  The stereoscopic reconstruction of air showers viewed by multiple imaging
atmospheric Cherenkov telescopes (IACTs) allows a more precise reconstruction
of shower energies and hence an improved determination of energy spectra.
Reconstruction techniques and in particular new systematic checks are discussed
on the basis of the large sample of gamma-rays from Mkn 501 detected with the
HEGRA IACT system. (Talk presented at the Workshop ``Towards a Major
Atmospheric Cherenkov Detector V'', Kruger Park, South Africa, 1997)
