  A recently proposed novel technique for the detection of cosmic rays with
arrays of Imaging Atmospheric Cherenkov Telescopes is applied to data from the
High Energy Stereoscopic System (H.E.S.S.). The method relies on the ground
based detection of Cherenkov light emitted from the primary particle prior to
its first interaction in the atmosphere. The charge of the primary particle (Z)
can be estimated from the intensity of this light, since it is proportional to
Z$^2$. Using H.E.S.S. data, an energy spectrum for cosmic-ray iron nuclei in
the energy range 13--200 TeV is derived. The reconstructed spectrum is
consistent with previous direct measurements and is one of the most precise so
far in this energy range.
