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Building a LIDAR for CTA

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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Building a LIDAR for CTA

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  1. Building a LIDAR for CTA

  2. What is the new “thing” at IFAE?

  3. 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

  4. Shower development From PAO

  5. 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

  6. 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

  7. 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

  8. LIDAR distance measurements

  9. 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

  10. 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 Photo-detector We measure the amount of backscattered light as a function of distance to the LIDAR

  11. The LIDAR equation • Some assumptions have to be made to solve the equation • Klett inversion has associated systematic uncertainty of around 30% Backscattering coefficient Rayleigh->Molecular Mie->Aerosol Extinction Coefficient Ozone Aerosol Clouds

  12. Typical response What’s this? Cloud, aerosol,…? Clean atmosphere Attenuation, when, why?

  13. 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

  14. Raman vs Rayleigh

  15. Aerosol coefficients extraction

  16. 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!

  17. CLUE @ LP

  18. Open CLUE container • Fully robotized lids, “petals” and telescope frame • Easy to transport • One still in La Palma

  19. CLUE Telescope Multiwire proportional chamber filled with C4H11NO Telescope d=1.8 m f/d=1 High FOV Excellent luminosity Big hole in the center Electronics behind mirror

  20. 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

  21. Telescope frame • Mechanical model redone from scratch • Finite elements simulation

  22. Laser • 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?

  23. 1st stage 2nd stage What else? Optical setup Can get very complicated!!! (from UPC)

  24. 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?

  25. 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 -> 3.75 m per sample • 30 Km -> 4000 samples memory • Dynamic range >16 bits (20 bits) • For rates of 1Khz, many channels ~50MB/s.

  26. 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.

  27. FIN

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