Enhanced H.E.S.S. II low energies performance thanks to the focus system

TL;DR

This paper demonstrates how an adjustable focus system on the H.E.S.S. II telescope improves gamma-ray detection and analysis by optimizing focus during observations, enhancing performance at low energies.

Contribution

Introduction of a focus system on H.E.S.S. II that adjusts camera position during data collection, improving gamma-ray detection and analysis.

Findings

Increased gamma-ray trigger rate with focus adjustment

Improved energy reconstruction accuracy

Enhanced gamma-ray effective area at low energies

Abstract

For the current generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), with their large mirrors and their cameras with fine segmentation of photodetectors, the focusing capability is a relevant issue. The optical system of an IACT has a limited depth of field. Therefore, focusing the telescopes close to the shower maximum in the atmosphere has a significant impact on the data acquisition and analysis. As the distance of the shower maximum to the telescope depends (among others) on the zenith angle, an adjustable focus would be desirable. The fifth Cherenkov telescope of the H.E.S.S. II array is equipped with a focus system which allows to adjust the position of the camera along the optical axis, possibly during data taking. This impact has been studied on gamma-ray Monte Carlo simulations, and the results in terms of gamma-ray trigger rate, energy reconstruction and gamma-ray effective area will be shown.