  MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes (IACT) that
observe Very High Energy (VHE) gamma ray sources. The PMTs in their cameras are
designed to operate under moonlight, but they are limited to Moon phases below
93% (300 Moon hours per year), as they can get damaged if the amount of light
they receive is too high. As a result, they cannot be used in the three to five
nights around full Moon. We have selected commercial inexpensive UV-pass
filters rejecting light above a wavelength of 420 nm, where the moonlight
intensity is stronger. We mounted them on light-weight frames that can be
easily installed on the telescope cameras. Test observations have been
performed during the last nine months, from which a moonlight transmission of
about 20% and a Cherenkov light transmission of about 45% are estimated. This
allows the observation of sources down to an angular distance of 5 degrees to
the Moon during Full Moon: essentially in the whole sky and all possible
moonlight conditions. Therefore, the duty cycle of MAGIC can be extended by
about 30%, including nights when VHE observations with IACTs are currently not
feasible. Here we evaluate the preliminary performance, in terms of sensitivity
and energy threshold, of the MAGIC telescopes equipped with the UV-pass filters
under different moonlight intensities, as inferred from Crab Nebula
observations and Monte Carlo simulations.
