July 2001 Zanjan, Iran 1
The Use of Modeling for Site SurveysThe Use of Modeling for Site Surveys
Marc SarazinEuropean Southern Observatory
July 2001 Zanjan, Iran 2
The Ideal Sites of the PastThe Ideal Sites of the Past
• In the 40’s– On the national territory– Close to a major educational center– Cloud free area
• In the 60’s– Cloud free area ++– Good seeing
• In the 80’s– Cloud free area +++ (higher operation cost)– Good seeing ++ (better optics)– Low precipitable water vapor (IR)
July 2001 Zanjan, Iran 3
The Ideal Sites of the PresentThe 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 2001 Zanjan, Iran 4
The Ideal Sites of the PresentThe 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 2001 Zanjan, Iran 5
The Ideal Sites of the (near) FutureThe 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 2001 Zanjan, Iran 6
The Ideal Sites of the (near) FutureThe 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 2001 Zanjan, Iran 7
The new parameters for site surveysThe 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 2001 Zanjan, Iran 8
The new parameters for Site SurveysThe 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 2001 Zanjan, Iran 9
The new parameters for site surveysThe 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 2001 Zanjan, Iran 10
The new parameters for site surveysThe 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 2001 Zanjan, Iran 11
The new parameters for site surveysThe 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 Collegehttp://op.ph.ic.ac.uk/research/
July 2001 Zanjan, Iran 12
The Ideal Sites of the FutureThe Ideal Sites of the Future
Additional Requirements
• Low Seismic ActivityAn ELT cannot be made stiff enough to survive earthquakes
• Low Wind at ground levelAn ELT is more sensitive to wind shake during tracking
• Crystal Clear SkiesLaser guide star efficiency depends on atmospheric
extinction
July 2001 Zanjan, Iran 13
The new tools for site surveysThe 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 2001 Zanjan, Iran 14
The new tools for site surveysThe new tools for site surveys
Source: http://www.seismo.ethz.ch/GSHAP/
Peak Ground Acceleration up to
5m/s2:10% probability of exceedance in
50 years
July 2001 Zanjan, Iran 15
The new tools for site surveysThe 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 2001 Zanjan, Iran 16
The new tools for site surveysThe 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 2001 Zanjan, Iran 17
The new tools for site surveysThe new tools for site surveys
Global Models
The relative humidity is hard to model
accurately
ECMWF analysis over northern Chile
July 2001 Zanjan, Iran 18
The new tools for site surveysThe new tools for site surveys
Global Models
Seasonal Variations are accurately represented
ECMWF analysis over the VLT Observatory Paranal in 1993
July 2001 Zanjan, Iran 19
The new tools for site surveysThe 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 2001 Zanjan, Iran 20
The new tools for site surveysThe 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 2001 Zanjan, Iran 21
The new tools for site surveysThe new tools for site surveys
Vertical cut of the turbulence Cn2 along the E-W direction centered on Paranal
2
342
z
LCT
First attempts to model the seeing at ESO Paranal Observatory using Meteo-France Meso-Nh model(Masciadri et al., 1997)
July 2001 Zanjan, Iran 22
The new tools for site surveysThe 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 2001 Zanjan, Iran 23
The new tools for site surveysThe new tools for site surveys
MM5, a mesoscale model is available as
freeware. It is used at the
Mauna Kea Weather Center (http://hokukea.soest.hawaii.edu/forecast/mko/)
to produce vertical profiles of the turbulence.
Comparison of MM5 profiles above Mauna Kea Observatory with in situ SCIDAR observations