Walter Alef was of invaluable help in all the steps of this work and Guidetta Torricelli made useful comments and suggestions. Dave Graham , Richard Porcas , Richard Schilizzi, Jürgen Neidhöfer, Sheperd Doeleman, Malcom Wamsley, Ambretta Falchi, Leslie Hunt, Klaus Reif and Claudio Codella were involved in many useful discussions. We acknowledge support for this research by the EU contract CHCGECT920011.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998