An image of a slightly extended source For this model the source has a total

An image of a slightly extended source for this model

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An image of a slightly extended source. For this model the source has a total flux density of 1 Jy. One arcsecond radio source
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Case 2 - a small source cont. Visibility Amplitude (Jy) 1.0 0.8 0.6 (u 2 + v 2 ) 0.5 (k )
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For a slightly extended source, the visibility amplitudes decrease with increasing baseline length. The largest amplitudes are measured for the shortest baselines. On the longest baseline at (100 k ), the visibility amplitude drops to about half of the peak value. This source is unresolved on the shorter baselines but is partially resolved on the longer baselines. Case 2 - a small source cont. long baselines short baselines Source (size = 1 arsecond)
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Case 3 - a larger source An image of a larger extended source. The source has a total flux density of 1 Jy. 3 arcseconds In this model the source again has a total flux density of 1 Jy but has a larger size of three arcseconds.
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Case 3 - a larger source cont. 1.0 (u 2 + v 2 ) 0.5 (k ) 0.6 0.2 Visibility Amplitude (Jy)
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Compare the visibility plots for case 2 and case 3. For the larger source, the visibility amplitudes decrease more steeply as the baselines increase in length. This source is well resolved. Note that almost no emission is detected on the longest baselines. Case 3 - a larger source cont. long baselines shortest baseline Source (size = 3 arseconds)
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To end this discussion consider two point sources which are separated from each other by one arcsecond in an east-west direction. Each of the sources has a flux density of 1 Jy. Case 4 - two point sources For this model the diagram on the next slide shows how the visibility amplitudes vary with time for the five longest baselines of the ATCA. 1 arcsec
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Case 4 - two point sources cont. Visibility Amplitude (Jy) Time from start of observation (hours) 0 4 8 12 Amplitude variations for a 6-km baseline . Amplitude variations for a 3-km baseline .
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For this model, the visibility amplitudes decrease during the first six hours of observing but then increase again. The maximum detected emission of 2 Jy corresponds to the total flux density of the two sources. For some baseline directions the wavefronts from the two sources combine in phase and the visibility amplitude from the correlated signal has a maximum value. For other directions the wavefronts from the two sources partly cancel each other giving smaller visibility amplitudes. Notice that the strongest variations occur for the longest baselines where the source is most resolved. Case 4 - two point sources cont.
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End of Activity Press the ESC (Escape) key to return to the home page for this Module.
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Image Credits The Australia Telescope Compact Array 13-cm radio emission from Jupiter Image credit: G. Dulk, Y. Leblanc, R. Sault, R. Hunstead Effelsberg 100-m radio telescope Aerial view of the Compact Array Photo credit: Stewart Duff, CSIRO
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  • Spring '16
  • Ali
  • Wavelength, Telescope, angular resolution, radio telescope

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