Astronomy and Meteorology III
Sensors mounting and powering up
F. Buffa, G.C. Cocco, I. Porceddu and M. Serrau
Stazione Astronomica di Cagliari
Foreword
In the two previous reports (TNG Newsletter no. 8 and 9) of the Galileo
telescope Astronomical Weather team - a branch of the Site Testing
Group - we discussed about the TNG approach and the interdisciplinary
activity we were starting. This progress report wants to be a short
communication concerning the successful installation of the meteorological
instrumentation on the meteo tower, a positive step ahead towards
the site monitoring. A forthcoming Technical Report will follow, carefully
describing the data collection and handling status. Here we shortly
report about hardware operations (sensor mounting and cabling) and
data logging procedure.
Introduction
Meteorological conditions are a de facto constraint on the
telescope observational capabilities: relative high wind speed and
relative humidity values can actually limit or halt the telescope.
Therefore a set of reliable basic meteorological parameters must be
available in order to support the observational facilities. Additional
sensors may also be installed, near by and inside the dome, if dome
and local seeing conditions are to be monitored.
The TNG project decided, in Phase 1, to build a robust meteo tower
which would be able to resist to the harsh conditions which are not
uncommon in winter at the Roque de Los Muchachos Observatory. The
meteo tower of the TNG is an iron-made structure, designed by Dario
Mancini, from the Capodimonte Astronomical Observatory (Naples). The
total height is 15 meters, which is the approximate height above the
ground of the TNG M1 mirror. The meteo tower, which is about 30 meters
far away from the DIMM, did not suffer any damage during last winter,
when lighter and less expensive structures crashed.
Sensors
The foreseen and now available initial set of sensors will provide
these basic meteorological measurements:
- Temperature. The temperature sampling is available
at four different locations (ground, 2, 5 and 10 meters above ground),
in order to have an extimate of the ground layer local gradient. We
use a Pt100 standard sensor for the temperature, with an accuracy
of 0.1 Celsius degrees.
- Relative humidity. This measurement is done at
2 meters, but in a further step more heights will be sampled; the
sensor, a Humicap Vaisala, performs with an accuracy of 0.1 % of RH.
- Atmospheric pressure measurement is obtained with
a Vaisala PTB200 digital barometer, having an accuracy of 0.01 hPa;
- Wind speed and direction. A special device is
used for these measurements, a Gill ultrasonic anemometer, mounted
on the top of the tower (a bit less than 15 m above ground). This
instrument shows two special features: a) it is able to provide
a 3-D wind field, i.e. the upwelling convective flux can be detected.
The aim is to monitor the air flux at the primary mirror height; b) it
can sample wind components at high rates, just over 10 Hz;
Data logging and realtime transmission
Sensor sampling and data logging is driven by a Vaisala QLC50 datalogger,
a fully programmable system with a onboard PCMCIA memory card for
data storage: the QLC50 can actually store up to 20 megabytes of data.
The memory card is only used as a local backup, where up to six months
of data can be stored with the actual data sampling setup.
Due to the close interaction between meteo and seeing measurements
and the need for dataflow optimization, realtime data communication
from the telescope to the meteo tower is done by mean of a software
routine which is part of the DIMM controller. The physical connection
between meteo tower and DIMM is accomplished through an optical fiber
link.
Power supply
The Astronomical Weather Station (AWS) will not be connected to the
TNG power supply network but will be powered by a solar panel, with
a backup pack of batteries. They should allow the AWS being able to
collect and send data without solar panel contribution for about one
month. The solar power supply choice should avoid most of the problems
which are related to the lightning features, and it will guarantee
a continuous monitoring of the data. The solar panel has been located
at about 6 meters above ground.
Mounting the instrumentation
The present safety regulations do not allow the meteo tower being lowered
down for maintenance operations. Therefore, the whole set of mounting
operations has been accomplished with the tower in vertical position,
as can easily seen from the enclosed photos. Special tools, normally
used for climbing, have been used in order to guarantee the maximum
level of working security for our team.
Operations have started on Monday, 7th of April, and the instrumentation
was finally mounted on Thursday, 17th of april. A few days have been
lost due to the extremely bad weather conditions we found. Drill and
other tools have been powered from the chillers section, which
is about 40 meters away from the tower. Due to the constraints
we had on our back trip to Italy, we did not able to perform any test
on the batteries charge/discharge duty cicle. Nevertheless, we had
the opportunity to verify the correct behaviour of the whole instrumentation
and to check the data logging.
Final remarks
The TNG meteo tower, will be finally tested soon. After checking the
power supply facilities, we will start the commissioning phase and,
hopefully, the TNG meteo data series will begin.
Meteo Tower Image
