Atmospheric Profilers Marc Sarazin European Southern Observatory July
Atmospheric Profilers Marc Sarazin (European Southern Observatory) July 2001 Zanjan, Iran 1
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 July 2001 Zanjan, Iran 2
Outline • Why do we need turbulence profiles? • Microthermal sensing • Sound back scattering • Scintillation • Mesoscale modeling July 2001 Zanjan, Iran 3
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 July 2001 Zanjan, Iran 4
Turbulence Profilers The various methods for generating atmospheric turbulence profiles • Full line: Mauna Kea Model (Olivier, 94) • Dots: SCIDAR, Paranal • Dashes: Baloon borne microthermal sensors Ref: PARCSA Campaign, Univ. of Nice, 1992 -1993 July 2001 Zanjan, Iran 5
Balloon Borne Profilers Two Microthermal sensors, 1 m apart are attached far below the load of a standard meteorological radiosondes Pros: • Provide Temperature, Humidity and Wind • High vertical resolution (5 m) Cons: • An ascent last one hour or more • The balloon drifts horizontally (30 to 100 km) The rms of the differential temperature fluctuations over a few seconds is computed onboard and transmitted to the ground • Expensive technique (1 k. US$/flight) July 2001 Zanjan, Iran 6
Doppler SODAR Profiler Sound Detection And Ranging Monitoring the backscattered acoustic energy from the atmospheric layers The acoustic backscattering cross-section is a function of acoustic wavelength, absolute temperature and temperature structure coefficient Source: http: //www. remtechinc. com/sodar. htm July 2001 Zanjan, Iran 7
Doppler SODAR Profiler Pros: • Provides Wind profile (design goal) • Good vertical resolution (30 m) • Fully automated Cons: • Only relative Cn 2 measurements: no absolute calibration (the sound absorption by air depends on T, Rh profiles which are unknown) • Limited altitude range (<1 km) when there is little turbulence Source: http: //www. remtechinc. com/sodar. htm July 2001 Zanjan, Iran 8
The SCIDAR 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 Ref: tutorial at the Imperial College Site: http: //op. ph. ic. ac. uk/scidar. html/ July 2001 Zanjan, Iran 9
The SCIDAR Practically, thousands of frames of <1 ms exposure are combined to generate one profile every minute Ref: tutorial at the Imperial College Site: http: //op. ph. ic. ac. uk/scidar. html/ July 2001 Zanjan, Iran 10
The SCIDAR 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 Source: A. Tokovinin, Study of the SCIDAR concept for Adaptive Optics Applications, ESO-VLT Report TRE-UNI-17416 -0003 July 2001 Zanjan, Iran 11
The SCIDAR 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 5 km below ground, as a function of turbulence altitude. The minimum characteristic size of the scintillation patterns is 3. 5 cm. Telescope diameter=1. 2 m Pixel size=2 cm Source: A. Tokovinin, Study of the SCIDAR concept for Adaptive Optics Applications, ESO-VLT Report TRE-UNI-17416 -0003 July 2001 Zanjan, Iran 12
The SCIDAR Scidar Profile, seeing 1” Scidar Profile, seeing 2” Ref: tutorial at the Imperial College Site: http: //op. ph. ic. ac. uk/scidar. html/ July 2001 Zanjan, Iran 13
The SCIDAR and the Models MM 5, 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 MM 5 profiles above Mauna Kea Observatory with in situ SCIDAR observations July 2001 Zanjan, Iran 14
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 (1 km instead of 200 m) A. Tokovinin, V. Kornilov; Measuring turbulence profiles from scintillation of single stars, IAU Site 2000 Workshop, Marrakech, Nov. 2000 July 2001 Zanjan, Iran 15
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