Site-testing using an aerosol, backscatter lidar at the Roque de los Muchachos Observatory

Abstract

The Roque de los Muchachos Observatory, located on the island of La Palma in the Canary Islands, is home of many astronomical facilities for its high sky quality. In the context of the Extremely Large Telescope Design Study, two intensive lidar campaigns were performed at the Roque de los Muchachos Observatory near the Jacobus Kapteyn Telescope between 9th and 11th July 2007 and between 26th May and 14th June 2008. The goal of the campaign was to characterize the atmosphere in terms of planetary boundary layer (PBL) height and aerosol stratification vs. synoptic conditions. Three typical synoptic situations were found which, respectively, privileged the intrusion of aerosols from marine/oceanic, anthropogenic and Saharan origin. All measurements revealed a multi-layer stratification with a mean PBL height of 546 ± 198 m agl and top layers as high as ~ 8400 m asl. As a by-product an estimate of the aerosol optical thickness was also obtained and compared to the total atmospheric extinction coefficient measured by the Carlsberg Meridian Telescope. Except in the presence of Saharan dust the aerosol optical thickness is very low: the average values are 0.0405 at 532 nm and 0.0055 at 1064 nm. In the presence of Saharan dust values of 0.233 and 0.157 were found at 532 and 1064 nm, respectively. The proportion of aerosol optical thickness contained in the layers above the PBL against that contained in the PBL is in all cases greater or equal to 50 % which emphasizes the importance of the upper layers in the scattering and absorption of astronomical signals. Additionally, for the first time spaceborne lidar measurements were also compared to a ground lidar for evaluating the use of a spaceborne active instrument for aerosol content monitoring in an astronomical site