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Laser Guide Stars

Laser Guide Stars. b y Roberto Ragazzoni INAF – Astronomical Observatory of Padova (Italy) r oberto.ragazzoni@oapd.inaf.it. What are Laser Guide Stars?. They are artificially generated sources of light.. Light is pumped into the atmosphere from the ground…

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Laser Guide Stars

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  1. Laser Guide Stars by Roberto Ragazzoni INAF – Astronomical Observatory of Padova (Italy) roberto.ragazzoni@oapd.inaf.it

  2. What are Laser Guide Stars? • They are artificially generated sources of light.. • Light is pumped into the atmosphere from the ground… • …in some way they produces some lights that fly back to the ground… • …and this is perceived as a sort of artificial star that can be used as reference for Adaptive Optics.

  3. Rayleigh vs. resonant scattering Only specific atoms Excited at a specific wavelength All particles and aerosols in the atmosphere

  4. Problems of LGSs • Tip-tilt indetermination problem • Conical anisoplanatism • Focus at a finite distance • Rayleigh fratricide effects • Actual distance depends upon altitude and layer local variations • Cyrrus can make large scattered light • Aircraft and satellite hazards • You need a working laser!

  5. UP tilt DOWN tilt

  6. Pulsed vs. CW lasers… • Rayleigh positively needs pulsed lasers and a gating system synchronized with the laser • Sodium lasers has to comply with saturations of Sodium layer so CW are superior, in general.. • Be warned that Lasers can have micro and macro pulses…

  7. Beam expander exit and folding flat Launch mirror Wavefront sensor Laser units Laser platform Laser & Launch system • System layout • Laser source availability • Launch layout • Mechanical layout • Electronics layout LBT Laser Phase A Review

  8. A dye laser…

  9. A dye laser…

  10. A z-invariant WFS…

  11. Polychromatic LGS • A combination of wavelengths close one to each other is sent up… • ..the resonant scattering pumps up some electrons level such that in the decay two very different wavelengths are sent back… • So the way-UP is wavelength independent while the way-DOWN is not… • …a little signal with absolute tilt information.

  12. Laser Guide Stars… • Are now in operation at Keck, Gemini and VLT… • Science is mostly with spectroscopy… • Some telescopes are more effective than others… • Foreseen in various telescopes • Tilt and low orders is made through NGSs • Still sky coverage is not 100% and some significant FoV is required… • …MCAO with NGSs…????

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