1. SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen and Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
2. Kapteyn Astronomical Institute, University of Groningen, 9747 AD Groningen, The Netherlands
3. Optics Group, Department of Imaging Physics, Delft University of Technology, 2628 CJ Delft, The Netherlands
4. Terahertz and Millimetre Wave Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden