Building a LIDAR for CTA

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Building a LIDAR for CTA
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What is the new thing at IFAE?
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Imaging Atmospheric Cherenkov Technique

• PHYSICS OF SHOWERS
• Cosmic rays and gammas impinge the atmosphere
• Electromagnetic cascades
• e-e pairs
• bremsstrahlung
• Cherenkov radiationand Hadronic cascades
• pions and muons
• Typical Cherenkov signal is bluish light with few
  ns duration

Particle shower
10-20 km
1o
Cherenkov light cone
120 m
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Shower development
From PAO
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Atmosphere in IACT

• Atmosphere is actually a part of the detector
• Need to characterize it for accurate
  measurements
• Atmospheric Profile
• Can change seasonally
• Affects first interaction point and Cherenkov
  yield for a given shower.
• Can be measured with Radiosondes.
• Aerosols
• High level (e.g. clouds) can occur around
  shower-max and so affect Cherenkov yield image
  shape etc.
• Low Level (near to ground level) which act as a
  filter, lowering the Cherenkov yield.
• Can be measured with LIDARs

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LIDAR

• Light Detection and Ranging
• Same name, many different applications
• Industry Automation, vehicle cruise control,
  video clips, traffic monitoring
• Geology Elevation models, terrain surveys
• Military Long range 3D imaging, missile guiding
• Nuclear physics Density profile of fusion
  reactors plasma
• Astronomy Distance to moon, relativity
  measurements
• Meteorology

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Basic LIDARs

• Mainly used to measure distances
• Pretty common use
• A short pulse is emitted and backscattered
• Distance is proportional to time between emission
  and reception
• Low energy laser, high rate
• Single / dual axis mirror systems

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LIDAR distance measurements
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Extended LIDARs

• LIDAR technique is continuously evolving
• Coherent detection
• Optical heterodyne techniques
• Inelastic scattering
• LIDARs can measure many things
• Distance
• Speed
• Rotation
• Chemical composition and concentration

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LIDAR for atmospheric measurements

• A short light pulse is emitted to the atmosphere
• A portion of the light is scattered back toward
  the lidar system
• The light is collected by a telescope and focused
  upon a photo detector.

Laser source
Photo-detector
We measure the amount of backscattered light as
a function of distance to the LIDAR
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The LIDAR equation
Backscattering coefficient Rayleigh-gtMolecular Mie
-gtAerosol
Extinction Coefficient Ozone Aerosol Clouds

• Some assumptions have to be made to solve the
  equation
• Klett inversion has associated systematic
  uncertainty of around 30

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Typical response
Whats this? Cloud, aerosol,?
Clean atmosphere
Attenuation, when, why?
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Inelastic scattering Raman

• Not all scattering is elastic
• In some cases molecules change their vibrational
  and/or rotational state (Raman process), adding
  or absorbing part of photons energy
• Shift on the wavelength of scattered light,
  depending on molecule states
• Raman nitrogen/oxygen signals can be used to
  retrieve aerosol extinction coefficients with low
  uncertainty
• Cross section for Raman is orders of magnitude
  smaller than elastic
• Powerful lasers, large telescopes, efficient
  detectors and photon counting are required

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Raman vs Rayleigh
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Aerosol coefficients extraction
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CLUE experiment

• Old experiment in La Palma, sharing space with
  HEGRA
• Aim to measure matter/antimatter ratio in cosmic
  radiation observing the Cherenkov light produced
  by air showers
• Not a big success
• But can be recycled for a Raman LIDAR!

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CLUE _at_ LP
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Open CLUE container

• Fully robotized lids, petals and telescope
  frame
• Easy to transport
• One still in La Palma

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CLUE Telescope
Multiwire proportional chamber filled with C4H11NO
Telescope d1.8 m f/d1 High FOV Excellent
luminosity Big hole in the center
Electronics behind mirror
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CLUE good / bad things

• ?
• Robotized housing for the LIDAR
• Motorized telescope frame with big mirror
• Space for electronics on the same frame
• ?
• Mirror may be even too big and in not so good
  shape
• Obsolete control electronics
• Almost no written documentation
• Tons of things to do, few experience

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Telescope frame

• Mechanical model redone from scratch
• Finite elements simulation

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Laser

• Raman LIDAR usually use NdYAG lasers 355 nm
  (tripled)
• Plan to buy one with adjustable power and firing
  rate for development.
• Two possible locations
• Installed on the center of the mirror, on the
  other side of the hole
• On the focal plane, behind photodetector / fiber
• Photosensor near laser to read the actual power
  and length of each pulse
• Powerful lasers and airports do not mix well
• Authorization required?

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Optical setup
Can get very complicated!!!
(from UPC)
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Optical setup II

• Build custom mechanical pieces for compact and
  precise optical setup.
• Fiber and setup are attached to the telescope
  frame, no relative movements.
• Easily extendable to receive extra wavelengths.
• Use narrow-band filters or diffraction grating?

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Readout

• Raman signal is much smaller than Rayleigh
• Dual DAQ systems standard digitization for low
  altitudes (big signals) and photon counting for
  extended range.
• DAQ with high dynamic range and fast data
  transfer, but not a lot of BW needed
• 40 MHz sampling rate -gt 3.75 m per sample
• 30 Km -gt 4000 samples memory
• Dynamic range gt16 bits (20 bits)
• For rates of 1Khz, many channels 50MB/s.

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The future

• Motor control for telescope movement and
  container aperture
• Ethernet based motor driver already in
  development
• Waiting for the container to know specific motor
  requirements
• Decide on a Laser, create control SW/HW.
• Decide on sensors, order components and build
  optical setup.
• Clean the telescope mirror, verify optical
  characteristics and modify mechanical structure
  to adapt to laser, optical setup and DAQ.
• Design/Order Acquisition HW and SW.

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FIN