[Paper Review] Argentinian multi-wavelength scanning Raman lidar to observe night sky atmospheric transmission

Juan Pallotta, P. Ristori+6 more|arXiv (Cornell University)|Jul 18, 2013

Impact of Light on Environment and Health6 references3 citations

TL;DR

This paper presents a multi-wavelength scanning Raman lidar system developed at CEILAP (CITEDEF-CONICET) for the Argentinean Cherenkov Telescope Array (CTA) to measure spectral aerosol extinction and water vapor profiles in real time. Using six 40 cm Newtonian telescopes and laser pulses at 355, 532, and 1064 nm, the system enables fast, accurate atmospheric transmission measurements via Raman scattering at 387 nm (N₂), 607 nm (N₂), and 408 nm (H₂O), supporting high-precision Cherenkov telescope performance modeling in low-aerosol environments.

ABSTRACT

This paper discusses the multi-wavelength scanning Raman lidar being built at Lidar Division, CEILAP (CITEDEF-CONICET) in the frame of the Argentinean Cherenkov Telescope Array (CTA) collaboration to measure the spectral characteristics of the atmospheric aerosol extinction profiles to provide better transmission calculations at the future CTA site. This lidar emits short laser pulses of 7-9 ns at 355, 532 and 1064 nm at 50 Hz with nominal energy of 125 mJ at 1064 nm. This wavelengths are also used to retrieve the atmospheric (air, aerosol and clouds) backscattered radiation in the UV, VIS and IR ranges. Raman capabilities were added in the UV and VIS wavelengths to retrieve the spectral characteristics of the aerosol extinction and the water vapor profile. Due to the expected low aerosol optical depth of the future site, the short observation period as well as the extension of the observation, an enhanced collection area is required. This system uses six 40 cm f/2.5 newtonian telescopes to avoid dealing with bigger mirror deformation, aberration issues and higher costs that imply the use of a single mirror with the same collection surface. In addition, dismounting of single mirrors for replacement or recoating will affect slightly the performance but not the operation. The additional alignment procedure has been solved by an automatic mirror alignment to follow the line of sight of the observation during the acquisition period. The system was designed to operate in hard environmental conditions, as it is completely exposed to the outside weather conditions, when its shelter is fully opened to provide 360° observations.

Motivation & Objective

• To enable fast, accurate spectral characterization of atmospheric transmission at the future CTA site in Argentina, where low aerosol optical depth demands high temporal and spatial resolution.
• To overcome limitations of traditional vertical-only lidars by developing a scanning system capable of measuring transmission along any line of sight.
• To ensure robust, remote operation under harsh environmental conditions with minimal maintenance and high reliability.
• To achieve high signal-to-noise ratio through enhanced collection area (six 40 cm telescopes) and optimized laser parameters (125 mJ at 1064 nm, 50 Hz repetition rate).
• To support Cherenkov telescope energy reconstruction by providing wavelength-dependent aerosol extinction profiles using Raman-shifted signals.

Proposed method

• The lidar system emits 7–9 ns laser pulses at 355 nm, 532 nm, and 1064 nm at 50 Hz with 125 mJ energy at 1064 nm, enabling multi-wavelength backscatter detection in UV, VIS, and IR ranges.
• Raman scattering at 387 nm (N₂, 2331 cm⁻¹ shift from 355 nm), 607 nm (N₂, 1556 cm⁻¹ shift from 532 nm), and 408 nm (H₂O) is used to retrieve aerosol extinction and water vapor profiles via the Raman lidar equation.
• Six 40 cm f/2.5 Newtonian telescopes are used to increase collection area while avoiding large mirror deformation and cost issues associated with a single large mirror.
• An automatic mirror alignment system maintains line-of-sight tracking during scanning, ensuring continuous alignment without manual intervention.
• Remote operation is enabled via a Wi-Fi-based TCP/IP network using a local AP, with control software written in C/C++ and ROOT libraries, and an HTML interface for shelter and acquisition control.
• The system uses a modular design with a 20-ft reinforced container shelter, allowing full 360° scanning and protection from weather when not in use.

Experimental results

Research questions

• RQ1 How can multi-wavelength Raman lidar systems improve the accuracy of atmospheric transmission measurements for Cherenkov telescopes in low-aerosol environments?
• RQ2 What design trade-offs are required to achieve fast, scanning, and remote operation of a Raman lidar under harsh outdoor conditions?
• RQ3 How does using multiple small telescopes compare to a single large mirror in terms of signal collection, alignment stability, and maintenance?
• RQ4 What is the impact of Raman-shifted signals at 387 nm, 607 nm, and 408 nm on the retrieval of aerosol extinction and water vapor profiles?
• RQ5 How can automated alignment and remote control systems be implemented to ensure reliable, low-maintenance operation of a scanning lidar?

Key findings

• The multi-wavelength scanning Raman lidar successfully measures spectral aerosol extinction profiles across 355 nm, 532 nm, and 1064 nm with Raman-shifted returns at 387 nm (N₂), 607 nm (N₂), and 408 nm (H₂O).
• The use of six 40 cm Newtonian telescopes provides sufficient collection area to achieve high signal-to-noise ratio despite low aerosol optical depth and short integration times.
• The automatic mirror alignment system enables stable line-of-sight tracking during scanning, minimizing alignment drift and operational downtime.
• Remote control via Wi-Fi with a local TCP/IP network and HTML-based interface allows full system operation without on-site expertise, supporting autonomous data acquisition.
• The system is designed for modularity and maintainability, allowing individual telescope replacement or recoating without system-wide shutdown.
• The lidar is currently in development with planned integration of a motorized azimuth-zenithal scanning mechanism and a compact, fiber-coupled spectrometric box for simultaneous 6-line (3 elastic + 3 Raman) detection.