1. Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
2. Department of Physics, Florida State University, 77 Chieftan Way, Tallahassee, FL 32306, USA
3. The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St, Pasadena, CA 91101, USA
4. Department of Physics & Astronomy, Vanderbilt University, 6301 Stevenson Center Lane, Nashville, TN 37235, USA
5. Department of Physics, Fisk University, 1000 17th Avenue N., Nashville, TN 37208, USA
6. School of Physics, Trinity College Dublin, the University of Dublin, Dublin, Ireland