Local thermal seeing modeling validation through observatory measurements

Abstract

Dome and mirror seeing are critical effects influencing the optical performance of ground-based telescopes. Computational Fluid Dynamics (CFD) can be used to obtain the refractive index field along a given optical path and calculate the corresponding image quality utilizing optical modeling tools. This procedure is validated using measurements from the Keck II and CFHT telescopes. CFD models of Keck II and CFHT observatories on the Mauna Kea summit have been developed. The detailed models resolve all components that can influence the flow pattern through turbulence generation or heat release. Unsteady simulations generate time records of velocity and temperature fields from which the refractive index field at a given wavelength and turbulence parameters are obtained. At Keck II the C_n² and l₀ (inner scale of turbulence) were monitored along a 63m path sensitive primarily to turbulence around the top ring of the telescope tube. For validation, these parameters were derived from temperature and velocity fluctuations obtained from CFD simulations. At CFHT dome seeing has been inferred from their database that includes telescope delivered Image Quality (IQ). For this case CFD simulations were run for specific orientations of the telescope respect to incoming wind, wind speeds and outside air temperature. For validation, temperature fluctuations along the optical beam from the CFD are turned to refractive index variations and corresponding Optical Path Differences (OPD) then to Point Spread Functions (PSF) that are ultimately compared to the record of IQ.