A facility to evaluate the focusing performance of mirrors for Cherenkov Telescopes

TL;DR

This paper introduces a new open-air facility for evaluating the focusing performance of large segmented mirrors used in Cherenkov Telescopes, accounting for environmental conditions.

Contribution

The paper presents a novel metrological system specifically designed for in-situ mirror quality assessment in open-air conditions for Cherenkov Telescopes.

Findings

Accurate measurement of mirror radius of curvature

Assessment of focused and scattered light distribution

Demonstrated capability in real environmental conditions

Abstract

Cherenkov Telescopes are equipped with optical dishes of large diameter -- in general based on segmented mirrors -- with typical angular resolution of a few arc-minutes. To evaluate the mirror's quality specific metrological systems are required that possibly take into account the environmental conditions in which typically these telescopes operate (in open air without dome protection). For this purpose a new facility for the characterization of mirrors has been developed at the labs of the Osservatorio Astronomico di Brera of the Italian National Institute of Astrophysics. The facility allows the precise measurement of the radius of curvature and the distribution of the concentred light in terms of focused and scattered components and it works in open air. In this paper we describe the facility and report some examples of its measuring capabilities.