The image quality of ground-based solar telescopes depends on the amount of turbulence in the Earth’s atmosphere, which is strongest in layers close to the ground during daytime. Local optical turbulence affects the performance of adaptive optical systems and reduces the spatial resolution of solar observations. Increased turbulence which is caused by solar irradiation of the infrastructure close to the telescope and obstructions to a free airflow are major concerns, but difficult to detect and to monitor. We have conducted measurements of optical turbulence at the GREGOR solar telescope (Teide Observatory, Tenerife, Canary Islands) to assess quantitatively the influence of the infrastructure on the image quality.
The strength of optical turbulence is determined by the structure function parameter of the refractive index Cn2. We have measured the temporal behavior of the free flow optical turbulence about 30m above ground using a laser scintillometer between the towers of the Vacuum Tower Telescope and of GREOR. Local measurements of Cn2 were taken on the observing platform of GREGOR using three ultrasonic anemometers. Two anemometers are located at the north and south ends of the telescope building, a third one was be placed close to the telescope main mirror cell. Air temperature, pressure, humidity as well as wind speed and direction were measured along with refractive index measurements. An image quality indicator based on an estimate of the Fried parameter r0 was recorded whenever the adaptive optics system GAOS at the GREGOR telescope was operating. Recordings of the net solar radiation were obtained from the GONG experiment which is located a few 100 m from GREGOR. All data were taken between May 2015 and March 2016.
We investigate the relation between optical turbulence, solar irradiance and meteorological parameters. Under almost all conditions, optical turbulence nearby the telescope is stronger than free flow turbulence. We note significant dependencies of the strength and the horizontal gradient of Cn2 on wind direction. The moderate influence on image quality indicates that there is only a thin layer with strong turbulence just above the telescope.