ACONVEX, Manaus - AM

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The UV Raman Lidar is operational on the ACONVEX
site since July 2011. It uses a Quantel CFR-400 Nd-YAG
laser at 355 nm with 95 mJ per pulse and 10 Hz repetition
rate. Beam is expanded by 3 and final laser divergence is
0.25 mrad. The optical system uses a bi-axial setup with
a 400 mm separation between the cassegrain telescope
and the laser axis. The telescope’s primary mirror has
400 mm diameter, while the secondary has a diameter
of 90 mm. Focal length is 4000 mm resulting in a f/10
system. An iris is used at the focal plane which gives a
field of view of 1.75 mrad and an initial overlap at 85 m
and full overlap at 450 m.

No fiber optics are used and light passing through the iris
goes directly in the optical detection box. Interferomet-
ric filters separate the elastic back scattered signal and
the inelastic signals due to the raman cross-section of N2
(387 nm) and H2 O (408 nm) which are read collected in
different photo-multiplier-tubes. Signals from 355 and
387 nm are recorded in analog and photon-count modes,
while 408 nm only in photon count. The optical system
was designed to give an uniform signal on the cathode
surface almost independent of height of the detected sig-
nal. A neutral density filter is used to attenuate the elas-
tic signal avoiding saturation, and a good signal to noise
ratio (S/N) is found above 15 km depending on the at-
mospheric conditions. The N2 channel, 1-min average
signals have good S/N up to 15 km but only during night
time. For the H2 O channel, 1-min average signals have
good S/N only up to 6 km during night time.

The system is fully automated and includes a clock-
controlled shutter to cover the telescope and laser quartz
windows from direct sunlight exposure between 11 am
and 2 pm local time. As a backup system, a 10 mm
shutter is positioned just above the iris and kept in its
light-blocking position by a coil mechanism. Interlocks
are connected to the UPS and to the sun light sensor, a
small telescope with a 10o field of view around the large
telescope axis. Environmental conditions effects on the
electronics are minimized by continuously running an air
conditioning and a dehumidifier inside the instrumenta-
tion cabinet.