  The resolution power of current Imaging Atmospheric Cherenkov Telescopes is
presently restricted to scales of a few arcminutes. In the very high-energy
(VHE; E > 100 GeV) gamma-ray regime, the measurement of source sizes that are
comparable to or smaller than the resolution of the instrument is usually
limited by statistics and in particular by the uncertainties in the
characterisation of the instrument Point Spread Function (PSF). The PSF varies
strongly with observation and instrument conditions, demanding time-dependent
simulations of these conditions. Employing such simulations, we substantially
improve our understanding of the H.E.S.S. PSF and are now able to probe source
extensions well below one arcminute scale. We present the results of this new
approach applied to known VHE gamma-ray sources and show how this enables us to
reveal for the first time the extension of the Crab nebula at TeV gamma-ray
energies, with a width of 52 arcsec assuming a Gaussian source shape.
