ATMOSPHERIC TURBULENCE IN ASTRONOMY Marc Sarazin European Southern

ATMOSPHERIC TURBULENCE IN ASTRONOMY Marc Sarazin European Southern Observatory. Zanjan, 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 l Integral Monitoring Techniques – Seeing Monitoring – Scintillation Monitoring l Profiling Techniques – Microthermal Sensors – Scintillation Ranging l Modelling Techniques l Zanjan, July 2001 2

Modern Observatories The VLT Observatory at Paranal, Chile Zanjan, July 2001 3

Modern Observatories The ESO-VLT Observatory at Paranal, Chile Zanjan, July 2001 4

Why not bigger? 100 m diameter Effelsberg 100 m radiotelescope ESO OWL project Zanjan, July 2001 5

6 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) Vertical gradients of potential temperature and velocity determine the conditions for the production of turbulent kinetic energy Zanjan, July 2001 7

Atmospheric Turbulence In a turbulent flow, the kinetic energy decreases as the -5/3 rd power of the spatial frequency (Kolmogorov, 1941) within the inertial domain ]l, L[ Outer (injection) Scale: (L= 100 m or more in the free atmosphere, less if pure convection) Inner (dissipation) scale: (l~0. 1 mm in a flow of velocity u=10 m/s) = dissipation rate of turbulent kinetic energy (~u^3/L, m^2 s^-3) = kinetic viscosity (in air, 15 E-6 m^2 s^-1) Zanjan, July 2001 8

Atmospheric Turbulence Structure function of the temperature fluctuations (Tatarskii, 1961) 3 D Spectrum (Tatarskii, 1971) within the inertial domain ]2 /L, 2 /l[ but L is now the size of thermal eddies Zanjan, July 2001 9

$Atmospheric Turbulence Index of refraction of air Assuming constant pressure and humidity, n varies$

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 m. B, T in K, Cn 2 in m-2/3 Zanjan, July 2001 10

Optical Propagation The Signature of Atmospheric Turbulence The Long Exposure Parameters Zanjan, July 2001 11

Optical Propagation The Signature of Atmospheric Turbulence Seeing: (radian, ^-0. 2) Fried parameter: ( meter, ^6/5) Easy to remember: r 0=10 cm FWHM=1” in the visible (0. 5 m) Zanjan, July 2001 12

Optical Propagation The Signature of Atmospheric Turbulence Seeing = FWHM Strehl Ratio Zanjan, July 2001 S= 0. 7 à 2. 2 um FWHM=0. 056 “ S=0. 3 à 2. 2 um FWHM=0. 065 “ 13

Optical Propagation The Signature of Atmospheric Turbulence The Short Exposure Parameters Zanjan, July 2001 14

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 5 s exposure images in the K band on the ESO 3. 6 m telescope Zanjan, July 2001 15

Optical Propagation The Signature of Atmospheric Turbulence A Speckle structure appears when the exposure is shorter than the atmosphere coherence time 0 1 ms exposure at the focus of a 4 m diameter telescope Zanjan, July 2001 16

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) Zanjan, July 2001 17

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 22 m 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 Zanjan, July 2001 18

Optical Propagation The Signature of Atmospheric Turbulence Structure function for the phase fluctuations: The number of speckles in a pupil of diameter D is (D/r 0)^2 Zanjan, July 2001 19

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/r 0^2), less efficient (Strehl exp(r 0/D^2)) and more expensive to implement for bad seeing conditions Zanjan, July 2001 20

Local Seeing The many ways to destroy a good observing environment Zanjan, July 2001 21

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. Zanjan, July 2001 22

Mirror Seeing When a mirror is warmer that the air in an undisturbed enclosure, a convective equilibrium (full cascade) is reached after 10 -15 mn. The limit on the convective cell size is set by the mirror diameter Zanjan, July 2001 23

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 Zanjan, July 2001 24

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 -30 cm) the thickness turbulence over the whole diameter of the mirror. Zanjan, July 2001 25

Thermal Emission Analysis VLT East Landscape Access Asphalt Road l 19 Feb. 1999 l 0 h 56 Local Time l Wind summit: ENE, 7 m/s l Air Temp summit: 13. 5 C Zanjan, July 2001 26

Thermal Emission Analysis VLT Unit Telescope UT 3 Enclosure l 19 Feb. 1999 l 0 h 34 Local Time l Wind summit: ENE, 4 m/s l Air Temp summit: 13. 8 C Zanjan, July 2001 27

Thermal Emission Analysis VLT South Telescope Area Heat Exchanger l 10 Oct. 1998 l 11 h 34 Local Time l Wind summit: North, 3 m/s l Air Temp summit: 12. 8 C Zanjan, July 2001 28

CONCLUSION Until the 80’s, most astronomical facilities were not properly designed in order to preserve site quality Zanjan, July 2001 29