6 Conclusion

In this paper we have summarized the results of astroclimate studies at Maidanak observatory in Uzbekistan in 1996-1999, as well as some earlier data. The amount of clear night time hours and its distribution over the seasons for Mt. Maidanak is typical of Central Asian mountains. It is about 58% of available yearly dark time. In absolute units it corresponds to 2000 hours. But unlike Paranal where the seasonal variations of the available clear time are small, the summer observing capacity of Mt. Maidanak is two times higher than in winter (about 90% in July and up to 50% in February). These conditions appear very similar to La Silla, where the same 58% of yearly clear time has a maximum during the southern winter months.

A survey of the main parameter of atmospheric turbulence (seeing) was carried out at Mt. Maidanak for period from August 1996 to November 1999 with the DIMM, also used for site testing at ESO observatories of La Silla and Paranal in Chile. The last point allows us to characterize seeing conditions of Mt. Maidanak with an instrument calibrated at ESO observatories Pedersen et al.1988. From the whole data span, the mean value of seeing is 0.76 $^{\prime\prime}$, the median is 0.69 $^{\prime\prime}$. The best monthly median seeing, 0.62 $^{\prime\prime}$, was observed in November. A median seeing at Mt. Maidanak appears to be better than at La Silla and as good as at Paranal and Roque de los Muchachos observatories.

During the whole period of seeing measurements the temperature, wind velocity and direction were recorded on top of the 5 m high DIMM tower. A weak correlation of seeing with wind speed is found: the best seeing occurs with light wind up to 5 m/s.

In July 1998, a site testing campaign for the estimation of atmospheric parameters related to high angular resolution astronomical techniques was organized at Mt. Maidanak, by the common efforts of University of Nice, ESO, Sternberg Astronomical Institute and UBAI and under INTAS funding. During 9 nights the median wavefront outer scale ${\cal L}_0$ = 25.9 m and the median isoplanatic angle $\theta _0$ = 2.48 $^{\prime\prime}$ were measured with GSM. The corresponding median values of those parameters for La Silla, 25 m and 1.26 $^{\prime\prime}$ Martin et al.1998, Paranal, 22.0 m and 1.90 $^{\prime\prime}$ Martin et al.2000 and Cerro Pachon, 36 m and 2.8 $^{\prime\prime}$ Vernin et al.2000, confirm the suitability of Mt. Maidanak for high resolution observations. The median isoplanatic angle, presented in this paper, is in good agreement with the value of 2.30 $^{\prime\prime}$ obtained by Kornilov et al.2000 from stellar scintillation measurements at Mt. Maidanak in 1998 and 1999, and demonstrates the weak turbulence in the free atmosphere above Maidanak.

A specific feature of the atmosphere above Mt. Maidanak is a very low speed of wavefront motion. The wavefront coherence time $\tau_0$, estimated from GSM measurements, appears to be $\approx$ 12 ms, much larger than for Chilean observatories Sarazin1995,Vernin et al.2000.

The main contribution to the seeing at Mt. Maidanak is produced by the thick boundary layer. The surface layer is not important. On rare occasions when wind blows from the north, and due to the specific location of DIMM (13 m lower than the main 1.5 m telescope), the influence of the surface layer becomes noticeable.

Acknowledgements

The research presented in this paper was supported in part by INTAS-96-367 grant and NATO grant PST.CLG 975586. We are grateful to R. Sunyaev for his efforts in establishing efficient cooperation between ESO and UBAI on Maidanak site testing. We would like also to thank A. Aliev, K. Abdiev, T. Sadibekova, H. Sultanov for participation in the observations and technical assistance, then UBAI administrators A.U. Aliev and F. Uzakov for taking care of the instruments customs clearance and making things run smoothly. The important contributions of prof. F. Martin and R. Conan in GSM mission preparation and data reduction are acknowledged. D. Currie has kindly revised the English of this paper.