A&A 365, 324-329 (2001)
DOI: 10.1051/0004-6361:20000030
The first optical characterization of the Oukaïmeden site
with the Generalized Seeing Monitor (GSM)
A. Ziad1
- A. Jabiri2
- Z. Benkhaldoun2
- F. Martin1
- R. Conan1
- M. Lazrek3
- J. Borgnino1
Send offprint request: A. Ziad,
1 - UMR 6525
Astrophysique, Université de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 2, France
2 - Laboratoire de Physique des Hautes Énergies et Astrophysique, Département de Physique,
Université Cadi Ayyad, Faculté des
Sciences Semlalia, BP 2390, Marrakech, Morocco
3 - Astronomical
and Geophysical Laboratory, C.N.C.P.R.S.T., BP 1346 RP, Rabat, Morocco
Received 31 January 2000 / Accepted 17 October 2000
Abstract
The main atmospheric optical parameters (AOP) have been measured
during 10 nights in April 1998 with the GSM instrument at the
Moroccan site of Oukaïmeden. These parameters are of interest
for the optimization of high angular resolution techniques. During
this campaign the temporal evolution of the AOP and their
distributions have been studied. The outer scale presents a
log-normal histogram with a median value of 31 m which is rather
similar to the values obtained at other sites visited with GSM.
The selection of the Oukaïmeden site is the result of several
topographical and meteorological studies on the Atlas mountain
chain. Since 1988 this site has been chosen for the installation
of one of the IRIS (International Research of the Interior of the
Sun) stations. Here, we present the whole AOP data measured with
GSM during this campaign. The main photometric and meteorological
conditions of this site are also presented.
Key words: atmospheric effects - turbulence - site testing
Author for correspondance: ziad@unice.fr
Oukaïmeden is a 2700 m mountain in the Moroccan High Atlas.
This site (
North and
West) is located at 70
km from the city of Marrakesh; it is easily accessible by a fairly
good road. The selection of Oukaïmeden as a possible site for
the first Moroccan observatory was the result of meteorological,
topographical and daytime photometry studies during several years
(Kadiri 1983; Benkhaldoun 1994). In 1987, this site was included in the IRIS
network (Benkhaldoun et al. 1991) and later tested by the GONG project
(Hill 1994a; Hill 1994b). But, since all the previous studies
concerned daytime observations and meteorological
conditions (Benkhaldoun 1994); night-time optical characterization of
this site was needed in order to develop astronomical programs.
This characterization is related to the effect of atmospheric
turbulence on the wavefronts which severely reduces the
resolution of ground-based astronomical observations. Different
techniques have been developed to achieve the diffraction limited
resolution of observing instruments, namely speckle and long
baseline interferometry (SI, LBI) and adaptive optics (AO). But
the performance of these High Angular Resolution (HAR) methods
requires a better understanding of the behavior of wavefronts
perturbed by the atmospheric turbulence, more exactly a better
knowledge of atmospheric optical parameters (AOP). Among these
parameters are the Fried coherence size r0 which is related to
the seeing
(
), the
spatial coherence outer scale
,
the isoplanatic angle
and the wavefront coherence time
.
It is well
known that the performance of an AO system depends upon the seeing
conditions. On the other hand, for passive LBI the outer scale
is of interest because its value provides the
spectral bandwidth optimizing the signal-to-noise ratio
(Rodddier 1981; Ziad et al. 1994a). In addition, the
is also a
critical parameter for co-phasing an interferometer (Mariotti 1993).
In the case of a large telescope equipped with AO system, the
residual error after a tip-tilt correction grows for small
values (Voitsekhovich & Cuevas 1995). On the other hand, the choice of the
AO reference star must take into account the constraints related
to the isoplanatic angle. Finally the knowledge of the wavefront
coherence time
is of interest to optimize the exposure
time. Indeed, a compromise must be found between the flux, as a
function of increasing exposure time, and the degradation of the
image quality.
For all these reasons a measurement campaign has been performed at
the Oukaïmeden site with the Generalized Seeing Monitor. This
instrument, developed at the Département d'Astrophysique of Nice
University, consists of monitoring all of the optical parameters
and especially the outer scale
(Martin et al. 1994). This
monitor and data reduction procedures are succinctly described in
Sect. 3.1. Then, in Sect. 2, we
summarize the meteorological conditions of the Oukaïmeden
site and the previous daytime characterizations. Finally, in
Sect. 4 the summary of the measurement campaign
made with GSM at this site is presented and discussed in
Sect. 5.
2 Oukaïmeden site
The first step towards the creation of an observatory is the
selection of an optimal site. It must be a compromise between the
necessary remoteness from any light sources and the proximity to
scientific centers. This choice determines the attraction and
interest that this future observatory will be able to cause in
the international astronomical community.
Based on these various constraints and other topographical and
meteorological situations, a detailed study was carried out on the
Atlas mountains (Kadiri 1983). The selection of the
Oukaïmeden site resulted from the best compromise between
all the constraints and appears to be a good choice, when
compared to other former observatories, in terms of daytime sky
transparency, photometry, extinction and ground climatology. In
this section we present a summary of the meteorological
conditions and the previous daytime site-testing campaigns which
have been carried out at Oukaïmeden site.
As soon as Oukaïmeden was selected as a potential site for a
future observatory, daytime measurements were performed at
different wavelengths to estimate the diurnal quality of this
site.
In 1987, a simple Flux Integration Photometer (FIP) was built to
sample different values of atmospheric extinction coefficient.
The FIP instrument (Benkhaldoun et al. 1993) is a full disk sunlight triple
photometer. The analysis of the FIP database shows that the
Oukaïmeden appears to exhibit photometric conditions during
65% of daytime. The extinction coefficient at 700 nm wavelength
is k=0.13 mag/air-mass.
The same year brought the installation of an IRIS station at
Oukaïmeden. The IRIS instrument (Grec et al. 1991) measures a
Doppler shift integrated over the entire solar disk, using the
Sodium optical resonance spectrometer. The statistics from one
year of IRIS photometric data obtained at Oukaïmeden show
that the average extinction coefficient is about k = 0.1
mag/air-mass in the Sodium band and that the daily fraction of
clear weather is 64% (Benkhaldoun & Siher 1998).
In 1988, the site was also chosen to be tested by the GONG
project site survey controlled by the National Solar Observatory
of Tucson in Arizona. Thus, the GONG network installed on the
site a pyrheliometer to achieve a complementary photometric study
of the sunlight. The results obtained (Hill 1994b) presented a
lower value for the clear time fraction (50%) and an average
extinction coefficient of k=0.12 mag/air-mass. This value is
similar to that measured by Benkhaldoun et al. (1993) with the FIP instrument.
The values of the daytime extinction coefficients are
satisfactorily consistent. Their small differences depend on
details of the instrument calibrations, and also on the
different spectral bandwidths used by the different instruments.
The significantly smaller fraction of clear time measured by the
GONG site survey instrument
has simple explanations depending on data analysis
(Hill 1994a; Hill 1994b) as explained by Jabiri et al. (2000). On the other hand, the results obtained with the IRIS and the FIP instruments are
close, respectively,
and
of clear daytime. For
night-time conditions further measurements with the GSM are
needed for a long period (one year) to provide more statistics on
the clear time and to show the variations with the seasons.
In 1990 a meteorological station was installed to collect
information at the ground of the insolation, the pluviometry, the
wind speed and direction and the temperature. The analysis of
these meteorological data tends to confirm the choice of this site
(Jabiri et al. 2000). The temperature changes weakly during the night
and the variation does not exceed 2 or 3
C. We can thus
expect to have weak turbulence close to the ground.
Humidity undergoes a significant variation during the night
(Jabiri et al. 2000). It decreases until midnight and starts to
stabilize around the nightly average value (40%). This variation
of humidity is checked by daytime photometric measurements which
show a transparent sky in the morning, which covers with clouds in
the afternoon and starts to get clear at the beginning of
evening. The probability of a high local humidity is weak and for
night-time conditions this humidity is lower than 60%
(Jabiri et al. 2000).
During the night (Fig. 1), generally the wind is blowing
from the south and dispersed from east to west. This wind blows
predominantly from SE and most of the time it is lower than
5 m/s. This wind is responsible for Sahara dust transport; but
the presence of high mountains towards the East of the
Oukaïmeden protects the site from this dust wind.
![\begin{figure}
\par\includegraphics[width=8.8cm,clip]{DS1844f1.eps}
\par\end{figure}](/articles/aa/full/2001/02/aa1844/Timg12.gif) |
Figure 1:
Night-time windrose versus modulus at the Oukaïmeden
site. These data are the results of more than 2 year measurements
(December 1990 to March 1993) |
Open with DEXTER |
3.1 The Generalized Seeing Instrument
The GSM instrument consists of evaluating the
optical parameters of the perturbed wavefront by measuring Angle
of Arrival (AA) fluctuations. Indeed, the GSM uses the same
principle than a Shack-Hartmann, i.e., measuring AA at different
points of the wavefront and computing AA spatio-temporal
correlations leads to estimates of the seeing
,
outer
scale
,
isoplanatic angle
and coherence
time
.
The instrument consists of four 10-cm telescopes on equatorial
mounts (Fig. 2) equipped with detection modules measuring
the AA fluctuations and interfaced to a computer PC managing
simultaneously the 4 modules. Each telescope, pointing at the same
star, measures the AA fluctuations by means of flux modulation
which is produced by the displacement of the star image over a
Ronchi grating. Two telescopes are installed on a common mount on
a central pier (Fig. 2) working as a differential image
motion monitor (DIMM) with a 25 cm baseline. Two other telescopes
have different mounts on separate piers, located 0.8 m to the
south and 1 m to the east from the central pier, thus forming an
L-shaped configuration. This configuration has been chosen for
more sensitivity to the outer scale. The telescopes were situated
1.7 m above the ground.
The AA fluctuations are measured with 5 ms resolution time during
2 min acquisition time. Data are processed immediately after
each acquisition, allowing a quasi real-time monitoring of the
AOP. The data acquisition is repeated typically every 4 min.
The AA covariances are computed for each baseline (6 baselines
with 4 GSM modules) and normalized by the differential variance of
AA on the 25-cm baseline. They are compared to Von Kàrmàn
theoretical normalized covariances (Avila 1997) and the
appropriate
is found for each baseline. The final
value of
is taken as the median of the 6 individual
values and its error is estimated. The seeing
is calculated from the differential variance given by
the coupled modules as in the DIMM instrument (Sarazin & Roddier 1990). The
scintillation index
is computed during data
reduction and, as suggested by Loos & Hogge (1979) and Krause-Polstorff et al. (1993), an
estimate of the isoplanatic angle is deduced.
A quantification of the different GSM noises has been performed
and hence corrections of photon and scintillation noises are done
before data processing. Another correction for finite exposure
time is also performed; it consists in computing AA variance (or
covariance) for 5 ms and 10 ms and in extrapolating linearly to
the 0 ms exposure time. Finally, the statistical errors of the
computed variances and covariances are estimated and consequently
the errors of the AOP measured with GSM are provided.
In order to check the wind shake effect, r0 is computed from absolute image motion in each telescope,
corrected for finite
(Ziad et al. 1994b) and compared to r0 provided by the differential
technique. A good agreement is found for ground wind speed less than 10 m/s, showing that telescope
vibrations were not significant.
3.2 GSM configuration at the site
The on-site mission preparation consisted in building 4 piers to
support the GSM different modules. The IRIS building had been
arranged before our arrival to serve as GSM office. It was used
before observations to store the GSM equipment.
The GSM was installed on three piers located near the IRIS
instrument on a pre-existing platform. The piers had a compact
L-shaped configuration as described in Sect. 3.1.
The wind-induced telescope vibrations present a potential problem
in AA measurements. To reduce this effect, the instrument was
surrounded by a protective net (30% wind transparency) from 3
sides (south, east and west). The net was 2 m high, and
completely decoupled from piers.
The IRIS building with computers and observers was located at a
distance around 4 m from the central pier, to the south-west. With
a prevailing wind from the SE the GSM was supposed to be free from
the locally generated turbulence, and the building was not
troublesome either. On the other hand, with the southern-west wind
the GSM was in the turbulent lee created by its own building and
by the small rocks to the south.
![\begin{figure}
\par\includegraphics[height=6cm,width=8.5cm,clip]{DS1844f3.eps}
\par\end{figure}](/articles/aa/full/2001/02/aa1844/Timg15.gif) |
Figure 3:
The distribution of the total measuring time during
10 nights at the Oukaïmeden site |
Open with DEXTER |
![\begin{figure}
\par\includegraphics[height=6cm,width=8.5cm,clip]{DS1844f4.eps}
\par\end{figure}](/articles/aa/full/2001/02/aa1844/Timg16.gif) |
Figure 4:
The AOP measurements during one night (04/14/98) at the Oukaïmeden site |
Open with DEXTER |
![\begin{figure}
\par\includegraphics[height=6cm,width=8.5cm,clip]{DS1844f5.eps}
\par\end{figure}](/articles/aa/full/2001/02/aa1844/Timg17.gif) |
Figure 5:
The AOP measurements during the best night (04/18/98) at the
Oukaïmeden site. The median value of the seeing is 0.85 arcsec |
Open with DEXTER |
![\begin{figure}
\par\includegraphics[height=6cm,width=8.5cm,clip]{DS1844f6.eps}
\par\end{figure}](/articles/aa/full/2001/02/aa1844/Timg18.gif) |
Figure 6:
The summary of AOP data measured during 10 nights at the Oukaïmeden site. The reported values correspond to the AOP logarithmic means and bars indicate intervals containing 68% of parameter values for each night (
of the log-normal nightly distributions) |
Open with DEXTER |
![\begin{figure}
\par\includegraphics[height=5cm,width=5cm]{DS1844f7a.eps}\include...
...S1844f7b.eps}\includegraphics[height=5cm,width=5cm]{DS1844f7c.eps}
\end{figure}](/articles/aa/full/2001/02/aa1844/Timg19.gif) |
Figure 7:
Histograms of the AOP measured with GSM during 10 nights
at the Oukaïmeden site in April 1998. All these parameters
are well fitted with log-normal distributions (dashed-line) |
Open with DEXTER |
A stellar source was selected from the list of single bright stars
having from 2 to 3 magnitude and passing close to zenith at the
Oukaïmeden site during this season. Due to the limitation of
the possible hour angles (in the adopted configuration the
telescopes could not be pointed at hour angles less than 1 h
before meridian because they touched the piers) the selected star
was usually some 30-40 min. before meridian at the start, and 2-3 h after meridian at the end of its observations (depending on the
availability of a more suitable source). Thus, observations were
obtained at zenith angles from
to
.
After pointing to the source, the telescopes were focused by
maximizing the modulation contrast. Then, the acquisition sequence
was started, interrupted every 30-40 min for re-centering of
the star in the field of view. Signal was normally recorded during
2 mn, and these acquisitions were repeated every 4 mn.
Occasionally, longer or shorter acquisition times were used for
exploratory purpose. Immediately after acquisition the data were
transferred to the hard disk and processed. This enabled
assessment of data quality and results, including the seeing
,
outer scale
and isoplanatic angle
.
4 Results of atmospheric optical parameters with GSM
During this campaign continuous measurements
of the AOP (
,
,
)
were obtained
during 10 nights in the period of 10-25 April 1998. These
parameters were estimated for the wavelength
m.
Figure 3 shows the distribution of the total
measuring time during this campaign. We draw attention to the
larger number of measurements of the last night compared to the
others. This data recording time depends on the meteorological
conditions (cloud passages, strong wind...), on the observed star
change and on the instrument technical problems. Despite this,
for this campaign the mean measuring time was more than 4 hours
per night providing a high number of measurements of the seeing,
outer scale and isoplanatic angle.
A typical temporal evolution of the AOP during one night is shown in
Fig. 4. A rapid variation of
is usually
observed, as well as isolated "bursts'' of large outer
scale values. These bursts have been also noted at the other sites visited by
the GSM. They are typical of our data set, being more frequent on some
nights, less frequent or absent on some other nights. Burst duration
is about few minutes. Figure 5 shows the best night
measurements during this campaign. One can remark that the seeing is
usually under 1 arcsec and that all the AOP are less dispersed than the
results shown in Fig. 4 which indicates the stability
of the conditions during this night. The median value of the seeing is 0.85 arcsec and
the best measured value is 0.66 arcsec.
Figure 6 presents the summary of the AOP
measurements during this campaign. The seeing, outer scale and
isoplanatic angle are well fitted with log-normal distributions
(Fig. 7). So, for each night the reported AOP values in
Fig. 6 are deduced from the nightly log-normal
distributions (
). One can remark that contrary
to one night measurements (Fig. 4) all of the AOP
have a slow temporal variability from one night to another.
No significant correlation exists between outer scale and seeing but for some nights, as illustrated in Figs. 4 and 6, there is a correlation between seeing and isoplanatic angle which is due to the contribution of high turbulent layers on the seeing degradation.
5 Discussion and conclusion
For the first time the seeing
,
the outer scale
and the isoplanatic
angle
were monitored continuously during 10 nights at
the Oukaïmeden site.
The outer scale
data provided by GSM shows that
this parameter is well fitted by a log-normal distribution with a
rather similar median value compared to the other sites visited by
the GSM (Ziad et al. 2000). On the other hand, the
measurements during one night (Fig. 4) present a
strong temporal variability with isolated "bursts'' which suggests
a parallel
monitoring with HAR observations. The
seeing data present some excellent nights with values comparable
to La Silla observatory (Fig. 5) but during some
other nights the seeing was worse. For this parameter further
measurements are needed for a long period (one year) to provide
more statistics and to show the variations with the seasons. For
the isoplanatic angle
,
the data are well fitted with a
log-normal distribution having a median value of 1.32 arcsec
which is comparable to those found at La Silla and Paranal
observatories (Ziad et al. 2000). As shown in Fig. 5,
during some nights this site presents conditions comparable to
the best sites in the world but it would be interesting to know
how often this case is produced. No significant correlation
exists between the
and the seeing
but there is one between
and the isoplanatic angle
due to the high turbulent layer contribution.
The main meteorological results of this site obtained during more
than 2 years are characterized by a SE predominant wind generally
lower than 5 m/s. This site is protected from the dusts coming
from Sahara by the Atlas mountains. During the night weak
temperature variations are observed that do not exceed
or
showing the relative stability of this site. The humidity is
under
in the
of the nightly time.
On the other hand, the FIP, IRIS and GONG data show that the
extinction coefficient varies from 0.08 to 0.2 mag/air-mass, and
that
(FIP and IRIS) of the time is clear, showing that
this site has good photometric conditions.
Acknowledgements
This campaign has been organized in the framework of the
Franco-Moroccan cooperation. The authors would like to thank the
Rectorat de l'Université Cadi Ayyad and the Faculté Semlalia
for the efficient support of this campaign, the Royal Air Maroc
who helped us for financial support of the transport of the GSM
instrument. We express our thanks to Eric FOSSAT for his
suggestions to improve this manuscript.
-
Avila, R., Ziad, A., Borgnino, J.,
et al. 1997, J. Opt. Soc. Am., 14(11), 3070
In the text
NASA ADS
-
Benkhaldoun, Z. 1994, Ph.D. Thesis, Université Cadi Ayyad
In the text
-
Benkhaldoun, Z., Kadiri, S., Lazrek, M., & Touma, H. 1991,
Solar Phys., 133, 61
In the text
NASA ADS
-
Benkhaldoun, Z., Kadiri, S., Lazrek, M., & Vernin, J. 1993,
Exper. Astron., 2, 345
In the text
-
Benkhaldoun, Z., & Siher, E. 1-4 June 98,
Proceeding of the Soho/Gong98 workshop, Boston (USA), 109-113
In the text
-
Grec, G., Fossat, E., Gelly, B., & Schmider, F. 1991, Solar Phys., 133, 13
In the text
NASA ADS
-
Hill, F. 1994, Solar Phys., 152, 321
In the text
NASA ADS
-
Hill, F. 1994, Solar Phys., 152, 351
In the text
NASA ADS
-
Jabiri, A., Benkhaldoun, Z., Vernin, J., & Muñoz-Tuñon,
C. 2000, submitted to Astron. Astrophys.
In the text
-
Kadiri, S. 1983, Ph.D. Thesis, Université de Nice
In the text
-
Krause-Polstorff, J., Edmund, A., & Donald, L. W. 1993,
Appl. Opt., 32(21), 4051
In the text
NASA ADS
-
Loos, G., & Hogge, C. 1979, Appl. Opt., 18(15), 2654
In the text
NASA ADS
-
Mariotti, J. 1993, Adaptive Optics for Astronomy,
ed. D. Alloin, & J. M. Mariotti (Kluwer Academic Publishers), 309-320
In the text
-
Martin, F., Tokovinin, A., Agabi, A., Borgnino, J., & Ziad, A. 1994,
A&AS, 108, 173
In the text
NASA ADS
-
Roddier, F. 1981, in Progr. Opt., vol. XIX, ed. E. Wolf
In the text
-
Sarazin, M., & Roddier, F. 1990, A&A, 227, 294
In the text
NASA ADS
-
Voitsekhovich, V., & Cuevas, S. 1995, J. Opt. Soc. Am., 12(11), 2523
In the text
NASA ADS
-
Ziad, A., Borgnino, J., Agabi, A., & Martin, F. 1994a, Exp. Astron., 5, 247
In the text
NASA ADS
-
Ziad, A., Borgnino, J., Martin, F., & Agabi, A. 1994b, A&A, 282, 1021
In the text
NASA ADS
-
Ziad, A., Conan, R., Tokovinin, A., Martin, F., & Borgnino, J.
2000, Appl. Opt., in press
In the text
© ESO 2001