Daily Image

19-08-2009
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The intricacies of phased array telescopes

Submitter: Stefan Wijnholds
Description: With the roll-out of LOFAR progressing steadily, the full hierarchy of the LOFAR system becomes visible: the high band antenna (HBA) array consists of an array of stations, which are subarrays of 24, 48 or 96 HBA tiles, which in turn are composed of 16 HBAs. The left panel shows a map produced by a 24-tile subarray at CS302 by scanning the station beam over the sky integrating the data over the 148.4-158.8 MHz frequency range and 1 second for each pointing. The image shows the sun in the South-West and its grating lobe near the northeastern horizon as well as part of the galactic plane near the northwestern horizon and its grating response. So far, so good.

During this observation, the station correlator was running in parallel producing all inter-tile visibilities for the 195 kHz subband centered at 150.4 MHz integrated over 1 second for a full hour while the HBA tiles were scanning over the sky. From the raw visibilities, 3600 hemispheric snapshot images were produced using a DFT imager. One of these images, observed while the tiles were pointed at the galactic plane, is shown on the right. Although this picture may appear mind-boggling at first sight, everybody who thinks he or she understands how radio telescope arrays work, should be able to explain it. This is therefore left as an exercise for the reader.
Copyright: ASTRON / LOFAR
 
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