july 2001zanjan, iran1 the use of modeling for site surveys marc sarazin european southern...

July 2001 Zanjan, Iran 1

The Use of Modeling for Site SurveysThe Use of Modeling for Site Surveys

Marc SarazinEuropean Southern Observatory


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)


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)


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)


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


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)


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)


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)



Climate Change is a reality also for the astronomical seeing

Oscillations with periods from a few

months to a few decades



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?



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/


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


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).



Source: http://www.seismo.ethz.ch/GSHAP/

Peak Ground Acceleration up to

5m/s2:10% probability of exceedance in

50 years



Global Models

The wind at 700mb is not accurately

represented over mountains

ECMWF analysis over northern Chile, 60km (0.5 degree) grid.



Global Models

The wind at 500mb is accurately represented

ECMWF analysis over northern Chile, 60km (0.5 degree) grid.



Global Models

The relative humidity is hard to model

accurately

ECMWF analysis over northern Chile



Global Models

Seasonal Variations are accurately represented

ECMWF analysis over the VLT Observatory Paranal in 1993



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)



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)



Vertical cut of the turbulence Cn2 along the E-W direction centered on Paranal

2

342

z

LCT





Map of the integrated seeing in an area centered on Paranal



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 2001zanjan, iran1 the use of modeling for site surveys marc sarazin european southern...

Documents

s cloud free area good

limited observatories

free area spectroscopic

ideal sites

time good

use of modeling

national territory close