Muon rings enable precise calibration of telescopes at the Cherenkov Telescope Array Observatory

A new analytical solution has been developed to precisely calibrate dual-mirror Cherenkov telescopes at the Cherenkov Telescope Array Observatory (CTAO). The solution uses muon rings as a natural calibration source, addressing the challenge of determining how much Cherenkov light is detected by the camera, considering the shadows cast by the secondary mirror and the camera itself. The study, authored by Markus Gaug, Víctor Giráldez-Segalàs, and Fiona Redmen, is published in The Astrophysical Journal Supplement Series. The work provides an analytical method to calculate the fraction of Cherenkov light blocked by telescope components for each muon trajectory, with potential differences up to 40% from simplified models.

The study originated from a bachelor's thesis conducted by Víctor Giráldez-Segalàs, who extended analytical muon calibration methods for conventional single-mirror telescopes to the dual-mirror telescopes of the CTAO. The new formalism allows for precise calculation of the fraction of Cherenkov light blocked by various telescope components, which is critical for calibrating systems exposed to the elements. The method is particularly important for inclined muons and configurations with protective structures around the secondary mirror, where the predicted amount of light can differ by up to 40% from previous approximations.

The CTAO, a new-generation terrestrial observatory for very-high-energy gamma-ray astronomy, will have two locations: CTAO-North in La Palma and CTAO-South in the Atacama Desert. The new formalism provides the necessary tools to precisely calibrate the dual-mirror telescopes, including the Small-Sized Telescopes (SST), which use designs that offer wide fields of view and good image quality. The study also provides two results for conventional single-mirror telescopes, improving the accuracy of calibration and reducing systematic uncertainties.

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