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The Use of Modeling for Site Surveys
Marc Sarazin
European Southern Observatory
July 2001
Zanjan, Iran
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The 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
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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The new parameters for site surveys
Pacific decadal
oscillation, El Nino,
global warming?
Surveying a site
for a few years
long is not safe:
climate change
must be taken into
account
July 2001
Zanjan, Iran
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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
http://op.ph.ic.ac.uk/research/
July 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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The new tools for site surveys
Peak Ground
Acceleration up to
5m/s2:
10% probability
of exceedance in
50 years
Source: http://www.seismo.ethz.ch/GSHAP/
July 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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The new tools for site surveys
Global Models
The relative humidity is
hard to model
accurately
ECMWF analysis over
northern Chile
July 2001
Zanjan, Iran
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The 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
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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 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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The new tools for site surveys
Vertical cut of the
turbulence Cn2 along
the E-W direction
centered on Paranal
 
2
3
CT  L 
 z
4
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2
First attempts to model the seeing at
ESO Paranal Observatory using
Meteo-France Meso-Nh model
(Masciadri et al., 1997)
July 2001
Zanjan, Iran
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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 2001
Zanjan, Iran
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The 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
July 2001
Zanjan, Iran
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