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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,

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Presentation on theme: "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,"— Presentation transcript:

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/ Snow < 5g/cm/year 4. Limit for auroras 2 5. Limit of visibility fo geostationary satelites

4 Altitude level

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10 ConcordiAstro site testing : 3 experiments Balloons : Step 1 : PTU Step 2 : Cn 2 DIMM/GSM Step 1 : seeing Step 2 :  0, L 0,  0 To obtain a complete astronomical qualification of the site from the turbulence side  Goal : Mast Monitor the ground layer Cn 2

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

12 Concordiastro : 4 (+1) summer campaigns WhoStayBalloonTelescope 1995 J. Vernin 1 week5 (+1) A Agabi JM Clausse 1 week6 (+1) A Agabi J Dubourg 6 weeks A Agabi E Aristidi T. Travouillon 5 weeks A Agabi E Aristidi E Fossat T. Travouillon 3 months962

13 The experiments

14 Estimating the seeing : Differential Image Motion Monitor 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 Glass prism Overall cost ~30 k€

15 DIMM Principle The transverse (  t 2 ) and longitudinal (  l 2 ) variances of the spots position difference gives two estimates of the seeing . Assuming Kolmogorov turbulence (infinite outer scale), we have (Tokovinin, 2002, PASP 114, 1156)

16 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

17 Balloons In-situ soundings to obtain the turbulent energy profile C n 2 (h) RS80 radiosond Balloon r B =33 cm r A = 95 cm Thermometers Send :  T A,  T B P, T, U wind speed & direction  Measurement of  T A 2,  T B 2  Calculation of C T 2 = r A -2/3 C T 2 = r B -2/3  2 estimates of C n 2  Then… Principle (Borgnino et al., 1979, A&A 79, 184)

18 Inflating the Balloon In summer In winter

19 Preparing the sond…

20 Launching the balloon In summer In winter

21 Summer turbulence conditions

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

23 Summer seeing : statistics N data31597 Std deviation0.39 Mean seeing (arcsec)0.66 Seeing max5.22 Median seeing0.54 Seeing min (based on 2 summer campaigns)

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

25 Isoplanatic angle: statistics Maidanak2.47Ziad et al Oukaimeden1.58Ziad et al South Pole3.23Marks et al Paranal1.91Ziad et al La Silla1.25Ziad et al Pachon2.71Ziad et al N data6328 Mean (arcsec)6.8 Median6.8 Std dev2.4 Max17.1 Min

26 Comparison with other sites SiteSeeingIsoplan. angle Paranal La Silla Maidanak South Pole Dome C (summer) The best site of the world ?

27 Night seeing at Dome C SODAR + MASS Travouillon et al

28 Towards the winter  summer seeing 0.54 arcsec  AASTINO results : 0.27 arcsec in autumn We were very confident for the winter !

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

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

31 About the weather Statistics 2005 : about 85 % 2006 : systematic, visual, measurements about 80% in summer, 90 % in April 36 days74 days

32 Autumn seeing

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

34 How high is the surface layer ? 40 m 20 m  T=20°

35 Estimating turbulence parameters from balloon C n 2 (h) profiles Seeing Isoplanatic angle Coherence time C n 2 (h) h1h1  Parameters can be computed from C n 2 (h) and the wind profile v(h)  Changing h1 : compute parameters that would be observed at alt. h 1 wind speed

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

37 Optical/interferometric parameters Integrated from h=8m Balloons Seeing (arcs) 0.4  0 (ms) 11.2  0 (arcs) 5.3 Integrated from h =30m Balloons (10) Dimms (March- May 05) Seeing (arcs)  0 (ms) 7.0  0 (arcs) AASTINO 2004 data 0.27 ‘’ ‘’ s Interferometric coherence times    = 0.31 r 0 / v ~7 ms    = 0.31 L 0 / v ~775 ms L 0 = 10 m GSM h=3.5m 1 h > 30 m 3 h > 0 m  opd

38 Comparison with other sites Site   (arcs)  (ms)Lo(m) La Silla Paranal Pachon Maidanak Mauna Kea San Pedro South Pole Dome C (0) H> 30m

39 ECMWF (European Center for Medium range Weather Forecast) 60 pressure levels from surface 655mB to 0.1 mB60 pressure levels from surface 655mB to 0.1 mB 0h, 6h, 12h, 18h UT0h, 6h, 12h, 18h UT Parameters :Parameters : pressure (mB), pressure (mB), temperature ( o C), relative humidity (%), zonal et meridian wind speed projections (m/s)

40 Two types of sondes, RS 80 and RS 90 (more precise on the humidity and temperature parameters) Examples of the comparison between ECMWF analysis and balloons measurements RS 80 ____ balloons data ____ model RS 90

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43 Differences: Model – Data RS 80 – 168 used balloons RS 90 – 48 used balloons  Temperature rms ºC for RS ºC for RS 90  Relative humidity rms % for RS % for RS 90  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: 1.Tropause 2.Ground layer

44 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 February March April May June July August September October November December The Coherence time of the wavefront is defined by (Roddier, 1981):  o ~ 1/V o where V o is velocity of the turbulence - And Sarazin&Tokovin (2001) proposed an expression for V o which related to metrological variables only: V o = Max(0.4V 200Mb )

45 Atmospheric turbulence model H. 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

46 Auroras

47 SSS Photometer (v) MOSP DIMM GSM Pistonscope Mast Increase the statistics over more than one year  ,  0,  Cn²(z), L 0 (z) Cn²(z), V(z)  L 0,  0,  Cn²(h i ), up to 40m  opd,  opd Instruments for the next winter Extinction coefficient

48 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)

49 Observability


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