Improved sensitivity of H.E.S.S.-II through the fifth telescope focus system

TL;DR

This paper investigates how adjusting the focus of the H.E.S.S.-II telescope's fifth large dish improves sensitivity to low-energy gamma-ray events by optimizing the imaging of atmospheric Cherenkov radiation.

Contribution

It introduces the focus system for the H.E.S.S.-II CT5 telescope and analyzes its impact on sensitivity at low energies, addressing a key question in Cherenkov telescope imaging.

Findings

Focus adjustment enhances low-energy gamma-ray detection sensitivity.

Optimal focus depth improves Cherenkov image quality.

Focusing at the shower maximum yields better sensitivity.

Abstract

The Imaging Atmospheric Cherenkov Telescope (IACT) works by imaging the very short flash of Cherenkov radiation generated by the cascade of relativistic charged particles produced when a TeV gamma ray strikes the atmosphere. This energetic air shower is initiated at an altitude of 10-30 km depending on the energy and the arrival direction of the primary gamma ray. Whether the best image of the shower is obtained by focusing the telescope at infinity and measuring the Cherenkov photon angles or focusing on the central region of the shower is a not obvious question. This is particularly true for large size IACT for which the depth of the field is much smaller. We address this issue in particular with the fifth telescope (CT5) of the High Energy Stereoscopic System (H.E.S.S.); a 28 m dish large size telescope recently entered in operation and sensitive to an energy threshold of tens of GeVs. CT5 is equipped with a focus system, its working principle and the expected effect of focusing depth on the telescope sensitivity at low energies (50-200 GeV) is discussed.