  The large optical reflector (~ 100 m^2) of a H.E.S.S. Cherenkov telescope was
used to search for very fast optical transients of astrophysical origin. 43
hours of observations targeting stellar-mass black holes and neutron stars were
obtained using a dedicated photometer with microsecond time resolution. The
photometer consists of seven photomultiplier tube pixels: a central one to
monitor the target and a surrounding ring of six pixels to veto background
events. The light curves of all pixels were recorded continuously and were
searched offline with a matched-filtering technique for flares with a duration
of 2 us to 100 ms. As expected, many unresolved (<3 us) and many long (>500 us)
background events originating in the earth's atmosphere were detected. In the
time range 3 to 500 us the measurement is essentially background-free, with
only eight events detected in 43 h; five from lightning and three presumably
from a piece of space debris. The detection of flashes of brightness ~ 0.1 Jy
and only 20 us duration from the space debris shows the potential of this setup
to find rare optical flares on timescales of tens of microseconds. This
timescale corresponds to the light crossing time of stellar-mass black holes
and neutron stars.
