Long-term monitoring of the optical performance of the primary mirrors at the Coihueco and Loma Amarilla sites of the Pierre Auger Observatory

Authors

• A. Abdul Halim, The University of Adelaide
• P. Abreu, Instituto Superior Técnico
• M. Aglietta, Istituto Nazionale di Fisica Nucleare, Sezione di Torino
• M. Ahmed, Université Paris-Saclay
• I. Allekotte, Centro Atomico Bariloche
• K. Almeida Cheminant, Radboud Universiteit
• B. Fick, Michigan Technological UniversityFollow
• K. Nguyen, Michigan Technological UniversityFollow
• D. Nitz, Michigan Technological UniversityFollow
• et al.

Document Type

Article

Publication Date

6-22-2026

Department

Department of Physics

Abstract

This paper presents the results of an optical performance monitoring campaign for the primary mirrors installed in fluorescence telescopes at the Coihueco and Loma Amarilla sites at the Pierre Auger Observatory in Argentina. Since the end of 2003, this effort has developed into a unique long-term dataset that addresses the performance of the primary mirrors. We have focused on the scattering characteristics and specular reflectance of mirror segments, as well as their evolution over several years of operation. Despite being housed in climate-controlled buildings, dust accumulation, impurities, and natural aging — such as oxidation or other chemical or physical degradation processes — gradually deteriorate the specular reflectivity of the mirror segments. In this paper, we summarize data collected over eight years of fluorescence telescope operation since the last major cleaning in autumn 2016. During this period, periodic in situ measurements were performed of scattering characteristics using two techniques: (1) specular and diffuse reflectance measurements at a wavelength of 670 nm using a portable scatterometer, and (2) spectral reflectance measurements in the range from 300 nm to 700 nm using an integrating sphere and a spectrometer. This study builds upon a previous monitoring campaign conducted between 2012 and 2017. The measurements show that the primary mirrors remain in acceptable condition after more than 20 years of operation.

Publisher's Statement

© 2026 The Author(s). Published by IOP Publishing Ltd on behalf of Sissa Medialab. Publisher’s version of record: https://doi.org/10.1088/1748-0221/21/06/P06040

Publication Title

Journal of Instrumentation

Recommended Citation

Abdul Halim, A., Abreu, P., Aglietta, M., Ahmed, M., Allekotte, I., Almeida Cheminant, K., Fick, B., Nguyen, K., Nitz, D., & et al. (2026). Long-term monitoring of the optical performance of the primary mirrors at the Coihueco and Loma Amarilla sites of the Pierre Auger Observatory. Journal of Instrumentation, 21(6). http://doi.org/10.1088/1748-0221/21/06/P06040
Retrieved from: https://digitalcommons.mtu.edu/michigantech-p2/2755

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

Version

Publisher's PDF