  The idea of using stellar photometry for atmospheric monitoring for optical
experiments in high-energy astrophysics is seemingly straightforward, but
reaching high precision of the order of 0.01 in the determination of the
vertical aerosol optical depth (VAOD) has proven difficult. Wide-field
photometry over a large span of altitudes allows a fast determination of VAOD
independently of the absolute calibration of the system, while providing this
calibration as a useful by-product. Using several years of data taken by the
FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope at the Pierre
Auger Observatory in Argentina and about a year of data taken by a similar
instrument deployed at the planned future Southern site of the Cherenkov
Telescope Array in Chile, we have developed methods to improve the precision of
this measurement technique towards and possibly beyond the 0.01 mark. Detailed
laboratory measurements of the response of the whole system to both the
spectrum and intensity of incoming light have proven indispensable in this
analysis as the usual assumption of linearity of the CCD detectors is not valid
anymore for the conditions of the observations.
