Comparison with other sites

1
First DIMM seeing measurements
at Dome C
Eric Aristidi A. Agabi, J. Vernin, M. Azouit, F.
Martin, A. Ziad, E. Fossat
LUAN - Université de Nice Sophia-Antipolis, France
CONCORDIASTRO Program 3rd summer campaign Nov.
2002 Jan 2003
2
The  seeing 
A parameter characterising atmospheric
turbulence. Seeing is the angular resolution of
long exposure images through the atmosphere.
e 005 r02.5 m
e 01 r01 m
e 03 r030 cm
e 1 r010 cm
3
The Dome C DIMM
The Concordiastro site testing programme consists
in the qualification of the Dome C as an
astronomical site. The goal is to perform a
complete site testing and to provide optical
parameters such as seeing value or coherence
time. These quantities will be measured by
combining a set of 3 Differential Image Motion
Monitors (DIMM), based on the GSM (Generalised
Seeing Monitor) concept. The site-testing
program is based on several summer campaigns
dedicated to the preparation of winter
observations at Dome C.
Glass prism

• Celestron 11 diam28 cm foc 2.8 m
• 2 holes mask diam. D6 cm sep. B20 cm
• Glass prism deviation30 arcsec
• Digital CCD max sensitivity500 nm pixel
  size10 microns
• thermostated at 20C

CCD
4

5
DIMM Principle
Focal image
Hadar (b Cen), V0.6, 6 Dec 2002
The transverse (st2) and longitudinal (sl2)
variances of the spots position difference gives
two estimates of the seeing e.
Assuming BgtgtD, r0ltD and Kolmogorov turbulence
(infinite outer scale), we have (Tokovinin,
2002, PASP 114, 1156)
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Data acquisition /online processing
Target star Canopus (a Eri), V-0.9
2 mn time series (about 1000 frames, stat. error
on seeing is 5)

• Exposure time adjustable (useful range 1-20
  ms)
• Time between 2 successive images 121 ms

• Threshold the images
• Detect the two spots (on the whole image area for
  the first frame)
• Compute baycenters x1,x2,y1,y2 and save
• Place a twin window of 50x50 pixels centered on
  each spot that will reduce the spot detection
  area for next frame and gain speed
• When 2 minutes are ellapsed, compute the
  variances and Fried parameter and save

a 1 value of longitudinal/transverse seeing every
2 minutes
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Transverse vs longitudinal values
Longitudinal and transverse values should be
equal. a We eliminated points where the ratio
r0t/r0l lt0.99 or r0t/r0l gt1.10
3650 values have been retained (49.8 )
a
Data obtained on 7335 measurements between 06 dec
2002 and 02 jan 2003
8
Scale calibration
Image of double star a Cen
Orbit of a Cen (Baize, 1960)
Separation 12.03 arcsec
Autocorrelation (800 images)
Separation is given by the position of 2ndary
peaks
Scale 1 px0.751
Airy disc of subapertures 1.8
9
Measurements during the period 9 Dec 2002 2 Jan
20033650 seeing values (daytime) obtained on
Canopus
These data are the first daytime seeing value
obtained by DIMM !
10
Comparison with other sites
Night time (DIMM)
Daytime
Not corrected for zenith
11

Seeing measurements I
Longitudinal
o Transverse
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Seeing measurements II
Longitudinal
o Transverse
13
Balloon Temperature probing
a
Good seeing when temp. profile is flat !
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Effect of exposure time
Seeing calculations from image motion suppose an
exposure time of zero. In practical this value
is a few ms (from 5 to 20 ms in our case, mostly
10 ms). This cause systematic underestimation of
the seeing.
To estimate this effect, barycenters of
successive images (separated by T120 ms) were
rebined by 2, 3 and 4 to simulate exp. times of
T, 2T, 3T. Seeing is computed in each case. An
exponential fit is then applied to compute the
instantaneous seeing. We found a bias of 2
No decorrelation of images within 120 ms
large coherence time ?
15
Future next year ?

• Access to
• Coherence time
• Outer scale

(DIMM 3) In 2 years

• Also measuring
• Scintillation
• isoplanatic angle

DIMM 1
DIMM 2
16
Thank you !
17
Telescope turrbulence
The good case 
Seeing et0.8 and el 0.9
The bad case
Seeing is overestimated by large values of sl
and st Here et2.3 and el 2.9
18
Sky background and thresholding
b Cen (V0.6) Exp 7 ms 04/12/2002, 14h30
(alt59) About 60 from the Sun
Background b640 /-20
Canopus (V-0.9) Exp 10 ms 23/12/2002, 0h04
(alt50) About 180 from the Sun
Background b270/-10
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Summary of errors and bias
The Dome C DIMM still needs a calibration with
another seeing monitor. That will be done by
performing simultaneous observations with GSM.