1. Department of Physics & Astronomy, University College London , Gower St, London WC1E 6BT, UK
2. Highams Park School , Handsworth Avenue, Highams Park, London E4 9PJ, UK
3. Sterrenkundig Observatorium, Ghent University , Krijgslaan 281-S9, B-9000 Gent, Belgium
4. Department of Physics and Astronomy, Louisiana State University , Baton Rouge, LA 70803, USA
5. School of Physics and Astronomy, Cardiff University , Cardiff, Wales CF24 3AA, UK