Astroclimatology of ESO sites
Atmospheric Parameters Relevant for Adaptive Optics
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Atmosphere Knowledge and Adaptive Optics for 8 to 100 m telescopes
The statistics for the years 1999-2003 are obtained from
1mn resolution scintillation measurements built from the statistics on several hundreds individual 5ms
DIMM exposures taken in photometric sky. the scintillation data is
then passed through a 10mn gaussian filter and converted into isoplanatic angle following the method proposed
by Loos and Hogge (Appl. Optics 18, 15, 1979). A correction for finite exposure time is applied
following the algorithm computed by A. Tokovinin (Sarazin & Tokovinin, 2001).
The horizontal velocity of the
scintillation pattern is taken as half the wind speed at 200mB (hypothesis supported by an analysis of
dual exposure GSM measurements made at Paranal in December 1998). The average yearly exposure time bias
correction was about 30% in 2000 at Paranal. The accuracy of this method is of the order of 10% rms on
the isoplanatic angle.
Check for last night measurements at
Paranal and La Silla.
The statistics for the years 1999-2003 are obtained from the 10mn averaged DIMM seeing measurements made
in photometric sky along tau0=0.31 R0/V0 where V0 is
taken as 2/5th of the wind speed at 200mB (supported by balloon flight results at Paranal in 1992 and
Pachon in 1998, Sarazin & Tokovinin, 2001). The
accuracy of this method is estimated to about 20% rms.
Check for last night measurements at
Paranal and La Silla. Real time measurements are available at Paranal
and at La Silla (internal
access only) based on 1mn DIMM seeing measurements.
Constants at Paranal during VLTI VINCI & Siderostats Commissioning (pdf int. report). Tau0 time history
since VLTI first light up to August 2002, and the slowest wavefront of this period. First comparisons with
atmospheric phase fluctuation power spectra obtained with VLTI show a very good correlation (see discussion in the paper
SPIE48-35 by Di Folco et al.)
Wind at 200mB
The statistics for years 1999, 2000, 2001, 2002,
2003, 2004 and 2005
are computed from the 00hUT ECMWF
analysis and subsequent 06, 12, and 18hUT forecasts after linear rebinning. The ECMWF analyses
and forecasts from 1989 to 1993 were compared to Antofagasta radiosoundings at
were found out to be accurate within +/-20% (analyses) and +/-40% (forecasts) in more than
70% of the cases in the 100-350mB altitude range. Check for the
Dependency of Isoplanatic Angle
As illustrated in this zoom of the
Night Aug31 to Sep01, the seeing and the isoplanatic
angle are not directly correlated because of the variations in the vertical
profile of the turbulence. In this example, the mean altitude (hbar) of the turbulence
shows a wavelike structure which strongly influences the isoplanatic angle,
while being poorly correlated with the integral seeing.
On the long term however (1999-2003) a relation (one curve per year) appears between seeing and hbar because
the very bad seeeing comes generally from the first kilometer and the very good occurs often when only the high layers are
turbulent. The global statistics of hbar 1999-2003 show an average optimal conjugation altitude of
about 3.5km above Paranal, similar to what has been found above Mauna Kea by Flicker and Rigaut (PASP 114:
Dependency of Isoplanatic Angle and
The isoplanatic angle is relatively dependent on the
200mB wind speed which governs high
altitude turbulence. As a consequence, the largest isoplanatic angle values are
obtained for the smallest 200mB wind speed; they are however not systematically linked to
large coherence times which can be very small during bad seeing, regardless of the velocity.
On the other hand, the smallest
isoplanatic angles occurr more often with high 200mB wind speeds, which invariably
correspond to low coherence times (although very often with good seeing!).
-PARSCA (Paranal Seeing Campaigns, 1992 and 1993) results:
Balloon borne microthermal measurements led to estimate
the average vertical profile of the turbulenceas well as
isoplanatic angle and coherence time
1997 at La Silla (outer
scale), 1998 at Paranal
(outer scale and
layer turbulence) and 2007 at Paranal (outer
scale and surface layer).
-Balloons launching in 1998 at Cerro
Pachon, the site of Gemini South.
-Remote Optical Profilers:
MASS@Tololo started in April 2002
SCIDAR@LaSilla started in July 2002
MASS@Mauna Kea in October 2002
MASS@Pachon in January 2003
MASS@Paranal in March and September 2003.
MASS-LITE@LaSilla started in December 2003.
Using PARSCA data along with the output of global models such as
(European Center for Medium-Range Weather Forecasts)
1993 vertical wind profiles
Direction at Paranal, we may estimate the average
wavefront temporal characteristics (ps file).
More ambitious is the 3D modelling of the turbulence
in the atmosphere, using mesoscale non-hydrostatic models Meso-Nh or