5 Conclusion

Table 1 summarises the actual knowledge on the study of the mesospheric sodium layer for LGS-AO systems. The first column compiles the LGS parameters, and Col. 2 the "corresponding'' Na characteristics. The last column indicates ways of studying these characteristics.

Spectroscopic studies like the one presented in Sect. 3 are perfectly suited for statistical studies of mesospheric sodium column density. They allow to establish the minimum Na column density and thus to derive the laser power required to produce a LGS of specified magnitude. The interest of monitoring the sodium on a daily basis, therefore, is to give an indication of the expected LGS-AO correction. A comparison with the actual correction is also a way of checking on the system performances.

Due to their adverse effect on the stability of the mesospheric sodium centroid height, it is desirable to establish the statistics of the sporadics'appearance. If, as Kwon et al. (1988), one finds a LST slot where a huge majority of these sporadics appear for the observing site, LGS-AO observations can then be programmed outside this window. It is also very important to determine the time scale of these altitude variations. Indeed, if changes in focus, due to sporadics, happen on time-scale bigger than $\approx$ 10 min, one could then correct for it by recording the defocus on the wavefront sensor and averaging over few minutes to determine the correction needed. If however, as it seems from lidar observations, changes occur on seconds to few minutes time scale, one then needs a separate monitor to identify these periods. The previously mentioned way of correcting the defocus is not possible anymore since on short time scale one does not know how much of the defocus is due to the atmosphere and how much is linked to the motion of the sodium layer itself. Of course, what can be called the boundary between "mid-'' and "short-'' term variations depends on the defocus rms accepted by the AO systems used.

Acknowledgements

NUIG and ESO are part of the European Union (EU) TMR Network "Laser Guide Star for 8 m-class Telescopes'' (contract No. FMRX-CT96-0094). NA is grateful to the EU for financial support. The ESO NTT team is also thanked for its support during the recent observations and especially O. Hainaut.