Daily Image

20-04-2007
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Where is Cas A?

Submitter: Sarod Yatawatta & Jan Noordam
Description: There are still many things wrong with this test image made with LOFAR Core Station 1 (CS1), but it also has some highly encouraging features. For instance, the calibration is good enough to completely remove the bright source Cas A (20.000 Jy) from the field centre. This makes it possible to see about 30-40 fainter 3C sources (~50 Jy). The image noise level is ~3 Jy, still 6 times higher than the expected thermal noise. Much of this is caused by the incomplete subtraction (on the right) of the second-brightest source, Cygnus A (5000 Jy), which gets very close to the horizon during the observation. The resulting dynamic range is a rather respectable 1:6000.
The CS1 array consists of only 16 "stations": 15 individual dipoles and one phased array of 48 dipoles (half a standard LOFAR station). The maximum baseline was ~500m. The total bandwidth was 450 kHz, distributed over 3 subbands between 45 and 60 MHz. The total observing time was 29 hours. The primary beam of the 48-dipole station was pointed at Cas A, which was therefore observed with much greater sensitivity than the rest of the sky.
The calibration consisted of solving simultaneously for the complex gains (JJones) in the direction of the two brightest sources, Cas A and Cygnus A. The contributions of both sources were subtracted from the data. Before making an image from them, the residuals were corrected for the complex gain in the direction of Cas A. This procedure gives the best image quality for sources close to Cas A, but it is still surprisingly good elsewhere.
Watch this space for further LOFAR results.
Copyright: LOFAR
 
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