I agree in the sense that such a leakage has a corresponding "effective difference beam". However I have a feeling that such effective difference beams do not necessarily correspond simply with actual physical difference beams. There again given the constraint that the effect must be uniform across the focal plane to pass deck jack then perhaps they do. So it is unambiguous? Does a given TB pattern imply one - and only one - effective difference beam? Clem -- ********************************************************************** Clem Pryke - Associate Professor - Physics University of Minnesota, Room 313 Tate, 116 Church Street S.E. Minneapolis, MN, 55455 Tel: 612-624-7578 Fax: 612-624-4578 email: pryke(a)physics.umn.edu **********************************************************************