An important aspect for any observatory is the regular monitoring of their site conditions. In addition to the weather, monitoring of the atmospheric turbulence also provides a critical aspect to the overall characterization of the site. Ground based observations are severely affected by turbulence in the atmosphere, where the turbulence will blur an image through a process referred to as seeing. This blurring can be corrected through the use of adaptive optics, but optimization of the system requires understanding at what altitudes the turbulence is occurring in the atmosphere.
Laure Catala along with researchers at South African Astronomical Observatory and Center for Advance Instrumentation at the University of Durham has recently reported on the results of our first year of site monitoring in Sutherland. The monitoring was carried out with three instruments: a Differential Image Motion Monitor (DIMM), a Multi Aperture Scintillation Sensor (MASS), and a SLOpe Detection And Ranging (SLODAR) instrument. DIMM measures the total seeing based on the motion of two images of the same star. The MASS measures the differential seeing at different altitudes from 500m to 16 km via the scintillation of a source in different apertures. The SLODAR provides high resolution maps (55-80m) of the turbulence in the lower atmosphere (up to 500m) using measurement of the wavefront of a double star. The MASS-DIMM is used as a combined instrument attached to a 12-inch Meade telescope and Tim Pickering has deployed a new camera, named timDIMM, to provide the DIMM measurements along with automating the system.
For the first year of observations, the median seeing at the site was measured to be 1.32″. Although this is worse than other major observatories, the contribution from the free atmosphere (above 1 km) was only 0.41″, which is as good as any other observatory in the world. The majority of the seeing at the site was due to layers below 1 km. The atmospheric profile for Sutherland is shown in the figure on the right and the large excess of turbulence in layers at 500m and below is easily seen. The profiles are show for the median conditions as well as the 25% and 75% quartiles of the seeing distribution.
One of the strongest correlation that was seen in the data was the relationship between wind direction and seeing conditions. The worse seeing conditions were typically seen when the wind was coming from the South East. These winds also usually bring in high humidity. Good seeing conditions were usually most often associated with Westerly winds although good conditions were observed at winds from all directions.
Fig 2. Strength of the seeing as a function wind direction and speed during good, median, and bad seeing conditions. The worse seeing conditions predominately occur during winds from the South East.
Although the seeing is not as good as some other astronomical sites, the dominate nature of the ground layer does indicate that the deployment of adaptive systems on the telescopes at the site could lead to significant improvements in their performance. Monitoring of the site is still continuing and the most recent measurements are available online. For more information, please see the AMAO group at SAAO.
- 2013MNRAS.436..590C Optical turbulence characterization at the SAAO Sutherland site Monthly Notices of the Royal Astronomical Society, Volume 436, Issue 1, p.590-603 Nov 2013