Daily Image
23-12-2009
Cassiopeia A seen through LOFAR
| Submitter: | Sarod Yatawatta |
| Description: | We have observed Cassiopeia A (CasA) many times before, while LOFAR was in various stages of testing/completion. In all previous observations, we only saw a point source in the sky. But now, we do have long enough baselines to give us a resolution of a few arc seconds, so we can see many more details of this complex source. Obviously, this brings its own calibration and imaging problems. Using 4 core HBA stations (split into two) and 4 remote ones, we observed CasA for 6 hours with a large fractional bandwidth (115-160 MHz). The longest baseline is about 30 km. The corresponding uv coverage (for multi frequency synthesis) is shown on the left. The corresponding image is shown on the right. The (preliminary) image was obtained after calibration using a shapelet-based source model, and deconvolution using CLEAN. Imaging in Stokes I is straightforward, thanks to the fact that the dipoles in different stations have the same orientation. The dynamic range obtained after CLEAN is a few hundred. However, it is also possible to subtract the source model from the visibility data, to give us a dynamic range of about 10,000, and thus reveal many fainter off-axis sources. The very best result is obtained by updating the shapelet model from the visibility data itself, by solving for the frequency-dependent shapelet coefficients (This feature is only available in MeqTrees at the moment). As the rollout adds more LOFAR stations, and our calibration and imaging software matures, we will continue to get better images. Watch this space. |
| Copyright: | ASTRON |
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