Absolute reflectance of a concave mirror used for astro-particle physics experiments

TL;DR

This paper presents an improved in-situ method for monitoring the absolute reflectance and point spread function of concave mirrors used in astro-particle physics telescopes, accounting for weathering effects and scattering losses.

Contribution

It introduces a refined, precise in-situ measurement technique and compares two mirror technologies under real-world conditions in astro-particle physics experiments.

Findings

Refined measurement method with improved precision.

Significant light losses due to scattering identified.

Comparison of different mirror technologies in operational environments.

Abstract

The absolute reflectance of a reflector and its point spread function are the key parameters of a telescope for measuring light flux. Typically, one is using low-cost technologies for producing mirrors for the needs of astro-particle physics experiments. As a rule, these are operating telescopes in open air conditions at desert or mountainous locations, for cost reasons without protecting domes. The mirrors on such telescopes are exposed to sand in strong winds, precipitation and large temperature variations. Due to weathering, their reflectance is declining within few years. In this report we describe in a great detail the application of an in-situ method to the MAGIC imaging air Cherenkov telescopes for regularly monitoring their absolute reflectance and the point spread function. Compared to similar work that was previously performed, in this report we focus on important details of light losses due to scattering. These allowed us to further refine the method and significantly improve its precision. Also, we report on an in-situ comparison of two mirror types produced with different technologies.