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: (l~0.1mm in a flow of velocity u=10m/s) = dissipation rate of turbulent kinetic energy (~u^3/L, m^2s^-3) = kinetic viscosity (in air, 15E-6 m^2 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: r0=10cmFWHM=1” in the visible (0.5m)

13. Optical PropagationThe Signature of Atmospheric Turbulence S= 0.7 à 2.2 um FWHM=0.056 “ S=0.3 à 2.2 um FWHM=0.065 “ 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/Missions.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/r0^2), less efficient (Strehlexp(r0/D^2)) 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 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:

25. 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.

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