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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, Sadibekova T. www-luan.unice.fr/Concordia. South Pole 1979. Main characteristics of the site. 1. Altitude > 3000 m

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Site testing at Dome C: recent results

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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, Sadibekova T.

www-luan.unice.fr/Concordia


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South Pole 1979


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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


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Altitude level


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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


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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)


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Concordiastro : 4 (+1) summer campaigns


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The experiments


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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€


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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)


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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


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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)


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Inflating the Balloon

In winter

In summer


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Preparing the sond…


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In winter

Launching the balloon

In summer


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Summer turbulence

conditions


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November 2003: amazing days

Wow ! Excellent !

! Values not corrected from z and exposure time (10 ms)


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Summer seeing : statistics

(based on 2 summer campaigns)

0.54

3


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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


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Isoplanatic angle: statistics

6.8

6.8


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Comparison with other sites

The best site of the world ?


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Night seeing at Dome C

SODAR + MASS

Travouillon et al


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Towards the winter

  • summer seeing 0.54 arcsec

  • AASTINO results : 0.27 arcsec in autumn

We were very confident for the winter !


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First winterover

10 Feb: Deparure of the last plane

Karim Agabi :

The winter

astronomer


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Remote-controlling (useful at –70°C…)

Data acquisition

Concordia labo

300 m

To the mast (700 m)

Wi-Fi LAN+Fiber optics connection


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About the weather

36 days

74 days

Statistics

2005 : about 85 %

2006 : systematic, visual, measurements

about 80% in summer, 90 % in April


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Autumn seeing


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Some vertical profiles…

Everything is in

the surface layer !

Seeing in altitude:

<0.4 arcsec

Ground seeing:

>1 arcsec


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DT=20°

40 m

40 m

20 m

How high is the surface layer ?


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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


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Surface layer

  • South Pole : 220m

R.D. Marks, et al. 1999, A&A

  • Dome C : 30m


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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


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Comparison with other sites


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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)


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  • 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


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  • 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


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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)


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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


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Auroras


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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


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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)


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Observability


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