Seeing, wind, and outer scale effects on image quality at the magellan telescopes

We present an analysis of the science image quality obtained on the twin 6.5 m Magellan telescopes over a 1.5 yr period, using images of a??10 5 stars. We find that the telescopes generally obtain significantly better image quality than the DIMM-measured seeing. This is qualitatively consistent with expectations for large telescopes, where the wavefront outer scale of the turbulence spectrum plays a significant role. However, the dominant effect is found to be wind speed, with Magellan outperforming the DIMMs most markedly when the wind is strongest.