An essential part of the CTAO telescopes’ detectors is situated well above their actual mechanical structure: the Earth’s atmosphere. It is there that high-energy gamma rays interact with atmospheric molecules, giving rise to a cascade of particles that produce faint flashes of Cherenkov light. However, because atmospheric conditions dictate how this light reaches the ground, it is fundamental to characterise our skies continuously. A key part of this important task is performed by devices known as Raman LIDARs.
A Raman LIDAR works by firing powerful laser pulses into the atmosphere and using a telescope to collect the light that bounces back. By measuring the exact time it takes for this light to return, it can calculate the precise amount of aerosols present at different altitudes. An important feature of the CTAO’s Raman LIDARs is their ability to simultaneously fire two laser beams of different wavelengths, covering the spectrum of Cherenkov light detected by the telescopes. The backscattered signal is then separated into four specific channels. One of these channels is dedicated to capturing the so-called Raman-scattered light, which occurs when photons interact with atmospheric nitrogen, resulting in a slight shift in wavelength. This sophisticated configuration provides scientists with highly detailed atmospheric profiles, allowing them to pinpoint various aerosols.
The CTAO will benefit from two designs of these instruments, one for each array site: the Barcelona Raman LIDAR (BRL) for CTAO-North in La Palma, Spain, and the Montpellier Raman LIDAR (MRL) for CTAO-South in the Atacama Desert, Chile. The latter is being developed by researchers from the In-Kind Contribution (IKC) team at LUPM (Laboratoire Univers et Particules de Montpellier), France.
The MRL shares a similar foundational design with its northern counterpart, featuring a large 1.8-metre mirror, a robust, high-specification laser, and a bespoke detection module. However, what truly sets the MRL apart at this stage is its innovative, in-house technology. The LUPM team has designed and produced a specialised timing protocol for the light sensor that allows the device to capture atmospheric profiles at exceptionally low altitudes as low as 230 metres. Furthermore, the team has developed an automated alignment and calibration protocol for the entire detection chain. This self-sufficiency reduces the need for human intervention to a minimum, which is a tremendous advantage for operating and maintaining the instrument in the remote, high-altitude environment of the Chilean desert.
Recently, the MRL team concluded a 24-month testing phase at the Observatoire de Haute-Provence in France. Over this period, atmospheric profiles were recorded, analysed, and validated against data from LATMOS, a reference site for the European Aerosol Research Lidar Network (EARLINET), a collaborative scientific network comprising dozens of LIDAR observation stations across Europe. The results confirmed that all individual elements of the LIDAR performed successfully.
These positive evaluations mark a significant step forward in the MRL development, propelling the project toward its next fundamental stage: the Critical Design and Mechanical Review (CDMR). Conducted by the CTAO Central Organisation, a CDMR is a comprehensive evaluation designed to verify that an instrument’s design is fully mature, safe, and ready for final construction and deployment.
In the near future, a panel of experts will visit France to officially initiate this review process. Passing the CDMR is mandatory to ensure the MRL fulfils the technical requirements for installation at CTAO-South. As the teams prepare for this important step, the progress of the Montpellier Raman LIDAR serves as a reminder that an observatory requires many different pieces working in harmony. Beyond the telescopes, calibration and monitoring instruments like the MRL are necessary to guarantee the highest quality data and allow the site to operate at maximum efficiency.