Characterisation of Atmospheric Cherenkov Transparency with All-Sky Camera Measurements

摘要

The High Energy Stereoscopic System (H.E.S.S.) in Namibia measures gamma-ray emission via the detection of Cherenkov light in the optical waveband and is therefore highly sensitive to changes in the transparency of the atmosphere. This is especially true for aerosols, small dust particles covering the sky at the H.E.S.S. site and severely reducing the atmospheric transparency for blue Cherenkov light for several days each year. To quantify this effect, the Cherenkov Transparency Coefficient has been introduced as a hardware-independent parameter, which enables a correction of measured gamma-ray brightnesses. Neighbouring the Cherenkov array, the Automated Telescope for Optical Monitoring (ATOM) operates an all-sky cloud camera as secondary instrument. Due to its high exposure frequency, the cloud camera may act as a detection system, if image parameters indicating low Cherenkov transparency are identified. However, the current instrument - originally conceived as a weather warning system - only produces white-light frames in low resolution. This study examines all frames taken with the current instrument since 2008 which coincide with H.E.S.S. observations in order to characterise relations with the measured Cherenkov transparency. As a result of this preliminary study, trivial relations between the examined sky monitor observations and gamma-ray brightness can be excluded. However, it is planned to expand the scope of this activity with an upgraded device by introducing colour dependency and more advanced photometry with a larger number of objects in the near future.