  One of the problems in the design of a high energy e+e- linear collider is
the distribution of the luminosity as a function of real collision energy,
dL/dE, due to initial state radiation, beamstrahlung, and the energy spread of
the collider. These effects smear a sharp feature in the cross section of e+e-
-> hadrons such as the ttbar threshold into a flatter structure. This study
reviews the impact on dL/dE of these effects as a function of machine
parameters, explores some methods of measuring dL/dE with Bhabha scattering,
how the dL/dE flattens the ttbar cross section near threshold, and the effect
the dL/dE measurement has on extracting parameters of the top quark, such as
the mass and width, in a 2.5/fb scan of the ttbar threshold.
