Optical turbulence characterization at the SAAO Sutherland site

We present results from the first year of a campaign to characterize and monitor the optical turbulence profile at the South African Astronomical Observatory's Sutherland observing station in South Africa. A Multi Aperture Scintillation Sensor Differential Image Motion Monitor (MASS-DIMM) was commissioned in 2010 March to provide continuous monitoring of the seeing conditions. Over the first month of the campaign, a Slope Detection And Ranging (SLODAR) from Durham University was also installed, allowing an independent verification of the performance of the MASS-DIMM device. After the first year of data collection, the overall median seeing value is found to be 1.32 arcsec as measured at ground level. The ground layer which includes all layers below 1 km accounts for 84 per cent of the turbulence, while the free atmosphere above 1 km accounts for 16 per cent with a median value of 0.41 arcsec. The median isoplanatic angle value is 1.92 arcsec, which is similar to other major astronomical sites. The median coherence time, calculated from corrected MASS measurements, is 2.85 ms. The seeing conditions at the site do show a strong correlation with wind direction, with bad seeing conditions being associated with winds from the south-east.