Astronomical seeing conditions as determined by turbulence modelling and optical measurement

Abstract

Modern space geodetic techniques are required to provide measurements of millimetre-level accuracy. A new fundamental space geodetic observatory for South Africa has been proposed. It will house state-of-the-art equipment in a location that guarantees optimal scientific output. Lunar Laser Ranging (LLR) is one of the space geodetic techniques to be hosted on-site. This technique requires optical (or so-called astronomical) seeing conditions, which allow for the propagation of a laser beam through the atmosphere without excessive beam degradation. The seeing must be at ~ 1 arc second resolution level for LLR to deliver usable ranging data. To establish the LLR system at the most suitable site and most suitable on-site location, site characterisation should include a description of the optical seeing conditions. Atmospheric turbulence in the planetary boundary layer (PBL) contributes significantly to the degradation of optical seeing quality. To evaluate astronomical seeing conditions at a site, a two-sided approach is considered – on the one hand, the use of a turbulence-resolving numerical model, the Large Eddy Simulation NERSC (Nansen Environmental and Remote Sensing Centre) Improved Code (LESNIC) to simulate seeing results, while, on the other hand, obtaining quantitative seeing measurements with a seeing monitor that has been developed in-house.