Daily Image
09-09-2010
Redundancy of LOFAR HBA station visibilities
| Submitter: | Parisa Noorishad |
| Description: | Visualizing data in the form of a movie is probably the best way to understand your data when you have a lot of it. The LOFAR HBA station monitoring observations done to check the applicability of redundancy calibration to the highly redundant HBA station array is such a case. Todays image shows data from the HBA observation with station CS302 done on September 5, 2009, over 24 hour. Each frame in the left panel shows data from a single snapshot observation of about 9 minutes in which the station correlator swept over all 512 195-kHz subbands between 100 and 200 MHz with 1 s integration per subband. During this observation, the tile beams were pointed at the local zenith. The left panel shows the amplitude and phase of all available redundant visibilities on three distinct baselines. A redundant visibility is the correlation between two tiles whose baseline vector (the vector connecting the center of the two tiles) appears more than once in the array. During a large time fraction of this monitoring observation, the redundant visibilities are very similar, although with different levels of SNR. This result is expected, because they measure the same spatial information on the sky. There are also periods, in which the redundancy breaks down. This happens when there is no strong source in the main beam and the A-team sources and the Sun, the strongest sources in the sky, are in the side lobes. This is shown in the right panel, which shows the position of these sources for every snapshot superimposed on the tile beam pattern. At least two factors, both currently being investigated in more detail, cause this break down: 1. If the strong sources are in the side lobes, their SNR will be much lower and the noise may have a significant impact on the data. 2. Redundancy assumes that all tile beams are identical. EM simulations in CEASAR indicate that this may not be true for he side lobes of the tile beam pattern due to mutual coupling between the tiles. To fully understand and, ultimately, handle such effects, movies like these will play an instrumental role. |
| Copyright: | ASTRON / LOFAR |
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