ATMOSPHERIC TURBULENCE IN ASTRONOMY

1
ATMOSPHERIC TURBULENCEIN ASTRONOMY

• Marc Sarazin
• European Southern Observatory

2
List of ThemesHow 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

3
Modern Observatories
The VLT Observatory at Paranal, Chile
4
Modern Observatories
The ESO-VLT Observatory at Paranal, Chile
5
Why not bigger? 100m diameter
Effelsberg 100m radiotelescope
ESO OWL project
6
0.6 arcsec
7
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)
Vertical gradients of potential temperature and
velocity determine the conditions for the
production of turbulent kinetic energy
8
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 100m or more in the free atmosphere, less if
pure convection)
Inner (dissipation) scale (l0.1mm in a flow of
velocity u10m/s)
? dissipation rate of turbulent kinetic energy
(u3/L, m2s-3) ? kinetic viscosity (in air,
15E-6 m2 s-1)
9
Atmospheric Turbulence
Structure function of the temperature
fluctuations (Tatarskii, 1961)
3D Spectrum (Tatarskii, 1971)
within the inertial domain 2?/L,2 ?/l but L
is now the size of the thermal eddies
10
Atmospheric Turbulence
Index of refraction of air
Assuming constant pressure and humidity, n varies
only due to temperature fluctuations, with the
same structure function
P,e (water vapor pressure) in mB, T in K, Cn2 in
m-2/3
11
Optical PropagationThe Signature of
Atmospheric Turbulence
The Long Exposure Parameters
12
Optical PropagationThe Signature of
Atmospheric Turbulence
Seeing (radian, ??-0.2)
Fried parameter ( meter, ??6/5)
Easy to remember r010cm?FWHM1 in the visible
(0.5?m)
13
Optical PropagationThe Signature of Atmospheric
Turbulence
Seeing FWHM
Strehl Ratio
14
Optical PropagationThe Signature of
Atmospheric Turbulence
The Short Exposure Parameters
15
Optical PropagationThe 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
16
Optical PropagationThe Signature of
Atmospheric Turbulence

• A Speckle structure appears when the exposure is
  shorter than the atmosphere coherence time ? 0

1ms exposure at the focus of a 4m diameter
telescope
17
Optical PropagationThe 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)
18
Optical PropagationThe 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//www-astro.unice.fr/GSM/M
issions.html
19
Optical PropagationThe Signature of Atmospheric
Turbulence
Structure function for the phase fluctuations
The number of speckles in a pupil of diameter D
is (D/r0)2
20
Optical PropagationThe Signature of Atmospheric
Turbulence
Why looking for the best seeing if turbulence can
be corrected? Adaptive optics techniques are more
complex (N?D/r02), less efficient
(Strehl?exp(r0/D2)) and more expensive to
implement for bad seeing conditions
21
Local Seeing

• The many ways to destroy a good observing
  environment

22
Local SeeingFlow 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
  re-circulation inside the cavity zone is the main
  source of thermal turbulence in the wake.

23
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

24
LOCAL TURBULENCEMirror 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

25
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.

26
Thermal Emission AnalysisVLT East Landscape

• Access Asphalt Road
• 19 Feb. 1999
• 0h56 Local Time
• Wind summit ENE, 7m/s
• Air Temp summit 13.5C

27
Thermal Emission AnalysisVLT Unit Telescope

• UT3 Enclosure
• 19 Feb. 1999
• 0h34 Local Time
• Wind summit ENE, 4m/s
• Air Temp summit 13.8C

28
Thermal Emission AnalysisVLT South Telescope
Area

• Heat Exchanger
• 10 Oct. 1998
• 11h34 Local Time
• Wind summit North, 3m/s
• Air Temp summit 12.8C

29
CONCLUSION

• Until the 80s, most astronomical facilities were
  not properly designed in order to preserve site
  quality