4 Conclusions

The results shown here, in spite of their preliminarity and somewhat roughness of their derivation, are showing a scenario where LGSs are no longer necessary for giant telescopes. The borderline lies somewhere between few tens of m to one hundred meters, depending upon several assumptions. The exact threshold diameter depends upon a number of seeing and performance requirement parameters, and it is hard to outline further conclusions without assuming some specific case study. It is also noticeable that multiple wavefront sensing is still in its infancy and it cannot be excluded that some substantial improvement can be obtained by some novel technique. What are fundamental limits of this technique and is it possible that the threshold diameter could make this technique an attractive alternative even for current 8 m class telescopes is beyond the scope of this paper but it is, of course, a very interesting and somewhat fascinating issue to be attacked urgently.

Acknowledgements

Special thanks are due to F. Rigaut for useful discussions on the subject of this paper. Thanks are due for the several discussions on related topics to the 8 m Laser Guide Star European Training and Mobility Resources Network of the European Community. Many thanks also to M. Northcott for his suggestions and advices.