  The Cherenkov Telescope Array (CTA) is the next generation ground based
observatory for gamma ray astronomy at very high energies. Employing more than
100 Imaging Atmospheric Cherenkov Telescopes in the northern and southern
hemispheres, it was designed to reach unprecedented sensitivity and energy
resolution. Understanding and correcting for systematic biases on the absolute
energy scale and instrument response functions will be a crucial issue for the
performance of CTA. The LUPM group and the Spanish/Italian/Slovenian
collaboration are currently building two Raman LIDAR prototypes for the online
atmospheric calibration along the line of sight of the CTA. Requirements for
such a solution include the ability to characterize aerosol extinction at two
wavelengths to distances of 30 km with an accuracy better than 5%, within time
scales of about a minute, steering capabilities and close interaction with the
CTA array control and data acquisition system as well as other auxiliary
instruments. Our Raman LIDARs have design features that make them different
from those used in atmospheric science and are characterized by large
collecting mirrors (2.5 m2), liquid light guides that collect the light at the
focal plane and transport it to the readout system, reduced acquisition time
and highly precise Raman spectrometers. The Raman LIDARs will participate in a
cross calibration and characterization campaign of the atmosphere at the CTA
North site at La Palma, together with other site characterization instruments.
After a one year test period there, an in depth evaluation of the solutions
adopted by the two projects will lead to a final Raman LIDAR design proposal
for both CTA sites.
