Eliminating perspective elongation for LGS based AO-systems at ELTs

Laser Guide Star Adaptive Optics systems become in the moment reality at several 8...10m class telescope facilities. At these aperture diameters all effects, arising from the finite distance and vertical extend of the artificial excited guide star can still be neglected for the wavefront sensing process. This is changing completely when the telescope diameter become further increased as it is the case of next generation Extremely Large Telescopes. Effects such as perspective elongation become dominant and impact negatively the wavefront sensing process. This will result in severe quality restrictions for next generation Adaptive Optics systems. In this paper we want to introduce and explain in detail a novel kind of wavefront sensing techniques, optimized to overcome perspective elongation. Its basic idea is to project the generated LGS virtually to infinity. We will give a theoretical introduction and some background information about the proposed sensing concept and introduce a practical solution for a potential optical setup. The latter is based on inverting the so called Bessel Beam concept, producing diffraction less beam over a certain finite range. Furthermore we will characterize the new sensor type, compare the senor efficiency to alternative approaches and discuss finally both its advantages and disadvantages.