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تسجيل مستخدم جديدStatus and motivation of Raman LIDARs development for the CTA Observatory
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The Cherenkov Telescope Array (CTA) is the next generation of Imaging Atmospheric Cherenkov Telescopes. It would reach unprecedented sensitivity and energy resolution in very-high-energy gamma-ray astronomy. In order to reach these goals, the systematic uncertainties derived from the varying atmospheric conditions shall be reduced to the minimum. Different instruments may help to account for these uncertainties. Several groups in the CTA consortium are currently building Raman LIDARs to be installed at the CTA 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 photosensor, an optical module that spectrally select wavelengths of interest, and a read-out system. Raman LIDARs can reduce the systematic uncertainties in the reconstruction of the gamma-ray energies down to 5 % level. All Raman LIDARs subject of this work, have design features that make them different than typical Raman LIDARs used in atmospheric science, and are characterized by large collecting mirrors ($sim2~$m$^2$). They have multiple elastic and Raman read-out channels (at least 4) 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.
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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 desig
ned 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.
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