Daily Image
20-08-2009
LOFAR wide-field imaging with only 3 baselines !
| Submitter: | Ger de Bruyn for the LOFAR calibration team |
| Description: | Following the recent succesful LOFAR LBA/HBA calibration/imaging of CygA using the first 3 LOFAR stations (see AJDI of 16Jul09) two 60-hour observations of the much fainter compact quasar 3C196 (~100 Jy at 100 MHz) were made in the weekends of 17-20 July and 31July-2August. These were done with the same 3 stations: CS302, RS307 and RS503. The data were processed by Sarod Yatawatta in MeqTrees and by Louise Ker (ASTRON-summerstudent) using BBS. The images were compared with a WSRT-LFFE 139 MHz Newstar image of the same field (a 1x12h synthesis with 13 telescopes) yielding excellent agreement. The results are indeed spectacular and beyond our 'wildest' dreams (!) and bode well for the images to be produced during the commissioning efforts over the next 1-2 months. Both MeqTrees and BBS (with only 1 subband !) produced images where many background sources could be detected. In the MeqTrees image, which used very wide-frequency bandwidth synthesis, a total bandwidth of about 13 MHz spread over a very wide frequency range (30-73 MHz for the LBA and 115-185 MHz for the HBA band) about 100 (hundred !) sources were detected. This is remarkable, to say the least, because it involves a dataset with only 3 baselines ! The data were calibrated using a single Direction-Independent-Calibration on 3C196. Following the calibration the central source was subtracted from the data. Some of the brightest off-axis sources (typically 5-10 Jy) show some spiky residuals indicating mild non-isoplanaticity (this was daytime observation revealing ionospheric TIDs in the phase solutions). The LOFAR images have not been deconvolved. The strong sidelobes in the PSF of the LOFAR-images are obvious, as is the improvement produced by Multi-Frequency-Synthesis (MFS). The noise in the images, inbetween obvious artefacts that still need further thought and work, indicate a DR of about 1000:1 in both the HBA and LBA bands. Stay tuned for even more impressive results using a 5 station observation from last weekend ! |
| Copyright: | ASTRON |
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