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3C196 and the A-team

Submitter: Ger de Bruyn
Description: The WSRT has an excellent reputation in wide-field high-dynamic-range imaging. This property is due in large part to the equatorial mount of its 14 dishes, the on-axis receivers, the virtually negligible closure errors, and the East-West arrangement of the array. An unusual demonstration of wide-field imaging is given in the montage shown above.

In the centre is shown a 6x6 degree image, with a resolution of 2x3 arcmin, surrounding the bright source 3C196. The observing frequency is 139 MHz, in the middle of the LFFE-band (LFFE = Low Frequency Front Ends). The brightest sources in the sky, also known as the A-team, are also visible and need to be located, and subtracted, lest their sidelobes will spoil the central target image. The images at the locations of the A-team are displayed in the corners. The sources CasA and CygA are circumpolar for the WSRT, hence always visible, but also TauA and VirA are above the horizon for most of the 12h synthesis. The A-team images have not been individually selfcalibrated ('peeled'), nor have they been 'deconvolved'. The intense symmetric side-lobe patterns around CasA and CygA reveal how remarkably similar the WSRT far-field sidelobe response is, up to distances of 100 degrees or more from the pointing direction. The spiky pattern radiating from CygA reveals a time-dependent position and flux density presumably due to ionospheric scintillation (CygA is the most compact source among the A-team).

We are using these images to prepare ourselves for LOFAR all-sky imaging. For example, the positions of these distant sources should vary with frequency due to ionospheric refraction. A comparison between the MeqTrees/AIPS++ and NEWSTAR analysis has revealed subtle differences (at arcmin level) and together with Wim Brouw and Sarod Yatawatta I am trying to understand their origin.

Copyright: Ger de Bruyn
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