  It is demonstrated that the bootstrap kernel [\onlinecite{sharma11}] for
finite values of ${\bf q}$ crucially depends upon the matrix character of the
kernel and gives results of the same good quality as in the ${\bf q}
\rightarrow 0$ limit. The bootstrap kernel is further used to study the
electron loss as well as absorption spectra for Si, LiF and Ar for various
values of ${\bf q}$. The results show that the excitonic effects in LiF and Ar
are enhanced for values of ${\bf q}$ away from the $\Gamma$-point. The reason
for this enhancement is the interaction between the exciton and high energy
inter-band electron-hole transitions. This fact is validated by calculating the
absorption spectra under the influence of an external electric field. The
electron energy loss spectra is shown to change dramatically as a function of
${\bf q}$.
