Daily Image
30-10-2006
LOFAR imaging in a nutshell
| Submitter: | Oleg Smirnov, Jan Noordam |
| Description: | Calibration is the art of subtracting bright sources, so that we can study the faint ones underneath. As many sources as possible (thousands) will be subtracted quite satisfactorily from the uv-data. This simulation illustrates what happens to the rest. 25 point sources are placed on a regular grid with a spacing of 5 arcmin. They are observed with a LOFAR-like telescope, with 27 antennas and a maximum size of 30 km. During the 8-hour observation, a Travelling Ionospheric Disturbance (TID) makes it appear as though the sky is viewed from the bottom of a swimming pool, with waves travelling along the surface. This causes all sources to 'move' in different ways. It is possible to correct the data for this motion, but only for one point in the sky. Therefore, only the central source is imaged perfectly, while the (integrated) images of the others are distorted. Fortunately, these distortions can be predicted, so the sources can be largely reconstructed afterwards. However, part of their flux will be irretrievably distributed over the image, thus increasing the noise level, and thus the LOFAR sensitivity. Therefore, LOFAR images will be made as mosaicks of smaller facets, which minimizes the problem, at the cost of extra processing. The distortions are larger for lower observing frequencies, as can be seen as we pass from 30 MHz (large blobs) to 190 MHz (small blobs). The TID size was 100 km, and its waves propagated 'horizontally' over the image. In reality the TID propagating direction w.r.t. the sky would change during the observation, due to the apparent rotation of the sky. |
| Copyright: | The MeqTree Foundation 2006 |
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