ao - ForOT Optical Turbulence Forecasts

SODIUM (& RAYLEIGH) “LASER GUIDE STARS” (or “Laser Beacons”)
Keck Observatory
Uses Na-lasers at 589 nm wavelength
Scattered radiation on 95 km high atomic Sodium
layer creates artificial star of about 1 arcsec size
This “Laser Guide Star”, or LGS, is not at infinity.
Therefore (i) its focus is behind the nominal focus
of the telescope and (ii) the light received is a
cone rather than as cylinder (“Cone Effect”)
The Cone Effect requires multiple LGSs to infer
the wavefront coming from the actual star
Laser power limits (~ 10W) the LGS brightness to
V ~ 9 (depends strongly on laser properties and
on Mesospheric Na density)
Laser is best transmitted from
the telescope center to minimize
the elongation of the LGS due
to the 10 km Na-layer thickness
Rayleigh Scattering
laser location
LGS perspective elongation
Keck