  The Cherenkov Telescopes Array (CTA) is planned to be an Observatory for very
high energy gamma ray astronomy and will consist of several tens of telescopes
which account for a reflective surface of more than 10000 m$^2$. The mirrors of
these telescopes will be formed by a set of facets. Different technological
solutions, for a fast and cost efficient production of light-weight mirror
facets are under test inside the CTA Consortium. Most of them involve composite
structures whose behavior under real observing conditions is not yet fully
tested. An outdoor test facility has been built in one of the candidate sites
for CTA, in Argentina (San Antonio de los Cobres [SAC], 3600m a.s.l) in order
to monitor the optical and mechanical properties of these facets exposed to the
local atmospheric conditions for a given period of time. In this work we
present the preliminary results of the first Middle Size Telescope (MST)
mirror-monitoring campaign, started in 2013.
