  Three related features of the astrophysical S-factor, (S_{17}), for low
energies are an upturn as the energy of the proton goes to zero, a pole at the
proton separation energy and a long ranged radial integral peaking at
approximately 40 fm. These features, particularly the last, mean (S_{17}) at
threshold is largely determined by the asymptotic normalization of the proton
bound-state wave function. In this paper we show that by taking the pole
explicitly into account, (S_{17}) can be calculated in terms of purely short
range integrals involving the two-body potential. Much of the discusion is
relevant to other radiative capture processes especially those wth weakly bound
final states.
