Upload
chick
View
30
Download
0
Tags:
Embed Size (px)
DESCRIPTION
Atmospheric Profilers. 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 - PowerPoint PPT Presentation
Citation preview
July 2001 Zanjan, Iran 1
Atmospheric Profilers
Marc Sarazin(European Southern Observatory)
July 2001 Zanjan, Iran 2
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
Outline
• Why do we need turbulence profiles?
• Microthermal sensing
• Sound back scattering
• Scintillation
• Mesoscale modeling
July 2001 Zanjan, Iran 4
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
Atmospheric Turbulence
P,e (water vapor pressure) in mB, T in K, Cn2 in m-2/3
July 2001 Zanjan, Iran 5
Turbulence Profilers
Ref: PARCSA Campaign, Univ. of Nice, 1992-1993
The various methods for generating atmospheric turbulence profiles
•Full line: Mauna Kea Model (Olivier, 94)
•Dots: SCIDAR, Paranal
•Dashes: Baloon borne microthermal sensors
July 2001 Zanjan, Iran 6
Balloon Borne Profilers
Two Microthermal sensors, 1m apart are attached far below the load of a standard meteorological
radiosondesPros:
•Provide Temperature, Humidity and Wind
•High vertical resolution (5m)
Cons:
•An ascent last one hour or more
•The balloon drifts horizontally (30 to 100 km)
•Expensive technique (1kUS$/flight)
3
222()( rCrrTrT T
The rms of the differential temperature fluctuations over a few seconds is computed onboard and transmitted to the ground
July 2001 Zanjan, Iran 7
Doppler SODAR Profiler
Source: http://www.remtechinc.com/sodar.htm
Sound Detection And RangingMonitoring the backscattered acoustic
energy from the atmospheric layers
2
2310075.0T
CT
The acoustic backscattering cross-section is a function of acoustic wavelength, absolute temperature and temperature structure coefficient
July 2001 Zanjan, Iran 8
Doppler SODAR Profiler
Pros:
•Provides Wind profile (design goal)
•Good vertical resolution (30m)
•Fully automated
Cons:
•Only relative Cn2 measurements: no absolute calibration (the sound absorption by air depends on T,Rh profiles which are unknown)
• Limited altitude range (<1km) when there is little turbulence
Source: http://www.remtechinc.com/sodar.htm
July 2001 Zanjan, Iran 9
The SCIDAR
Ref: tutorial at the Imperial College Site: http://op.ph.ic.ac.uk/scidar/scidar.html/
SCIntillation Detection And Ranging
(J. Vernin, 1979)Analysis of the interference pattern produced
at the ground by the light of two closeby sources diffracted by a turbulence layer
The aurocorrelation of the pupil scintillation pattern shows a peak for the distance BC. The
separation of the double star scales the altitude of the turbulence layer
July 2001 Zanjan, Iran 10
The SCIDAR
Practically, thousands of frames of <1ms exposure are combined to generate one profile every minute
Ref: tutorial at the Imperial College Site: http://op.ph.ic.ac.uk/scidar/scidar.html/
July 2001 Zanjan, Iran 11
The SCIDAR
Source: A. Tokovinin, Study of the SCIDAR concept for Adaptive Optics Applications, ESO-VLT Report TRE-UNI-17416-0003
Optical Setup:
2: focal plane with field stop
3: collimator
4: chromatic filter
5: conjugate pupil plane
6: detector
Detector and pupil plane conjugate are collocated in non generalized mode
July 2001 Zanjan, Iran 12
The SCIDAR
Source: A. Tokovinin, Study of the SCIDAR concept for Adaptive Optics Applications, ESO-VLT Report TRE-UNI-17416-0003
The double star separation, and the telescope diameter set the altitude range
Fig: auto correlation shift X in generalized mode, with the detector conjugated at a plane 5km below ground, as a function of turbulence altitude.
The minimum characteristic size of the scintillation patterns is 3.5cm.
Telescope diameter=1.2m Pixel size=2cm
July 2001 Zanjan, Iran 13
The SCIDAR
Ref: tutorial at the Imperial College Site: http://op.ph.ic.ac.uk/scidar/scidar.html/
Scidar Profile, seeing 1” Scidar Profile, seeing 2”
July 2001 Zanjan, Iran 14
The SCIDAR and the Models
MM5, a mesoscale model is available as freeware. It is used at the
Mauna Kea Weather Center (http://hokukea.soest.hawaii.edu/forecast/mko/)
to produce vertical profiles of the turbulence.
Comparison of MM5 profiles above Mauna Kea Observatory with in situ SCIDAR observations
July 2001 Zanjan, Iran 15
The SCIDAR and the MASS
A single star profiler: the MASS: Multi Aperture Scintillation Sensor
A portable instrument for site surveys with a reduced altitude resolution (1km instead of 200m)
A. Tokovinin, V. Kornilov; Measuring turbulence profiles from scintillation of single stars, IAU Site 2000 Workshop, Marrakech, Nov. 2000