¹ Astronomy and Astrophysics Center, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637
² Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, Toulouse, F-31400, France
³ Current address:Yale University, P.O. Box 208101, New Haven, CT 06520; Universidad de Chile, Casilla 36-D, Santiago, Chile
⁴ Andes Fellow
⁵ Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15232
⁶ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125
⁷ Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510
⁸ Department of Physics and Astronomy, Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218
⁹ Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260
¹⁰ Princeton University Observatory, Princeton, NJ 08544
¹¹ Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349-0059
¹² Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195
¹³ Department of Physics of Complex Systems, Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary
¹⁴ Department of Astronomy and Research Center for the Early Universe, School of Science, University of Tokyo, Hongo, Bunkyo, Tokyo, 113-0033, Japan
¹⁵ Institute for Cosmic-Ray Research, University of Tokyo, Midori, Tanashi, Tokyo 188-8502, Japan
¹⁶ Institute for Advanced Study, Olden Lane, Princeton, NJ 08540
¹⁷ Remote Sensing Division, Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, DC 20375
¹⁸ Astronomy Center, University of Sussex, Falmer, Brighton BN1 9QJ, England, UK
¹⁹ Royal Observatory, Edinburgh, EH9 3HJ, England, UK
²⁰ US Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002-1149
²¹ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802
²² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany