Seeing, Wind and Outer Scale Effects on Image Quality at the Magellan Telescopes

TL;DR

This study analyzes image quality at Magellan telescopes, revealing that wind speed and outer scale effects significantly influence image quality, with telescopes outperforming DIMM measurements especially during strong winds, and identifying a negative bias in DIMM data.

Contribution

First comprehensive analysis linking wind, outer scale, and image quality at large telescopes, including detection of DIMM bias and quantification of outer scale effects.

Findings

Magellan telescopes outperform DIMM seeing measurements.

Wind speed strongly affects image quality, especially during strong winds.

Negative bias in DIMM data linked to optical aberrations and upper atmospheric turbulence.

Abstract

We present an analysis of the science image quality obtained on the twin 6.5 metre Magellan telescopes over a 1.5 year period, using images of ~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. Excluding data taken during strong wind conditions (>10 m/s), we find that the Magellan telescopes still significantly outperform the DIMM seeing, and we estimate the site to have L_0 ~ 25 m on average. We also report on the first detection of a negative bias in DIMM data. This is found to occur, as predicted, when the DIMM is affected by certain optical aberrations and the turbulence profile is dominated by the upper layers of the atmosphere.