The new sodium laser is used to excite fluorescence of a thick layer of sodium atoms which are typically observed between 80 and 100km above sea level.
The wavefront distortions caused by the turbulent atmosphere in the returned laser light were measured using CANARY 150 times a second,
and are compared to
the distortions measured from a constellation of background stars.
Unlike other LGS systems in operation, the laser is situated 40m off-axis from the WHT, recreating the LGS geometry expected for
the upcoming
European Extremely Large Telescope with the WHT simulating a 4.2m diameter
segment of this giant telescope.
Recreating this geometry is important because the
variable thickness and density of the sodium layer means that the LGS suffers from 20-30 arcseconds of perspective elongation in the
CANARY wavefront sensors.
LGS systems on existing 8-10m
class telescopes typically observe 2-5 arcseconds of elongation, which is inside the isoplanatic patch size of the atmosphere.
The first generation of instruments proposed for the E-ELT
all rely on adaptive optics system to fulfil their scientific goals, and errors in the wavefront sensing coming from the
highly-elongated LGS will affect performance. There are several proposed methods for wavefront sensing from such an elongated LGS,
but these have never been investigated on-sky in a real-world environment.
