building a lidar for cta
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Building a LIDAR for CTA. What is the new “thing” at IFAE?. Imaging Atmospheric Cherenkov Technique. PHYSICS OF SHOWERS Cosmic rays and gammas impinge the atmosphere Electromagnetic cascades e-e+ pairs bremsstrahlung Cherenkov radiation and Hadronic cascades pions and muons

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Presentation Transcript
imaging atmospheric cherenkov technique
Imaging Atmospheric Cherenkov Technique
  • 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



~ 10-20 km

~ 1o

Cherenkov light cone

~ 120 m

atmosphere in iact
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
  • 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
basic lidars
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
extended lidars
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
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.

LIDAR for atmospheric measurements

Laser source


We measure the amount of backscattered light as a function of distance to the LIDAR

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

Backscattering coefficient



Extinction Coefficient




typical response
Typical response

What’s this?

Cloud, aerosol,…?

Clean atmosphere

Attenuation, when, why?

inelastic scattering raman
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 photon’s 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
clue experiment
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!
open clue container
Open CLUE container
  • Fully robotized lids, “petals” and telescope frame
  • Easy to transport
  • One still in La Palma
clue telescope
CLUE Telescope

Multiwire proportional chamber filled with C4H11NO


d=1.8 m


High FOV

Excellent luminosity

Big hole in the center

Electronics behind mirror

clue good bad things
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
telescope frame
Telescope frame
  • Mechanical model redone from scratch
    • Finite elements simulation
  • Raman LIDAR usually use Nd:YAG 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?
optical setup
1st stage

2nd stage

What else?

Optical setup

Can get very complicated!!!

(from UPC)

optical setup ii
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?
  • 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 -> 3.75 m per sample
    • 30 Km -> 4000 samples memory
    • Dynamic range >16 bits (20 bits)
    • For rates of 1Khz, many channels ~50MB/s.
the future
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.