******************************************************
   PR #   161
   The principal goal is to seperate and measure the contribution of
   circumsourceand dust scattering to the total X-ray spectrum of Vela
   X-1 in eclipse. Theexpected data from the SIS on the halo and core
   spectrum, combined with theknown optical extinction (Av=2.2), will
   enable a determination of the X-rayscattering efficiency of
   interstellar grains. Information about thedistribution and composition
   of circumsource matter will also be obtained.
 ******************************************************