  The Cherenkov Telescope Array (CTA) is the next generation of ground-based
very high energy gamma-ray instruments and is planned to be built on two sites
(one in each hemisphere) in the coming years, with full array operation
foreseen to begin 2020. The goal of performing high precision gamma-ray energy
measurements while maximizing the use of observation time demands detailed and
fast information about atmospheric conditions. Besides LIDARs designed to
monitor clouds and aerosol content of the atmosphere in the pointing direction
of the CTA telescopes, we propose to use the "FRAM" (F(/Ph)otometric Robotic
Atmospheric Monitor) device, which is a small robotic astronomical telescope
with a large field of view and a sensitive CCD camera that together ensure
precise atmospheric characterization over the complete field-of-view of the
CTA. FRAM will use stellar photometry to measure atmospheric extinction across
the field of view of the CTA without interfering with the observation (unlike
laser-based methods). This allows FRAM to operate with high temporal resolution
and provide both real-time data for on-the-fly scheduling decisions and an
offline database for calibration and selection of scientific data. The fast
robotic mount of the telescope supports quick observation of multiple fields
when the array is split and even a check of the conditions in the directions of
the upcoming observations is possible. The FRAM concept is built upon
experience gained with a similar device operated at the Pierre Auger
Observatory. A working prototype of FRAM proposed for CTA is being built in
Prague for extensive testing before deployment on site; first results and
experiences with this prototype are presented.
