Download Atmospheric Turbulence in Astronomy

ATMOSPHERIC TURBULENCE
IN ASTRONOMY
Marc Sarazin
European Southern Observatory
List of Themes
How to find the ideal site...and keep it good?

Optical Propagation through Turbulence
– Mechanical and Thermal
– Index of Refraction
– Signature on ground based observations
– Correction methods
 Integral Monitoring Techniques
– Seeing Monitoring
– Scintillation Monitoring
 Profiling Techniques
– Microthermal Sensors
– Scintillation Ranging
 Modelling Techniques
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Modern Observatories
The VLT Observatory at Paranal, Chile
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Modern Observatories
The ESO-VLT Observatory at Paranal, Chile
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Why not bigger? 100m diameter
Effelsberg 100m radiotelescope
ESO OWL project
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0.6 arcsec
Atmospheric Turbulence
Big whorls have little whorls,
Which feed on their velocity;
Little whorls have smaller whorls,
And so on unto viscosity.
L. F. Richardson (1881-1953)
g d dz
Ri 
0.25
2
T du dz 
Vertical gradients of potential temperature and velocity
determine the conditions for the production of turbulent
kinetic energy
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Atmospheric Turbulence
In a turbulent flow, the kinetic energy decreases as the -5/3rd
power of the spatial frequency (Kolmogorov, 1941)
within the inertial domain ]l, L[
Outer (injection) Scale:
L
(L= 100m or more in the free atmosphere, less if pure convection)
Inner (dissipation) scale:
(l~0.1mm in a flow of velocity u=10m/s)
 3 
l   
 
1
4
= dissipation rate of turbulent kinetic energy (~u^3/L, m^2s^-3)
= kinetic viscosity (in air, 15E-6 m^2 s^-1)
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Atmospheric Turbulence
Structure function of the temperature fluctuations
(Tatarskii, 1961)
DT r    T (r )  T (r  r    C r
2
2
T
2
3
3D Spectrum (Tatarskii, 1971)
T ( f )  0.033CT2 f 11/ 3
within the inertial domain ]2/L,2 /l[
but L is now the size of the thermal eddies
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Atmospheric Turbulence
Index of refraction of air
77.6 10
n 1 
T
6
e

1  7.52 10   P  4810 T 


3
2
Assuming constant pressure and humidity, n varies only due to
temperature fluctuations, with the same structure function
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
6 P 
2
C   80 10
CT at   0.5 m
2 
T 

2
n
P,e (water vapor pressure) in mB, T in K, Cn2 in m-2/3
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Optical Propagation
The Signature of Atmospheric Turbulence
The Long Exposure Parameters
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Optical Propagation
The Signature of Atmospheric Turbulence
Seeing:
(radian, ^-0.2)
FWHM ( )  0.98

r0



 2 
2
Fried parameter: r0 ( )  0.423
 sec( )  Cn (h)dh
( meter, ^6/5)
  


0
2
3
5
Easy to remember: r0=10cmFWHM=1” in the visible (0.5m)
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Optical Propagation
The Signature of Atmospheric Turbulence
Seeing = FWHM
S= 0.7 à 2.2 um
FWHM=0.056 “

FWHM 
r0
Strehl Ratio
S=0.3 à 2.2 um
FWHM=0.065 “
I
S
I0
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Optical Propagation
The Signature of Atmospheric Turbulence
The Short Exposure Parameters
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Optical Propagation
The Signature of Atmospheric Turbulence
Shorter exposures allow to freeze some atmospheric effects
and reveal the spatial structure of the wavefront corrugation
Sequential 5s exposure images in the K band on the ESO 3.6m
telescope
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Optical Propagation
The Signature of Atmospheric Turbulence
A Speckle structure appears when the exposure is
shorter than the atmosphere coherence time  0
r0
 0  0.31
V
 2

53
  Cn ( h ) V (h ) dh 

V  0 


2
Cn ( h )dh



0



1ms exposure at the focus of a 4m diameter telescope
3
5
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Optical Propagation
The Signature of Atmospheric Turbulence
How large is the outer scale?
A dedicated
instrument, the
Generalized Seeing
Monitor
(GSM, built by the Dept. of
Astrophysics, Nice
University)
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Optical Propagation
The Signature of Atmospheric Turbulence
How large is the outer
scale?
Overall Statistics for the
Wavefront Outer Scale
At Paranal: a median value
of 22m was found.
Ref: F. Martin, R. Conan, A.
Tokovinin, A. Ziad, H. Trinquet, J.
Borgnino, A. Agabi and M. Sarazin;
Astron. Astrophys. Supplement,
v.144, p.39-44; June 2000
http://wwwastro.unice.fr/GSM/Missions.html
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Optical Propagation
The Signature of Atmospheric Turbulence
Structure function for the phase fluctuations:
 f
D  f   6.88  
 r0



5
3
The number of speckles in a pupil of
diameter D is (D/r0)^2
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Optical Propagation
The Signature of Atmospheric Turbulence
Why looking for the best seeing if turbulence can be
corrected?
Adaptive optics techniques are more complex (ND/r0^2),
less efficient (Strehlexp(r0/D^2))
and more expensive to implement
for bad seeing conditions
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Local Seeing
The many
ways to
destroy a
good
observing
environment
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Local Seeing
Flow Pattern Around a Building
Incoming neutral flow
should enter the
building to contribute
to flushing, the height
of the turbulent
ground layer
determines the
minimum height of
the apertures.
Thermal exchanges
with the ground by recirculation inside the
cavity zone is the
main source of
thermal turbulence in
the wake.
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Mirror Seeing
When a mirror is warmer that the air in an undisturbed enclosure, a convective
equilibrium (full cascade) is reached after 10-15mn. The limit on the convective cell
size is set by the mirror diameter
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LOCAL TURBULENCE
Mirror Seeing
The contribution to seeing due to turbulence over the mirror is given by:
The warm mirror seeing varies slowly with the thickness of the convective layer:
reduce height by 3 orders of magnitude to divide mirror seeing by 4, from 0.5 to 0.12
arcsec/K
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Mirror Seeing
The thickness of the
boundary layer over a
flat plate increases with
the distance to the edge
in the and with the
flow velocity.
When a mirror is warmer that the air in a flushed enclosure, the convective cells
cannot reach equilibrium. The flushing velocity must be large enough so as to
decrease significantly (down to 10-30cm) the thickness turbulence over the whole
diameter of the mirror.
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Thermal Emission Analysis
VLT East Landscape
*>14.9°C
14.0
12.0
10.0
8.0
6.0
4.0
Access Asphalt Road
 19 Feb. 1999
 0h56 Local Time
 Wind summit:
ENE, 7m/s
 Air Temp summit:
13.5C
2.0
0.0
*<-1.3°C
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Thermal Emission Analysis
VLT Unit Telescope
*>15.0°C
14.0
12.0
10.0
8.0
UT3 Enclosure
 19 Feb. 1999
 0h34 Local Time
 Wind summit:
ENE, 4m/s
 Air Temp summit:
13.8C
6.0
4.0
2.0
*<1.8°C
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Thermal Emission Analysis
VLT South Telescope Area
*>25.1°C
25.0
20.0
15.0
10.0
Heat Exchanger
 10 Oct. 1998
 11h34 Local Time
 Wind summit: North,
3m/s
 Air Temp summit:
12.8C
5.0
0.0
*<-1.6°C
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CONCLUSION
Until the 80’s, most astronomical
facilities were not properly
designed in order to preserve
site quality
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