- SITE TESTING
present many instruments conceived to measure one or a few
astroclimatic parameters exist. These instruments are based on
different physical principles and can be divided in four big
profilers: Generalized Scidar (Fuchs et al.
et al. 1997), Multi Aperture
Scintillation Sensor - MASS (Kornilov et
al. 2003, Tokovinin et
al. 2003), radiosoundings with macrothermal sensors for CN2
measurements (Azouit &
Vernin 2005), SLope Detection and
Renging - SLODAR (Wilson, 2002)
Image Motion Monitor - DIMM (Sarazin
& Roddier 1990), Generalized
Seeing Monitor - GSM, (Martin et
dedicated to monitoring
turbulence developed in the boundary layer: SHABAR
et al. 2010),
MAST equipped with microthermal
sensors for CN2
measurements (Azouit &
Vernin 2005), HVR-Generalized Scidar (Egner &
Masciadri 2007), Low Layer Scidar
- LOLAS (Avila et al.
2008), SLope Detection and Ranging - SLODAR at
high resolution (Osborn et al.
(4) Instruments detecting the height
of the turbulent surface layer: SOnic Detection and
Ranging - SODAR, SNODAR (Bonner et al.
All these instruments have some specificities and can be suitable to be
used for different goals.
An extended site testing campaign
(43 nights) has been recently completed above Mt. Graham (Masciadri et
al. 2010) using two instruments: the Generalized Scidar (GS) and
High Vertical Resolution Generalized Scidar (HVR-GS), a recent method
we recently proposed (Egner &
The GS is an optical
instrument that measures the scintillation maps produced by binary
stars on the pupil of the telescope (at least 1.5 m size). In this case
the GS has been run at the focus of the Vatican Adavnced Technology
Telescope (VATT). The optical
turbulence vertical distribution (CN2
profiles) is retrieved from the auto-correlation of
the scintillation maps. The binary stars have the following properties:
separation θ within the (3" - 14") range; m1, m2
≤ 5.8 mag;
≤ 1.5 mag. Wind speed vertical profiles can be retrieved from the
scintillation maps taken with a time lag of typically 20 - 40
Using the GS it has been possible
to retrieve the vertical distribution of the optical turbulence with a
resolution of ~ 1 km all along the 20 km. Using the HVR-GS it has been
possible to retrieve the turbulence spatial distribution in the first
kilometer from the ground with a vertical resolution of 25-30 m.
The astronomical site
Graham is located in Arizona (USA). A few moments of the ForOT
group activity on the summit can be find here.
Generalized Scidar runs
1. [28/5/2007 - 4/6/2007]: 8
2. [15/10/2007 - 28/10/2007]:
3. [21/2/2008 - 3/3/2008]: 11
4. [9/11/2008 - 20/11/2008]: 11
Fig 1-Left: Generalized
mounted to the focus of the Vatican Advanced Technology Telescope (D =
2m) at Mt. Graham.
Fig 1-Right: Temporal evolution
extended on the 20 km (whole
atmosphere) during a night .
The turbulence evolves in intermittent way in the low as well as
high atmosphere producing turbulence bumps at different heights in
different instants during the night.
INTEGRATED ASTROCLIMATIC RESULTS: STATISTICAL RESULTS
Fig 2: (from
et al. 2010): Cumulative
(43 nights) of four integrated
astroclimatic parameters. Top left: the
seeing in the total atmosphere (including the dome contribution). Top-right: the seeing in the free
atmosphere (h > 1 km). Bottom-left:
the isoplanatic angle. Bottom
right: the wavefront
coherence time. Thick lines: the whole
Dotted lines: summer time. Thin line: winter time.
Fig 3: (from
et al. 2010): Cumulative distribution of
dome seeing for all the 43 nights (thick line), the summer (dotted
line) and the winter (thin line) period.
Table 1: (from
et al. 2010): Median,
first and throd quartiles values of the main integrated astroclimatic
parameters above Mt. Graham (43 nights): seeing in the total
atmosphere (included the dome seeing), isoplantaic angle, wavefront
coherence time, integrated
equivalent wind speed.
that the median seeing in the whole atmosphere (included the dome
seeing) is ε = 0.95" and that the median dome seeing is εd =
follows that the median seeing related to the whole atmosphere without
the dome contribution for the richest statistic we collected so
nights) is ε = 0.72".
OPTICAL TURBULENCE VERTICAL
The optical turbulence distribution has been estimated with a
Generalized Scidar above Mt. Graham with a statistical sample of 43
et al. 2010): Median CN2
obtained with a sample of 43 nights and a Generalized Scidar
OPTICAL TURBULENCE VERTICAL
DISTRIBUTION AT HIGH RESOLUTION IN THE FIRST KILOMETER FROM THE GROUND
The optical turbulence vertical distribution has been estimated above
Mt. Graham in the first kilometer with a vertical resolution of 25-30 m
with a HVR-GS. The statistical
sample is constituted by 43 nights.
Fig. 5: (from
et al. 2010): Red
line: optical turbulence vertical distribution (J
profile, 50 % case)
above Mt. Graham in the first kilometer above the ground. J is
the integral of the CN2 on the thickness equal to h. For a precise
J (see Eq.9, Masciadri
et al. 2010). The vertical resolution is 25-30 m (HVR-GS). Black line: optical turbulence vertical
distribution above Mauna Kea (Chun et al. 2009) in the first 600 m
above the ground. The vertical resolution is in the range (15 m -
80 m). The higher resolution is obtained with LOLAS (Avila et al.
2008) in the first 60 m. The lower resolution is obtained with a SLODAR
(Wilson et al.
2009). At ~ 45 m from the ground, it is visible a local minimum above
Mauna Kea. This height corresponds, more or less, to the abrupt
detection break due to the sensitivity threshold from LOLAS (Table 3,
Chun et al., 2009). The evident huge vacuum zone between 600 m and 1 km
above Mauna Kea simply means that the turbulence is not measured in
this vertical slab. Conclusions: the
optical turbulence vertical distribution appears very similar above Mt.
Graham and Mauna Kea.
Using the HVR-GS it has been observed that the turbulence decreases,
sites in nightly stable conditions, in a much sharper way than what has
been believed and predicted so far by models (such as the Hufnagel
model). We discovered that
50% of the turbulence develops
typically in the first 80 m +/- 15 m from the ground. The
spatial structure of the turbulence as monitored by the HVR-GS appears
concentrated in thin
layers. It appears evident that the higher the vertical
of the instrument, the sharper is the decreasing of the optical
turbulence distribution with the height from the ground.
Fig. 6: (from
et al. 2010): Percentage of turbulence developed
between the ground and the height h with respect to the turbulence
developed in the whole atmosphere (~ 20 km) as retrieved from the HVR-GS
measurements and extended to the first kilometre.
Acknowledgments: This work is
funded by the Marie Curie Excellence Grant ForOT - MEXT-CT-2005-023878