Abstract
Instruments applying the IACT method, such as H.E.S.S. (High Energy Stereoscopic System), observe VHE (very high energy, E > 100 GeV) photons indirectly, using the Earth's atmosphere as a calorimeter. In the H.E.S.S. data reconstruction, the properties of this component are estimated by Monte Carlo simulations of a yearly averaged atmosphere density profile. Deviations of the real atmospheric conditions from this assumed atmospheric model will result in a biased reconstruction of the primary gamma-ray energy and thus the resulting source spectrum. In order to keep the corresponding systematic effects to a minimum, H.E.S.S. operates a set of atmospheric monitoring devices that allows it to characterise the atmospheric conditions during data taking. This information in turn is then used in data selection. Here, a short overview with respect to their usage during source observation and a posteriori analysis data selection will be presented.
Highlights
In recent years, observations with Imaging Atmospheric Cherenkov Telescopes (IACT) have opened up a new window to the non-thermal universe, leading to the discovery more than a hundred sources in the VHE band
In order to keep the corresponding systematic effects to a minimum, H.E.S.S. operates a set of atmospheric monitoring devices that allows it to characterise the atmospheric conditions during data taking
The reconstruction of gamma-rays in this method is based on the proper understanding and characterisation of Extended Air Showers (EAS) that VHE gamma-rays create in the upper atmosphere
Summary
Observations with Imaging Atmospheric Cherenkov Telescopes (IACT) have opened up a new window to the non-thermal universe, leading to the discovery more than a hundred sources in the VHE band. The reconstruction of gamma-rays in this method is based on the proper understanding and characterisation of Extended Air Showers (EAS) that VHE gamma-rays create in the upper atmosphere These showers are observable via the Cherenkov radiation the secondary particles emit. The 26th of July 2012 was the date of first light of the new 28 m H.E.S.S. telescope which is optimised for observations at lower energies than the H.E.S.S. phase I telescopes This instrument stands an impressive 50 m tall when pointing at zenith. The data were used to spot incoming clouds and to locate low-level aerosol layers They were used in a detailed atmospheric study [5], see Sect. This instrument can be operated together with the H.E.S.S. phase I telescopes, which increases the sensitivity of the array by roughly a factor of 2 and features an energy threshold to a typical value of 80 GeV
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