Daily Image
12-05-2009
LOFAR station 302
| Submitter: | Jan Noordam |
| Description: | An aerial view of LOFAR station 302, which is close to the LOFAR core. The picture was taken from a hand-tethered balloon, floating at an altitude of a few hundred meters. The controller and the cable can just be gleaned at the bottom. This typical LOFAR station has two patches of 24 5x5m "HBA tiles", each of which has a 4x4 array of dual-polarization dipoles. These High Band Antennae (HBA) are sensitive to frequencies between 110 and 240 MHz, which corresponds to wavelengths between 3 and 1.2 meter. Between the HBA patches is an array of ~50 Low Band Antennae (LBA), which are sensitive to 20-90 MHz (15-3 m). Each dual-polarization LBA antenna is placed on a 2x2m conducting ground-plane. Note that the LBA configuration is randomized, and has its greatest density in the centre. This is to minimize the effects of the station beam sidelobes. The planes of all LOFAR stations close to the LOFAR core are parallel to the plane of the core, but they can be a few meters above or below it. Also, the dipoles in all LOFAR station have the same orientation. These precautions make it easier to calibrate the instrument, and to make accurate wide-field images. Obviously, it will be more difficult to maintain these symmetries for the outlying stations, due to the curvature of the Earth surface. Fortunately, the Earth is quite large, so that the deviations are small for baselines up to several thousands of km. It will be clear that LOFAR, with its stations of horizontal dipole arrays, presents a much greater calibration and imaging challenge than radio telescopes made with steerable parabolic dishes like the WSRT. However, the LOFAR approach has considerable advantages: it is much cheaper per square meter collecting area, and the sky can be observed in many directions simultaneously. And it could be argued that, of all the radio astronomy institutes in the world, ASTRON is best equipped to meet such challenges. |
| Copyright: | topfoto (Assen) |
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