July 2001Zanjan, Iran1 The Use of Modeling for Site Surveys Marc Sarazin European Southern Observatory

July 2001Zanjan, Iran2 The Ideal Sites of the Past In the 40s –On the national territory –Close to a major educational center –Cloud free area In the 60s –Cloud free area ++ –Good seeing In the 80s –Cloud free area +++ (higher operation cost) –Good seeing ++ (better optics) –Low precipitable water vapor (IR)

July 2001Zanjan, Iran3 The Ideal Sites of the Present Common Trends –Telescopes and instruments are seeing limited –Observatories operate in flexible scheduling modes –Observatories are equipped with seeing monitoring facilities –Observatories are equipped with cloud monitoring facilities –Observatories receive meteorological forecasts Basic Site Requirements –Cloud free area (spectroscopic sky > 80% of the time) –Good seeing (median <1 at 0.5 micron)

July 2001Zanjan, Iran4 The Ideal Sites of the Present Requirements by Observing Technique Photometry and Spectroscopy –Photometric sky (stable and isotropic extinction coefficients) –Low light pollution (evolution controlled by state laws) –Good seeing Classical Imaging –Good seeing ++ and slowly varying Imaging assisted by classical NGS adaptive optics –Good seeing +++ (nr. actuators) and low wavefront velocity –Low high altitude turbulence (large isoplanatic angle)

July 2001Zanjan, Iran5 The Ideal Sites of the (near) Future Requirements by Observing Technique Observations assisted by classical LGS adaptive optics –Same as above –Photometric sky (photon return of sodium laser guide star) Observations assisted by multi-conjugate LGS adaptive optics –Same as above ++ –The increase in the corrected field of view scales as the isoplanatic angle

July 2001Zanjan, Iran6 The Ideal Sites of the (near) Future Have all good sites been discovered? Can a continental site provide excellent seeing conditions? Comparison of the seeing distribution at Maidanak (Uzbekistan) with conditions at ESO Observatories (Ehgamberdiev et al., A&A suppl, Aug. 2000)

July 2001Zanjan, Iran7 The new parameters for site surveys The wind velocity at the tropopause is a good indicator of the global isoplanatic angle ESO-Paranal DIMM isoplanatic angle compared to ECWMF 200mB wind during 9 months. (correction for exposure time averaging of scintillation after A. Tokovinin, ESO internal memo, July 2000)

July 2001Zanjan, Iran8 The new parameters for Site Surveys The performance of AO systems is ultimately determined by the real time vertical distribution of the turbulence Gemini Web site: Scidar profiles at Mauna Kea (F. Roddier et al. SPIE Vol 1236, 485, 1990)

July 2001Zanjan, Iran9 The new parameters for site surveys Climate Change is a reality also for the astronomical seeing Oscillations with periods from a few months to a few decades

July 2001Zanjan, Iran10 The new parameters for site surveys Surveying a site for a few years long is not safe: climate change must be taken into account Pacific decadal oscillation, El Nino, global warming?

July 2001Zanjan, Iran11 The new parameters for site surveys The apparent magnitude of Sodium Laser Guide Stars is naturally variable in time. It also depends on the atmospheric transmission (A. Quirrenbach, session VII) Variation of the sodium profile through the night of August 31, 2000 at La Palma. Source: Dainty et al., Imperial College

July 2001Zanjan, Iran12 The Ideal Sites of the Future Additional Requirements Low Seismic Activity An ELT cannot be made stiff enough to survive earthquakes Low Wind at ground level An ELT is more sensitive to wind shake during tracking Crystal Clear Skies Laser guide star efficiency depends on atmospheric extinction

July 2001Zanjan, Iran13 The new tools for site surveys Coarse Cloudiness Maps are readily available. A few km resolution is accessible (A. Erasmus, session V) Source: Surface Meteorology and Solar Energy Data Set (SSE) of NASA's Earth Science Enterprise Program (1x1 degree grid).

July 2001Zanjan, Iran14 The new tools for site surveys Source: Peak Ground Acceleration up to 5m/s2: 10% probability of exceedance in 50 years

July 2001Zanjan, Iran15 The new tools for site surveys Global Models The wind at 700mb is not accurately represented over mountains ECMWF analysis over northern Chile, 60km (0.5 degree) grid.

July 2001Zanjan, Iran16 The new tools for site surveys Global Models The wind at 500mb is accurately represented ECMWF analysis over northern Chile, 60km (0.5 degree) grid.

July 2001Zanjan, Iran17 The new tools for site surveys Global Models The relative humidity is hard to model accurately ECMWF analysis over northern Chile

July 2001Zanjan, Iran18 The new tools for site surveys Global Models Seasonal Variations are accurately represented ECMWF analysis over the VLT Observatory Paranal in 1993

July 2001Zanjan, Iran19 The new tools for site surveys Mesoscale models reveal local orographic effects on the flow The terrain model, covers 120x120 km with 500 m resolution. The initialization is done at one corner with the grid point values of a global model First attempts to model the seeing at ESO Paranal Observatory using Meteo-France Meso-Nh model (Masciadri et al., 1997)

July 2001Zanjan, Iran20 The new tools for site surveys Non-hydrostatic models can account for local orographic effects on the potential temperature because they allow vertical motion First attempts to model the seeing at ESO Paranal Observatory using Meteo-France Meso-Nh model (Masciadri et al., 1997)

July 2001Zanjan, Iran21 The new tools for site surveys Vertical cut of the turbulence Cn2 along the E-W direction centered on Paranal First attempts to model the seeing at ESO Paranal Observatory using Meteo-France Meso-Nh model (Masciadri et al., 1997)

July 2001Zanjan, Iran22 The new tools for site surveys First attempts to model the seeing at ESO Paranal Observatory using Meteo-France Meso-Nh model (Masciadri et al., 1997) Map of the integrated seeing in an area centered on Paranal

July 2001Zanjan, Iran23 The new tools for site surveys MM5, a mesoscale model is available as freeware. It is used at the Mauna Kea Weather Center ( to produce vertical profiles of the turbulence. Comparison of MM5 profiles above Mauna Kea Observatory with in situ SCIDAR observations