AO4ELT5 Proceedings

The Generalized FASS (Full Aperture Seeing Sensor): filling the lower altitudes of the Cn2 profile

Béchet, Clémentine (Engineering School, Pontificia Universidad Católica, Santiago, Chile, Santiago, Chile), Ayancán, Boris (Engineering School, Pontificia Universidad Católica, Santiago, Chile), Badinez, Rodrigo (Engineering School, Pontificia Universidad Católica, Santiago, Chile; European Southern Observatory, Garching, Germany), Guesalaga, Andrés (Engineering School, Pontificia Universidad Católica, Santiago, Chile), Sarazin, Marc (European Southern Observatory, Garching, Germany), Perera, Saavidra (Durham University, Durham, United Kingdom), Osborn , James (Durham University, Durham, United Kingdom), Wilson, Richard (Durham University, Durham, United Kingdom)

Turbulence monitors based on scintillation are in common use among the astronomical community. One of their weaknesses is that they are blind to turbulences located near the ground where the wavefront traversing these layers have not propagated over sufficiently long distances to generate scintillation patterns with significant contrast. One approach to solve this problem is to allow propagation below the telescope pupil by means of additional optics. The problem with such approach is the diffraction caused by the telescope aperture on the negatively conjugated images. We evaluate the impact of this undesired effect on a recently proposed technique called FASS (Full Aperture Seeing Sensor) that aims to monitor the atmospheric turbulence with a dedicated small telescope of typically 30-centimeter diameter. The method uses a Fourier analysis of the scintillation patterns to estimate the turbulence strength distribution in altitude. So far, the FASS concept and experimental results have been applied to pupil conjugation only, limiting the reconstruction of the turbulence profile to altitudes higher than 500m and requiring a total seeing monitor such as DIMM (Differential Image Motion Monitor) to fill the gap. Our goal is to develop a generalized version of FASS (negative conjugation), which will include the ground layer and where DIMM will not be necessary. In this article, we simulate this configuration, assessing the impact of the diffraction caused by annular aperture affects the profile estimation as a function of the conjugation altitude. We show that one of the features of the Fourier method used for profiling, which is the use of one-dimensional transforms of the image points along concentric rings, makes the technique very robust with respect to the diffraction rings generated by the annular aperture. We also define a method to find the most adequate negative altitude for conjugation, in order to design the future generalized FASS monitor.

DOI: 10.26698/AO4ELT5.0025- Proceeding PDF

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