atmospheric turbulence in astronomy l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
ATMOSPHERIC TURBULENCE IN ASTRONOMY PowerPoint Presentation
Download Presentation
ATMOSPHERIC TURBULENCE IN ASTRONOMY

Loading in 2 Seconds...

play fullscreen
1 / 29

ATMOSPHERIC TURBULENCE IN ASTRONOMY - PowerPoint PPT Presentation


  • 159 Views
  • Uploaded on

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

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'ATMOSPHERIC TURBULENCE IN ASTRONOMY' - kagami


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
atmospheric turbulence in astronomy

ATMOSPHERIC TURBULENCEIN ASTRONOMY

Marc Sarazin

European Southern Observatory

list of themes how to find the ideal site and keep it good
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
modern observatories
Modern Observatories

The VLT Observatory at Paranal, Chile

modern observatories4
Modern Observatories

The ESO-VLT Observatory at Paranal, Chile

why not bigger 100m diameter
Why not bigger? 100m diameter

Effelsberg 100m radiotelescope

ESO OWL project

atmospheric turbulence
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

atmospheric turbulence8
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)

atmospheric turbulence9
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

atmospheric turbulence10
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

optical propagation the signature of atmospheric turbulence12
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)

optical propagation the signature of atmospheric turbulence13
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

optical propagation the signature of atmospheric turbulence15
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

optical propagation the signature of atmospheric turbulence16
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

optical propagation the signature of atmospheric turbulence17
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)

optical propagation the signature of atmospheric turbulence18
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

optical propagation the signature of atmospheric turbulence19
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

optical propagation the signature of atmospheric turbulence20
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

local seeing
Local Seeing

The many ways to destroy a good observing environment

local seeing flow pattern around a building
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.

mirror seeing
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

local turbulence mirror seeing
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:

mirror seeing25
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.

thermal emission analysis vlt east landscape
Thermal Emission AnalysisVLT East Landscape

Access Asphalt Road

  • 19 Feb. 1999
  • 0h56 Local Time
  • Wind summit: ENE, 7m/s
  • Air Temp summit: 13.5C
thermal emission analysis vlt unit telescope
Thermal Emission AnalysisVLT Unit Telescope

UT3 Enclosure

  • 19 Feb. 1999
  • 0h34 Local Time
  • Wind summit: ENE, 4m/s
  • Air Temp summit: 13.8C
thermal emission analysis vlt south telescope area
Thermal Emission AnalysisVLT South Telescope Area

Heat Exchanger

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

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