  We investigate the decoherence of Er$^{3+}$ in Y$_2$SiO$_5$ at low magnetic
fields using the photon-echo technique. We reproduce accurately a variety of
the decay curves with a unique coherence time by considering the so-called
superhyperfine modulation induced by a large number of neighbouring spins.
There is no need to invoke any characteristic time of the spin fluctuations to
reproduce very different decay curves. The number of involved nuclei increases
when the magnetic is lowered. The experiment is compared with a model
associating 100 surrounding ions with their exact positions in the crystal
frame. We also derive an approximate spherical model (angular averaging) to
interpret the main feature the observed decay curves close to zero-field.
