Tufts/CfA/MIT Cosmology Seminar, at CfA:

                     Tuesday January 28, 1997
                             2:30 pm
                         Phillips Auditorium

              "Gravitational Lensing by Clusters of Galaxies: 
	       Effect on the X-Ray Background and on Measurements 
	             of the Sunyaev-Zel'dovich Effect"

                          Alexandre Refregier

			  Columbia University

Abstract: 
Gravitational lensing by clusters of galaxies alters the observed
flux and density distribution of background sources. In practice, this
can affect the statistics both of resolved sources and of the
residual unresolved background. The implications of this effect for
the cosmic X-Ray Background, and the prospects of its observation with
the future X-ray mission AXAF will be presented. The lensing effect
can also affect radio sources, and thus systematically bias
measurements of the Sunyaev-Zel'dovich effect and the resulting
estimate of the Hubble constant.