The Pathfinder Barcelona Raman LIDAR (pBRL), developed as a prototype system for atmospheric monitoring at the CTAO, has completed its performance evaluation phase. Its design and capabilities have now been thoroughly assessed and published in two comprehensive papers in the Remote Sensing journal. The evaluation confirmed strong overall performance, with just a few aspects to be optimised in the final design.
Cherenkov telescopes, like those used by the CTAO, rely on the Earth’s atmosphere as part of the detector. It is in the atmosphere, between altitudes of 5 and 18 km, where high-energy gamma rays interact and create particle showers that emit Cherenkov light. Since atmospheric conditions affect how this light reaches the telescopes, the atmosphere must be continuously monitored to meet the CTAO’s demanding accuracy requirements.
As part of this monitoring strategy, researchers from In-Kind Contribution teams in Spain (IFAE and UAB/CERES-IEEC), Slovenia (University of Nova Gorica), and Italy (INFN-Padova), in collaboration with members from the CTAO Central Organisation, developed the pBRL as an atmospheric calibration pathfinder device.
LIDAR (Light Detection and Ranging) systems use a laser and telescope to probe the atmosphere. The laser sends short pulses of light upward, which are scattered by aerosols and molecules. The telescope collects the returning light, and by measuring the delay, researchers can determine the amount and type of atmospheric content at different altitudes. This information is then used to correct atmospheric effects in CTAO observations.
What sets the pBRL apart from other LIDAR systems is its large mirror of 1,8 metres diameter, dual-laser configuration and uniquely designed detection units. The pBRL emits two laser beams at different wavelengths simultaneously, and the returning signal is split into four distinct wavelength channels. One of these channels isolates the Raman-scattered light — light that has interacted with nitrogen molecules in the air, causing its wavelength to shift slightly. This advanced setup enables researchers to extract much more detailed information about the atmosphere, including the identification of specific gases and aerosol types.
The prototype was installed on the CTAO-North site in La Palma, Spain, in 2021 for a one-year testing period. With the data collected, the pBRL team assessed its performance, design, data analysis and software. Their findings are detailed in two scientific papers published this year: the first article focuses on the system design and technical performance, while the second presents a newly developed data analysis software suite and results from the measurement campaigns. The results also include an assessment of the pBRL’s performance under challenging atmospheric conditions caused by dust from the eruption of the Tajogaite volcano in La Palma in 2021, providing valuable data that helped validate the system’s reliability.
With the evaluation of the pathfinder now complete, the teams are incorporating the lessons learned into the final design of the Barcelona Raman LIDAR system. These improvements will ensure even greater robustness and accuracy for long-term operation, supporting the Observatory’s goal of delivering high-quality gamma-ray data under different atmospheric conditions.