Laser interferometry guided adaptive optics for large telescopes

Novel laser guide star techniques are required if one wishes to avoid the technological and computational hurdles of implementing today's LGS designs on extremely large telescopes. Similar difficulties arise for correction at shorter wavelengths, or for multi-conjugate systems on 8-m class telescopes. To overcome many of the limitations, we propose to overlap coherently pulsed laser beams that are expanded over the full aperture of the telescope and hence travel upwards along the same path that light from the astronomical object travels downwards. Imaging the resultant interference pattern, and making use of the two polarization states, we are able to use phase shifting interferometry to retrieve the local wavefront gradients along both axes simultaneously. The technique can be generalized for shorter wavelength or wide field correction, and is applicable on any size of telescope. In this contribution we describe our experimental laboratory test-bed which we will use to verify our theoretical expectations, and to resolve a number of practical issues associated with implementation on a telescope.