  The presence of weak intergalactic magnetic fields can be studied by their
effect on electro-magnetic cascades induced by multi-TeV gamma-rays in the
cosmic radiation background. Small deflections of secondary electrons and
positrons as the cascade develops extend the apparent size of the emission
region of distant TeV gamma-ray sources. These gamma-ray halos can be
resolvable in imaging atmospheric Cherenkov telescopes and serve as a measure
of the intergalactic magnetic field strength and coherence length. We present a
method of calculating the gamma-ray halo for isotropically emitting sources by
treating magnetic deflections in the cascade as a diffusion process. With this
ansatz the moments of the halo follow from a set of simple diffusion-cascade
equations. The reconstruction of the angular distribution is then equivalent to
a classical moment problem. We present a simple solution using Pade
approximations of the moment's generating function.
