Atmospheric turbulence profiling using multiple laser star wavefront sensors

Abstract

This paper describes the data pre-processing and reduction methods together with SLOpe Detection And Ranging (SLODAR) analysis and wind profiling techniques for the Gemini South Multi-Conjugate Adaptive Optics System (GeMS).The wavefront gradient measurements of the five GeMS Shack-Hartmann sensors, each pointing to a laser guide star, are combined with the deformable mirror (DM) commands sent to three DMs optically conjugated at 0, 4.5 and 9 km in order to reconstruct pseudo-open loop slopes.These pseudo-open loop slopes are then used to reconstruct atmospheric turbulence profiles, based on the SLODAR and wind-profiling methods. We introduce the SLODAR method, and how it has been adapted to work in a closed-loop, multi-laser guide star system. We show that our method allows characterizing the turbulence of up to 16 layers for altitudes spanning from 0 to 19 km. The data pre-processing and reduction methods are described, and results obtained from observations made in 2011 are presented. The wind profiling analysis is shown to be a powerful technique not only for characterizing the turbulence intensity, wind direction and speed, but also as it can provide a verification tool for SLODAR results. Finally, problems such as the fratricide effect in multiple laser systems due to Rayleigh scattering, centroid gain variations, and limitations of the method are also addressed.

Online estimation of the wavefront outer scale profile from adaptive optics telemetry

“…3, right), but they also change in shape, especially for larger outer scales. These differences in shape are caused by the finite nature of the telescope pupil and the WFS fratricide effects (Cortés et al 2012). These characteristics are nicely captured via simulation.…”

“…The atmospheric turbulence profile is recovered from the correlation of wavefront slopes of two stars (A and B) with a known angular separation. The common turbulence patch between two WFSs for a single layer (left) decreases as a function of the layer altitude; the slant range to the sodium layer (zenith angle); and the angular separation between the artificial stars (see Cortés et al 2012). This causes the correlation peak to shift along the WFS baseline (bottom right).…”

“…As GeMS is a closed-loop system, the next step is to construct the pseudo-open-loop slopes (POLSs) from on-sky data using the actuator voltages, the interaction matrix and the residual slopes (see Cortés et al 2012 andGuesalaga et al 2014 for a detailed explanation). After subtracting tip and tilt, these slopes are used to construct the auto-correlation of slopes using the normalization of overlapping subapertures described in Butterley et al (2006) and Wang, Schöck & Chanan (2008).…”

Scintillation correction for astronomical photometry on large and extremely large telescopes with tomographic atmospheric reconstruction

“…We can estimate the atmospheric optical turbulence profile using the AO telemetry from the WFSs using the slope detection and ranging (SLODAR) method with either natural guide stars (Wilson 2002) or the laser guide stars (LGS; Cortés et al 2012). SLODAR works by correlating the wavefront slopes from two or more WFSs, and a turbulent layer will appear in the covariance function as a peak with an offset from the centre proportional to the altitude of the turbulent layer and a magnitude proportional to the strength of the layer.…”

“…However, as no AO system is perfect, the residual phase aberration can result in a complicated spatially and temporally varying point spread function (PSF; for example Cagigal & Canales 2000;Currie et al 2000;Osborn, Myers & Love 2009;Baena Gallé & Gladysz 2011). Even in medium-to high-order correction regimes, quasi-static speckles due to non-common path errors and uncorrected aberrations can add complexity to the PSF (Fitzgerald & Graham 2006;Soummer et al 2007;Osborn 2012). These variations in the PSF make separating the light from different objects difficult.…”