ATMOSPHERIC TURBULENCE IN ASTRONOMY

ATMOSPHERIC TURBULENCE

IN ASTRONOMY

Marc Sarazin

European Southern Observatory

2Zanjan, July 2001

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

3Zanjan, July 2001

Modern Observatories

The VLT Observatory at Paranal, Chile

4Zanjan, July 2001

Modern Observatories

The ESO-VLT Observatory at Paranal, Chile

5Zanjan, July 2001

Why not bigger? 100m diameter

Effelsberg 100m radiotelescope

ESO OWL project

6

0.6

arcs

ec

7Zanjan, July 2001

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

25.02

dzdu

dzd

T

gRi

8Zanjan, July 2001

= dissipation rate of turbulent kinetic energy (~u^3/L, m^2s^-3)

= kinetic viscosity (in air, 15E-6 m^2 s^-1)

4

13

l


Inner (dissipation) scale:(l~0.1mm in a flow of velocity u=10m/s)

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)

9Zanjan, July 2001

Structure function of the temperature fluctuations

(Tatarskii, 1961)

3D Spectrum (Tatarskii, 1971)

3/112033.0)( fCf TT

3

222()( rCrrTrTrD TT


within the inertial domain ]2/L,2 /l[ but L is now the size of the thermal eddies

10Zanjan, July 2001

Index of refraction of air

Assuming constant pressure and humidity, n varies only due to temperature fluctuations, with the same structure function

matCT

PC Tn 5.01080 2

2

262

T

eP

Tn 48101052.71

106.771 23

6


P,e (water vapor pressure) in mB, T in K, Cn2 in m-2/3

11Zanjan, July 2001

Optical Propagation

The Signature of Atmospheric Turbulence

The Long Exposure Parameters

12Zanjan, July 2001

Fried parameter: ( meter, ^6/5)

Seeing: (radian, ^-0.2) 0

98.0)(r

FWHM

53

0

22

0 )()sec(2

423.0)(

dhhCr n

Optical Propagation


Easy to remember: r0=10cmFWHM=1” in the visible (0.5m)

13Zanjan, July 2001

Optical Propagation

The Signature of Atmospheric TurbulenceS= 0.7 à 2.2 um FWHM=0.056 “

S=0.3 à 2.2 um FWHM=0.065 “

0I

IS

0rFWHM

Seeing = FWHM

Strehl Ratio

14Zanjan, July 2001

Optical Propagation


The Short Exposure Parameters

15Zanjan, July 2001

Optical Propagation


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

16Zanjan, July 2001

Optical Propagation

The Signature of Atmospheric TurbulenceA Speckle structure appears when the exposure is

shorter than the atmosphere coherence time 0

1ms exposure at the focus of a 4m diameter telescope

V

r00 31.0

53

0

2

0

352

)(

)()(

dhhC

dhhVhC

V

n

n

17Zanjan, July 2001

Optical Propagation

The Signature of Atmospheric TurbulenceHow large is the outer scale?

A dedicated instrument, the Generalized Seeing Monitor

(GSM, built by the Dept. of Astrophysics, Nice University)

18Zanjan, July 2001

Optical Propagation

The Signature of Atmospheric TurbulenceHow 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/Missions.html

19Zanjan, July 2001

Structure function for the phase fluctuations:

3

5

0

88.6

r

ffD

The number of speckles in a pupil of diameter D is (D/r0)^2

Optical Propagation


20Zanjan, July 2001

Why looking for the best seeing if turbulence can be corrected?

Adaptive optics techniques are more complex (ND/r0^2),

less efficient (Strehlexp(r0/D^2))

and more expensive to implement

for bad seeing conditions

Optical Propagation


21Zanjan, July 2001

Local Seeing

The many ways to

destroy a good

observing environment

22Zanjan, July 2001

Local Seeing

Flow Pattern Around a BuildingIncoming 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.

23Zanjan, July 2001

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

24Zanjan, July 2001

LOCAL TURBULENCE

Mirror Seeing

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

The contribution to seeing due to turbulence over the mirror is given by:

25Zanjan, July 2001

Mirror Seeing

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.

The thickness of the boundary layer over a flat plate increases with the distance to the edge in the and

with the flow velocity.

26Zanjan, July 2001

Thermal Emission Analysis

VLT East Landscape

Access Asphalt Road 19 Feb. 1999 0h56 Local Time Wind summit:

ENE, 7m/s Air Temp summit:

13.5C

*>14.9°C

*<-1.3°C

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

27Zanjan, July 2001


VLT Unit Telescope

UT3 Enclosure 19 Feb. 1999 0h34 Local Time Wind summit:

ENE, 4m/s Air Temp summit:

13.8C

*>15.0°C

*<1.8°C

2.0

4.0

6.0

8.0

10.0

12.0

14.0

28Zanjan, July 2001


VLT South Telescope Area

Heat Exchanger 10 Oct. 1998 11h34 Local Time Wind summit: North,

3m/s Air Temp summit:

12.8C

*>25.1°C

*<-1.6°C

0.0

5.0

10.0

15.0

20.0

25.0

29Zanjan, July 2001

CONCLUSIONUntil the 80’s, most astronomical

facilities were not properly designed in order to preserve

site quality

ATMOSPHERIC TURBULENCE IN ASTRONOMY

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