  We consider the collision of two Bose Einstein condensates at supersonic
velocities and focus on the halo of scattered atoms. This halo is the most
important feature for experiments and is also an excellent testing ground for
various theoretical approaches. In particular we find that the typical reduced
Bogoliubov description, commonly used, is often not accurate in the region of
parameters where experiments are performed. Surprisingly, besides the halo pair
creation terms, one should take into account the evolving mean field of the
remaining condensate and on-condensate pair creation. We present examples where
the difference is clearly seen, and where the reduced description still holds.
