FASS: a turbulence profiler based on a fast, low-noise camera

Abstract

The measurement of the atmospheric optical turbulence with a new scintillation profiler is described and demonstrated. Theinstrument, FASS (Full Aperture Scintillation Sensor), uses new fast and low-noise detectors to record and process sequencesof scintillation images. Statistical processing of these data is based on the calculation of power spectra of intensity in annularpupil zones over the angular coordinate. The angular power spectra are used to measure the optical turbulence intensity of 14layers located at logarithmically spaced distances from 0.3 to 25 km. The reference functions relating turbulence strength to theangular power spectra are computed by numerical simulation. Measurement of the ground-later turbulence and total seeing withFASS is possible when the detector is conjugated to a negative (below ground) distance. Results of measurement campaignsat Paranal are reported, documenting a good agreement of turbulence profiles measured by FASS with two other instruments,SCIDAR and MASS.