Ground-based gamma-ray astronomy made a considerable progress with the adoption of Imaging Atmospheric Cherenkov Telescopes (IACTs). A IACT is sensitive to Cherenkov light created by particles in gamma-ray induced air showers. Compared to other high energy detectors, the experiments that relay on air-shower measurements use atmosphere as a calorimeter. Even though IACTs are usually located at sites, where atmospheric conditions are very stable, local atmosphere should be continuously monitored, in terms of molecular density profiles, aerosol extinction profiles, and clouds. A next generation instrument, which will increase sensitivity of gamma-ray detection in 100 GeV to 10 TeV range for a factor of 5-10 is the Cherenkov Telescope Array (CTA).
For measurements of atmospheric properties above the CTA a LIDAR (LIght Detection And Ranging, a remote sensing device that measures backscattering of emitted laser pulses on aerosols and molecules along its field of view) system will be used. Operating at 355 nm, where the maximum efficiency of CTA is reached, it will be able to detect fine structures in the lowest part of the atmosphere and provide input data for CTA telescope calibration. Our LIDAR device has to meet the requirements imposed by the observatory. To calibrate CTA for any atmospheric effects along its line of sight, the LIDAR must measure extinction profile up to the altitude of 15 km a.s.l. where extended air-showers are typically formed. Since the CTA can operate down to the zenith angle of 60◦, this means LIDAR range must be at least 30 km. To cover the typical air-shower range of sever kilometers, the LIDAR range resolution must be at least or better than 300 m. The atmosphere must be characterized using at least two wavelengths present in the Cherenkov light spectrum, to be able to measure the transmittance with absolute accuracy of 0.03. Working at the CTA maximum sensitivity, laser light from the LIDAR would blind the CTA telescope, so the LIDAR data collection must be performed a few minutes before and after the CTA observation period, and during Wobble position. To achieve this, LIDAR must perform full characterization of an extinction profile during a minute long time interval.
The Barcelona LIDAR design is the most promising prototype and was awarded the status of a CTAO Pathfinder by the CTAO Council. It is in the final test and adjustment stage before shipping to the Roque de los Muchachos Observatory at La Palma. After its placement near the LAT- 1 telescope, the LIDAR will be tested in real conditions of the CTA-North site for a year. During this time the performance of all LIDAR systems will be investigated and first trials of the LST-1 data calibration with LIDAR atmospheric data will be made.
The UNG Barcelona LIDAR team includes M. Živec, S. Stanič, M. Zavrtanik.