracy comparable to the angular resolution. To make an image of a more complex radio source, it is necessary to use an array of radio telescopes which has a number of baselines of different lengths. The longer baselines resolve the small-scale structure in the source. The shorter baselines provide information on the larger scale structures. The technique of using arrays of radio telescopes is called aperture synthesis interferometry. Why use arrays?
To illustrate how an array of radio telescopes works, consider some of the properties of the Australia Telescope Compact Array (ATCA).

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• Spring '16
• Ali
• Wavelength, Telescope, angular resolution, radio telescope

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