Re-calibrated Generalized-Scidar measurements at Cerro Paranal (VLT's site)

TL;DR

This study re-calibrates Generalized Scidar measurements at Cerro Paranal to correct biases, providing more accurate turbulence profiles and error estimates crucial for astronomical site assessment.

Contribution

It introduces a re-calibration method for GS data that corrects biases and improves turbulence profile accuracy at Cerro Paranal.

Findings

Relative errors of CN2 profiles can reach 60% in some cases.

Total seeing errors are up to 12%, with median errors around 0.05-0.06 arcsec.

Absolute error of the median isoplanatic angle is 0.13 arcsec.

Abstract

Generalized Scidar (GS) measurements taken at the Paranal Observatory in November/December 2007 in the context of a site qualification for the future European Extremely Large Telescope E-ELT are re-calibrated to overcome the bias induced on the CN2 profiles by a not correct normalization of the autocorrelation of the scintillation maps that has been recently identified in the GS technique. A complete analysis of the GS corrected measurements as well as of the corrected errors is performed statistically as well as on individual nights and for each time during all nights. The relative errors of the CN2 profiles can reach up to 60% in some narrow temporal windows and some vertical slabs, the total seeing up to 12% and the total integrated turbulence J up to 21%. However, the statistic analysis tells us that the absolute errors of the median values of the total seeing is 0.06 arcsec (relative error 5.6%), for the boundary seeing 0.05 arcsec (relative error 5.6%) and for the seeing in the free atmosphere 0.04 arcsec (relative error 9%). We find that, in spite of the fact that the relative error increases with the height, the boundary and the free atmosphere seeing contribute in an equivalent way to the error on the total seeing in absolute terms. Besides, we find that there are no correlations between the relative errors and the value of the correspondent seeing. The absolute error of the median value of the isoplanatic angle is 0.13 arcsec (relative error 6.9%).