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Unshrouding LOFAR's station vision

Submitter: Stefan Wijnholds
Description: These two all-sky images (Stokes I) demonstrate the considerable progress we have made over the last years in imaging with a single LOFAR station. In this mode, the signals from the individual elements (LBA dipoles or HBA tiles) are cross-correlated with each other into "visibility samples".

The image on the left was made in the classical way, by applying a direct Fourier transform to the raw visibilities of a single 156 kHz subband. Successive snapshot images were added to increase the sensitivity.

To produce the right-hand image, the data of each subband were self-calibrated on the bright celestial radio sources Cas A and Cyg A. After this, the flux in each image pixel was estimated in the least squares sense using all available data simultaneously. This procedure implies a deconvolution, and thus removes the instrumental Point-Spread Function (PSF).

The positions of a number of well-known sources have been indicated to emphasize that the new technique "unshrouds" fainter features that are otherwise hidden by the PSF sidelobes around the bright sources. Even the third galactic loop (thanks to Michiel Brentjens for pointing this out) emerging from Cyg A, which can also be found in the Haslam map, is now clearly visible. Another major improvement is that all spurious emission from the galactic plane near the western horizon, caused by the side-lobes of the station beam, is removed.

The observations were made with the 48 antennas of the old test-station CS10, using 27 156 kHz subbands evenly distributed between 45.3 and 67.3 MHz, and 10 seconds of integration. The data were collected on 8 November 2008 between 10:21:59 and 10:26:45 UTC.

Copyright: ASTRON / LOFAR
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