Raman LIDARs and atmospheric calibration for the Cherenkov Telescope Array

Abstract: The Cherenkov Telescope Array (CTA) is the next generation of Imaging Atmospheric Cherenkov Telescopes. It will reach a sensitivity and energy resolution never obtained until now by any other high energy gamma-ray experiment. Understanding the systematic uncertainties in general will be a crucial issue for the performance of CTA. It is well known that atmospheric conditions contribute particularly in this aspect.Within the CTA consortium several groups are currently building Raman LIDARs to be installed on the two sites. Raman LIDARs are devices composed of a powerful laser that shoots into the atmosphere, a collector that gathers the backscattered light from molecules and aerosols, a photo-sensor, an optical module that spectrally selects wavelengths of interest, and a read--out system.Unlike currently used elastic LIDARs, they can help reduce the systematic uncertainties of the molecular and aerosol components of the atmosphere to <5% so that CTA can achieve its energy resolution requirements of<10% uncertainty at 1 TeV.All the Raman LIDARs in this work have design features that make them different than typical Raman LIDARs used in atmospheric science and are characterized by large collecting mirrors (2.5m2) and reduced acquisition time.They provide both multiple elastic and Raman read-out channels and custom made optics design.In this paper, the motivation for Raman LIDARs, the design and the status of advance of these technologies are described.