  We present a new method to measure the vertical aerosol optical depth (VAOD)
during clear nights using a wide-field imager - a CCD camera with a
photographic lens on an equatorial mount. A series of 30-second exposures taken
at different altitudes above the horizon can be used to measure the VAOD with a
precision better than 0.008 optical depths within a few minutes. Such a
measurement does not produce any light and is thus suitable for use at sites
where other astronomical instruments are located. The precision of the VAOD
measurement depends on laboratory calibration of spectral properties of the
system and of the response of the camera electronics to varying illumination
levels, as well as careful considerations of details of stellar photometry and
modelling of the dependence of measured stellar fluxes star color and position
within the field of view. The results obtained with robotic setups at the
future sites of the Cherenkov Telescope Array show good internal consistency
and agreement with the simultaneous measurements from a Sun/Moon Photometer
located at the same site.
