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Mapping LOFAR grating lobes

Submitter: George Heald
Description: In previous daily images, we've used the so-called "beam mapping" technique to take a look at the shape of the main lobe of the LOFAR station beams (here and here). We can also take a look at other properties of the station beams. For example, especially at the highest frequencies, the HBA station beams are expected to include strong grating responses because of the regular layout of the tiles. It is for this reason that the HBA stations have layouts which are all rotated relative to each other. (The dipoles inside the tiles are back-rotated to align with a common polarimetric axis.)

We've now been able to observationally measure this grating lobe pattern, using the same technique as in the previous tests. The new maps were measured at 240 MHz. In the right-hand side of the image, a sequence of 20 beam maps is shown. Each beam map corresponds to a unique core station. The probe source, Cygnus A, is located at the bottom left of each frame. At that location, you can see the response from the main station beam, which is where the telescope is digitally "pointing" in an observation of Cygnus A. The animation clearly shows the grating lobes rotating around the main beam. The position of the grating lobes is directly related to the rotation angle of the station layout - this is illustrated to the left, which shows a core station oriented at the rotation angle of the station whose beam is mapped on the right. (The image was captured from bing, see also here).

Each beam map is approximately 15 degrees by 15 degrees. The grating lobes are located at large separations from the main beam, and in normal observations will be weakened by the analog tile beam. The analog beamformer limits the field of view to about 20 degrees at this frequency, but the effect of the analog beam is not shown in this image since the tile is always "pointing" at Cygnus A during the beam mapping observation. Stations will be sensitive to sources which happen to coincide with the grating lobes, but not quite as much as is shown here! Since the grating lobes of stations do not line up, this effect is further minimized in the correlated signal on individual baselines, but it will still be present at a low level.
Copyright: ASTRON/LOFAR; Station image from bing
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