Photometric observations of a polychromatic laser guide star.
We report the photometric observation of a polychromatic laser guide star (PLGS) using the AVLIS laser at the Lawrence Livermore National Laboratory (LLNL). The process aims at providing a measurement of the tilt of the incoming wave front at a telescope induced by atmospheric turbulence. It relies on the two-photon coherent excitation of the 4D5/2 energy level of sodium atoms in the mesosphere. We used two laser beams at 589 and 569 nm, with a maximum total average output power of approximately 350 W. For the purpose of photometric calibration, a natural star was observed simultaneously through the same instrument as the PLGS at the focus of the LLNL 50-cm telescope. Photometric measurements of the 330-nm return flux confirm our previous theoretical studies that the PLGS process should allow us at a later stage to correct for the tilt at wavelengths as short as approximately 1 microm at good astronomical sites. They show also that, at saturation of two-photon coherent absorption in the mesosphere, the backscattered flux increases by a factor of approximately 2 when the pulse repetition rate decreases by a factor of 3 at constant average power. This unexpected behavior is briefly discussed.
Foy
R
R
Centre de Recherche Astronomique de Lyon/Observatoire de Lyon, France. foy@obs.univ-lyon1.fr
Tallon
M
M
Tallon-Bosc
I
I
Thiébaut
E
E
Vaillant
J
J
Foy
F C
FC
Robert
D
D
Friedman
H
H
Biraben
F
F
Grynberg
G
G
Gex
J P
JP
Mens
A
A
Migus
A
A
Weulersse
J M
JM
Butler
D J
DJ