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A Large Size air Cherenkov Telescope (LST) prototype, proposed for the Cherenkov Telescope Array (CTA), is under construction at the Canary Island of La Palma (Spain) this year. The LST camera, which comprises an array of about 500 photomultipliers (PMTs), requires a precise and regular calibration over a large dynamic range, up to $10^3$ photo-electrons (pe's), for each PMT. We present a system built to provide the optical calibration of the camera consisting of a pulsed laser (355 nm wavelength, 400 ps pulse width), a set of filters to guarantee a large dynamic range of photons on the sensors, and a diffusing sphere to uniformly spread the laser light, with flat fielding within 3%, over the camera focal plane 28 m away. The prototype of the system developed at INFN is hermetically closed and filled with dry air to make the system completely isolated from the external environment. In the paper we present the results of the tests for the evaluation of the photon density at the camera plane, the system isolation from the environment, and the shape of the signal as detected by the PMTs. The description of the communication of the system with the rest of detector is also given., Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC 2017), Busan, Korea. All CTA contributions at arXiv:1709.03483

The Cherenkov Telescope Array (CTA) will be the next generation observatory employing different types of Cherenkov telescopes for the detection of particle showers initiated by very-high-energy gamma rays. A good knowledge of the Earth's atmosphere, which acts as a calorimeter in the detection technique, will be crucial for calibration in CTA. Variations of the atmosphere's transparency to Cherenkov light and not correctly performed calibration of individual telescopes in the array result in large systematic uncertainties on the energy scale. The Cherenkov Transparency Coefficient (CTC), developed within the H.E.S.S. experiment, quantifies the mean atmosphere transparency ascertained from data taken by Cherenkov telescopes during scientific observations. Provided that atmospheric conditions over the array are uniform, transparency values obtained per telescope can be also used for the calibration of individual telescope responses. The application of the CTC in CTA presents a challenge due to the greater complexity of the observatory and the variety of telescope cameras compared with currently operating experiments, such as H.E.S.S. We present here the first results of a feasibility study for extension of the CTC concept in CTA for purposes of the inter-calibration of the telescopes in the array and monitoring of the atmosphere., Comment: All CTA contributions at arXiv:1709.03483