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Site testing at Dome C: recent results PowerPoint Presentation
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Site testing at Dome C: recent results

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  1. Site testing at Dome C: recent results CONCORDIASTRO Project E. Aristidi, A. Agabi, E. Fossat, T. Travouillon, M. Azouit, J. Vernin, A. Ziad, F. Martin, Sadibekova T. www-luan.unice.fr/Concordia

  2. South Pole 1979

  3. Main characteristics of the site 1. Altitude > 3000 m 2. Slope < 1/1000 3. Snow < 5g/cm/year 4. Limit for auroras2 5. Limit of visibility fo geostationary satelites

  4. Altitude level

  5. ConcordiAstro site testing :3 experiments To obtain a complete astronomical qualification of the site from the turbulence side  Goal : DIMM/GSM Step 1 : seeing Step 2 : q0, L0, t0 Mast Monitor the ground layer Cn2 Balloons : Step 1 : PTU Step 2 : Cn2

  6. Wind Speed Profiles Vol 186 Vol 45 Vol 118 Altitude (Km) Altitude (Km) Altitude (Km) Wind Speed Profiles at Paranal ESO Chili (1992) Wind Speed Profiles at Gemini NOAO Chili (1998) Wind Speed Profiles at Dome C (Dec 2000)

  7. Concordiastro : 4 (+1) summer campaigns

  8. The experiments

  9. Estimating the seeing :Differential Image Motion Monitor Glass prism • Celestron 11 d=28 cm, f = 2.8 m, tube in INVAR • 2 holes mask on pupil • diam. D=6 cm sep. B=20 cm • glass prism deviation=30  arcsec • CCD max sensitivity=500 nm pixel size=10 microns • thermostated at –20°C Overall cost ~30 k€

  10. DIMM Principle The transverse (st2) and longitudinal (sl2) variances of the spots position difference gives two estimates of the seeing e. Assuming Kolmogorov turbulence (infinite outer scale), we have (Tokovinin, 2002, PASP 114, 1156)

  11. Estimating isoplanatic angle Principle : scintillation measurement with a circular 10cm diameter pupil with 4 cm central obstruction Ziad et al., 2000, Appl. Opt. 39, 30

  12. Principle • Measurement of DTA2 , DTB2 Balloon • Calculation of • CT2=< DTA2 > rA-2/3 • CT2=< DTB2 > rB-2/3 rA= 95 cm rB=33 cm • Then… RS80 radiosond Thermometers Send : DTA , DTB P, T, U wind speed & direction • 2 estimates of Cn2 Balloons In-situ soundings to obtain the turbulent energy profile Cn2(h) (Borgnino et al., 1979, A&A 79, 184)

  13. Inflating the Balloon In winter In summer

  14. Preparing the sond…

  15. In winter Launching the balloon In summer

  16. Summer turbulence conditions

  17. November 2003: amazing days Wow ! Excellent ! ! Values not corrected from z and exposure time (10 ms)

  18. Summer seeing : statistics (based on 2 summer campaigns) 0.54 3

  19. Seeing as function of time Good seeing when surface layer temperature gradient vanishes No temp. gradient Temp. Gradient (6°/100m) (Aristidi et al., A&A 2005) • Good news for solar astronomy : seeing below 0.5 almost every day at tea time during ~6h

  20. Isoplanatic angle: statistics 6.8 6.8

  21. Comparison with other sites The best site of the world ?

  22. Night seeing at Dome C SODAR + MASS Travouillon et al

  23. Towards the winter • summer seeing 0.54 arcsec • AASTINO results : 0.27 arcsec in autumn We were very confident for the winter !

  24. First winterover 10 Feb: Deparure of the last plane Karim Agabi : The winter astronomer

  25. Remote-controlling (useful at –70°C…) Data acquisition Concordia labo 300 m To the mast (700 m) Wi-Fi LAN+Fiber optics connection

  26. About the weather 36 days 74 days Statistics 2005 : about 85 % 2006 : systematic, visual, measurements about 80% in summer, 90 % in April

  27. Autumn seeing

  28. Some vertical profiles… Everything is in the surface layer ! Seeing in altitude: <0.4 arcsec Ground seeing: >1 arcsec

  29. DT=20° 40 m 40 m 20 m How high is the surface layer ?

  30. Seeing Isoplanatic angle Coherence time Estimating turbulence parametersfrom balloon Cn2(h) profiles Cn2(h) wind speed h1 • Parameters can be computed from Cn2(h) and the wind profile v(h) • Changing h1 : compute parameters that would be observed at alt. h1

  31. Surface layer • South Pole : 220m R.D. Marks, et al. 1999, A&A • Dome C : 30m

  32. GSM h=3.5m sopd h > 0 m 3 l L0 = 10 m h > 30 m 1 l Interferometric coherence times t0= 0.31 r0/ v ~7 ms T0= 0.31 L0/ v ~775 ms Optical/interferometric parameters Integrated from h=8m Integrated from h =30m s

  33. Comparison with other sites

  34. ECMWF (European Center for Medium range Weather Forecast)http://www.ecmwf.int • 60 pressure levels from surface 655mB to 0.1 mB • 0h, 6h, 12h, 18h UT • Parameters : pressure (mB), temperature (oC), relative humidity (%), zonal et meridian wind speed projections (m/s)

  35. Two types of sondes, RS80 and RS90 (more precise on the humidity and temperature parameters) • Examples of the comparison between ECMWF analysis and balloons measurements RS80 ____ balloons data ____ model RS90

  36. Differences: Model – Data • RS80 – 168 used balloons • RS90 – 48 used balloons • Temperature rms 1.5 - 3 ºC for RS80 • 1 - 1.5 ºC for RS90 • Relative humidity rms 1 - 10 % for RS80 • 1 - 2 % for RS90 • Wind speed rms  1m/s at all altitudes (figure). • Optical turbulence forecast ? • Turbulence = temperature gradient + wind • Turbulence above Dome C can be produced mostly at: • Tropause • Ground layer

  37. Tropause (4-6km above ground): - In summer inversion of the temperature gradient; - No tropopause in winter!!! Average monthly wind speed (m/s) at: 200mB     250mB      300mB_________________________________________ January   6.57     8.15       9.91 February   10.26   14.18     15.89 March       9.39   10.88     12.21 April       10.47   10.94     12.01 May         12.60   12.95     13.54 June       12.93   13.93     14.21 July       12.94   13.45     13.68 August     17.56   17.84     16.70 September   12.69   13.64     13.56 October   10.85   10.65     10.76 November   12.08   13.46     14.89 December     6.40     8.66   11.21 - The Coherence time of the wavefront is defined by (Roddier, 1981): o~ 1/Vo where Vo is velocity of the turbulence - And Sarazin&Tokovin (2001) proposed an expression for Vo which related to metrological variables only: Vo = Max(0.4V200Mb)

  38. Atmospheric turbulence modelH. Gallee, M. Swain • Instantaneous (“snap shot”) profiles show strong and fast boundary layer seeing nearly always present over Antarctic ice sheets. • Models predicts large improvement in seeing and coherence time above boundary layer. • Model predicts Dome C has 1.16“ average seeing at 8 m elevation. • Dome C boundary layer most probable elevation is ~22 m. • Good agreement between model and observations for elevations below 1000 m. For best results, place telescope above blue line

  39. Auroras

  40. Instruments for the next winter 2006 - 2007 • SSS • Photometer (v) • MOSP • DIMM • GSM • Pistonscope • Mast Increase the statistics over more than one year Cn²(z), V(z) q0, t0, e Extinction coefficient Cn²(z), L0(z) e L0, t0, e sopd, qopd Cn²(hi), up to 40m

  41. Future instruments • AIRBUS (Near IR sky brightness) • IRAIT (80 cm IR telescope, general user) • A-STEP (40 cm telescope 30’x30’ photometer) • ICE-T (2x80 cm wide-field photometer) • MYKERINOS (Prototype interferometer 3x40 cm)

  42. Observability