  Laser guide stars with adaptive optics allow astronomical image correction in
the absence of a natural guide star. Single guide star systems with a star
created in the earth's sodium layer can be used to correct the wavefront in the
near infrared spectral regime for 8-m class telescopes. For possible future
telescopes of larger sizes, or for correction at shorter wavelengths, the use
of a single guide star is ultimately limited by focal anisoplanatism that
arises from the finite height of the guide star. To overcome this limitation we
propose to overlap coherently pulsed laser beams that are expanded over the
full aperture of the telescope, traveling upwards along the same path which
light from the astronomical object travels downwards. Imaging the scattered
light from the resultant interference pattern with a camera gated to a certain
height above the telescope, and using phase shifting interferometry we have
found a method to retrieve the local wavefront gradients. By sensing the
backscattered light from two different heights, one can fully remove the cone
effect, which can otherwise be a serious handicap to the use of laser guide
stars at shorter wavelengths or on larger telescopes. Using two laser beams
multiconjugate correction is possible, resulting in larger corrected fields.
With a proper choice of laser, wavefront correction could be expanded to the
visible regime and, due to the lack of a cone effect, the method is applicable
to any size of telescope. Finally the position of the laser spot could be
imaged from the side of the main telescope against a bright background star to
retrieve tip-tilt information, which would greatly improve the sky coverage of
the system.
