A novel image-correction method for cloud-affected observations with Imaging Atmospheric Cherenkov Telescopes

¹ Department of Astrophysics, University of Lodz, Pomorska 149, Łódź, Poland
e-mail: natalia.zywucka@fis.uni.lodz.pl; jsitarek@uni.lodz.pl; dorota.Sobczyńska@uni.lodz.pl
² Ruđer Bošković Institute, Bijenička cesta 54, Zagreb, Croatia
³ University of Rijeka, Faculty of Physics, Radmile Matejčić 2, Rijeka, Croatia
⁴ J. J. Strossmayer University of Osijek, Department of Physics, Trg Ljudevita Gaja 6, Osijek, Croatia

Received: 22 January 2024
Accepted: 19 February 2024

Abstract

Context. The presence of clouds during observations with Imaging Atmospheric Cherenkov Telescopes can strongly affect the performance of the instrument due to additional absorption of light and scattering of light beyond the field of view of the instrument. If not corrected for, the presence of clouds leads to increased systematic errors in the results.

Aims. One approach to correct for the effects of clouds is to include clouds in Monte Carlo simulations to produce models for primary particle classification, and energy and direction estimation. However, this method is challenging due to the dynamic nature of cloudy conditions and requires extensive computational resources. The second approach focuses on correcting the data themselves for cloud effects, which allows the use of standard simulations. However, existing corrections often prioritise the limitation of systematic errors without optimising overall performance. By correcting the data already at the image level, it is possible to improve event reconstruction without the need for specialised simulations.

Methods. We introduce a novel analysis method based on a geometrical model that can correct the data already at the image level given a vertical transmission profile of a cloud. Using Monte Carlo simulations of an array of four of the Large-Sized Telescopes of the Cherenkov Telescope Array, we investigated the effect of the correction on the image parameters and the performance of the system. We compared the data correction at the camera level with the use of dedicated simulations for clouds with different transmissions and heights.

Results. The proposed method efficiently corrects the extinction of light in clouds, eliminating the need for dedicated simulations. Evaluation using Monte Carlo simulations demonstrates improved gamma-ray event reconstruction and overall system performance.