  The sensitivity of an Imaging Atmospheric Cherenkov telescope is calibrated
by shining, from a distant pulsed monochromatic light source, a defined photon
flux onto the mirror. The light pulse is captured and reconstructed by the
telescope in an identical fashion as real Cherenkov light. The intensity of the
calibration light pulse is monitored via a calibrated sensor at the telescope;
in order to account for the lower sensitivity of this sensor compared to the
Cherenkov telescope, an attenuator is inserted in the light source between the
measurements with the calibrated sensor, and with the telescope. The resulting
telescope sensitivities have errors of 10%, and compare well with other
estimates of the sensitivity.
