  Determination of the energy and flux of the gamma photons by Imaging
Atmospheric Cherenkov Technique is strongly dependent on optical properties of
the atmosphere. Therefore, atmospheric monitoring during the future
observations of the Cherenkov Telescope Array (CTA) as well as anticipated
long-term monitoring in order to characterize overal properties and annual
variation of atmospheric conditions are very important. Several instruments are
already installed at the CTA sites in order to monitor atmospheric conditions
on long-term. One of them is a Sun/Moon photometer CE318-T, installed at the
Southern CTA site. Since the photometer is installed at a place with very
stable atmospheric conditions, it can be also used for characterization of its
performance and testing of new methods of aerosol optical depth (AOD)
retrieval, cloud-screening and calibration. In this work, we describe our
calibration method for nocturnal measurements and the modification of
cloud-screening for purposes of nocturnal AOD retrieval. We applied these
methods on two months of observations and present the distribution of AODs in
four photometric passbands together with their uncertainties.
