1. Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
2. Leiden Observatory, Leiden University, Oort Gebouw, PO Box 9513, NL-2300 RA Leiden, the Netherlands
3. INAF – Instituto di Radioastronomia, via P. Gobetti, 101, I-40129 Bologna, Italy
4. Institute for Computational Cosmology, Department of Physics, Durham University, Durham DH1 3LE, UK
5. ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, the Netherlands
6. Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, the Netherlands