Characterization of atmospheric conditions at the future Cherenkov Telescope Array sites


The Cherenkov Telescope Array (CTA) will be the world's largest ground-based gamma-ray observatory to detect cosmic gamma-ray photons using indirect measurements of Cherenkov light emitted by secondary shower particles, which are produced in primary gamma-ray interactions with atomic nuclei in the upper atmosphere. CTA will be the most sensitive gamma-ray observatory, which exceeds the performance of all existing Cherenkov telescopes in the energy range between 20 GeV - 300 TeV by an order of magnitude. To cover the entire sky and reach the highest possible sensitivity, CTA will consist of several dozens of telescopes built on two sites - La Palma island (CTA-N), and the Atacama desert in Chile (CTA-S). Determination of the energy and flux of the gamma photons by Cherenkov telescopes is strongly dependent on the optical properties of the atmosphere. Therefore, atmospheric monitoring during the future observations of the Cherenkov Telescope Array (CTA) as well as anticipated long-term monitoring to characterize overall properties and annual variation of atmospheric conditions are essential. Institute of Physics currently operates two precise Sun/Moon photometers at both future CTA sites. These instruments can determine the integral optical depth of the atmosphere and aerosol concentration at night by comparison of measured Lunar irradiance with a model dependent on Lunar phase and libration angles. This method, however, is still under development and suffers from systematic uncertainties mainly resulting from uncertainties in the Lunar irradiance model. The thesis aims to develop a reliable method of calibration of the photometer using data from other instruments for atmospheric monitoring installed at both future CTA sites.