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Durham University CfAI developed the SLODAR (SLOpe Detection And Ranging) technique for characterisation of the vertical profile of atmospheric optical turbulence. 

SLODAR is a crossed beams method based on observations of double stars using a Shack-Hartmann wavefront sensor. The optical turbulence profile (OTP) is recovered from the cross-correlation of the wavefront slope measurements for the two stars. 

Conventional SLODAR can provide measurements of the turbulence profile for the whole atmosphere. Here we present a new implementation of the technique to enable profiling with very high resolution of the surface layer turbulence (only) by observing targets with very large angular separations. These widely separated targets cannot be observed simultaneously within the field of view of a single WFS. Hence, we use a reflective wedge to divert the light for the two target stars into separate WFSs with independent detectors. With this modification, a vertical resolution of ∼10 m was achieved, and the surface layer of turbulence was resolved. We refer to the modified technique as surface-layer SLODAR or SL-SLODAR.

Photograph of the SL-SLODAR on the 16-inch Meade telescope
at Paranal (Photo: Tim Butterley)
The median surface layer profile for data acquired on 14 nights in 2009 February and April, at Cerro Paranal, Chile. The subplot on the left shows the surface layer profile which has a strong boundary layer and a decay height of ∼11 m. The subplot on the right shows the integrated surface layer (resolved) turbulence and the integrated (unresolved) contribution from higher altitudes, and also the median total turbulence strength for all altitudes.
Comparison of the seeing values (corresponding to the integrated turbulence strength for the whole atmosphere) for SL-SLODAR and DIMM for the night of 2009 April 9. The solid line without error bars shows the seeing from the Paranal DIMM site monitor and the higher line with error bars is the seeing as calculated by SL-SLODAR. The lower line with error bars is the seeing for the SL-SLODAR calculated by subtracting the first bin to give an estimate of the seeing from 6 m, to match the altitude of the