ASTRI-Horn Cherenkov gamma-ray telescope: the average reflectivity of the primary mirror based on and “ad hoc” multilayer coating

Abstract

ASTRI-Horn is a 4-m class Cherenkov telescope located on Mt. Etna, Serra La Nave, Italy, operated at the Italian National Institute for Astrophysics (INAF) “M.G. Fracastoro” observing station. It is the end-to-end prototype for the nine telescopes of the ASTRI Mini-Array, a collaborative international effort led by INAF under construction at the Teide Observatory on Tenerife, in the Canary Islands. The ASTRI Mini-Array is based on nine Image Atmospheric Cherenkov Telescopes (IACTs) and it is devoted to perform astronomical observations in the high-energy gamma-ray band, above 1 TeV. ASTRI-Horn is characterized by a Schwarzschild-Couder dual-mirror optical design with a 4.3 m diameter primary mirror (M1), segmented in 18 hexagonal panels, and a monolithic 1.8 m diameter secondary mirror. The focal plane camera is based on arrays of Silicon Photo-Multiplier (SiPM) sensors, working in the wavelength band of 300-900 nm. The panels of the upgraded version of the telescope’s primary mirror are based on a multilayer dielectric coating instead of the classical Aluminium (with a thin SiO2 protection layer) coating. This multilayer coating has been designed to strongly reduce the reflectivity above 700 nm, where the diffuse night sky background dominates over the Cherenkov signal from showers and the SiPM sensors are still effective. In this work we present the computation of the average reflectivity of ASTRI-Horn primary mirror starting from reflectivity measurements performed on each panel over a wide wavelength range (200-1000 nm). This experimental average reflectivity curve will be adopted in the ASTRI-Horn simulation chain, which does not handle the reflectivity of the single panels.