1 Introduction

Figure 1: Maidanak Observatory layout

Maidanak Observatory (longitude 66 $^{\circ}56'$ E, latitude 38 $^{\circ}41'$ N) is located at the south-east of the Republic of Uzbekistan at a distance of about 120 km south of Samarkand. It lies on the spurs of the Pamir and Alai mountain system at 2600 m above sea level. The landscape of the observatory is mainly smooth and rock outcrops do not prevent from building over the whole 30 hectares of its territory. The vegetation of Mt. Maidanak is typical of high-mountain dry sub-tropics, bushes being prevailing. This summit was selected for astronomical observations in the late 60-s as a result of a ten-year long site assessment campaign organized at the Ulugh Beg Astronomical Institute (UBAI) of the Uzbek Academy of Sciences by V.S. Shevchenko and his co-workers Slutsky & Khetselius1970,Kardopolov et al.1971,Shevchenko1973. The crucial role of V.S. Shevchenko in the development of Maidanak Observatory must be stressed here.

In the early 70-s, when the high-quality atmospheric conditions of Mt. Maidanak have been established, the summit was occupied by a Satellite Laser Ranging (SLR) station Scott1995. Nevertheless, UBAI astronomers were allowed to continue astronomical observations with two 60-cm and one 48-cm telescopes in the neighborhood of SLR station. Meanwhile the Moscow State University (MSU) and a few other Soviet Union institutions had erected their astronomical facilities at a neighboring summit situated 5 km to the west of Mt. Maidanak. So, there are actually two "Maidanak'' summits - the Eastern, hosting SLR station and UBAI Observatory, and the Western, hosting all other observatories. By the early 90-s about 10 telescopes as well as the corresponding observatory infrastructure (roads, buildings, mechanical shops, etc.) were equipping the Western summit. At present all astronomical facilities of both summits belong to UBAI and are operated in the framework of scientific agreements between UBAI and MSU, Ukrainian and Lithuanian Astronomical Institutions. Hereafter we call "Maidanak Observatory'' the Western summit where all the site testing studies described below have been carried out. The layout of Maidanak Observatory with the location of its telescopes is shown in Fig. 1.

The studies of the Maidanak astroclimate continued through the 80-s and up to the beginning of the 90-s in close cooperation between UBAI and MSU astronomers Artamonov et al.1987,Shcheglov & Gur'yanov1991. They provided a detailed meteorological information, such as the amount of clear time, temperature variations, wind speed, humidity and their seasonal distribution Novikova1970,Gladyshev & Shirokova1987. The measurements of extinction coefficient Zheleznyakova1984, sky background Kardopolov & Philip'ev1979, as well as comprehensive optical and aerophysical studies Gur'yanov et al.1992 (hereafter Gur92) were carried out as well. Measurements of the seeing, a basic parameter of the atmosphere of an astronomical site, have been repeatedly conducted at Mt. Maidanak by several groups of researchers using different methods and instruments Shevchenko1973,Artamonov et al.1987,Shcheglov & Gur'yanov1991,Ilyasov et al.1992,Gur'yanov et al.1992.

All these studies indicated that Mt. Maidanak was one of the most favorable sites worldwide for astronomical observations. Not only this fact, but also an exceptional geographical location at a mid-distance between the main international astronomical facilities of Chile, Hawaii and Canaries, make Mt. Maidanak very attractive for observational programs requiring continuous monitoring of astronomical objects. By now several observational programs using focal instrumentation received from advanced western institutions are already being carried out at Mt. Maidanak Schulte-Ladbeck1997,Herbst1999,Nelsen1999.

However, for many observational programs it was necessary to establish whether atmospheric conditions of Mt. Maidanak match the requirements of modern high-resolution astronomical observation. In this paper we present the results of the statistical analysis of seeing measurements carried out at Mt. Maidanak with the ESO DIMM during the period from August 1996 up to October 1999 (Sect. 2). In Sect. 2 we present also a comparison of the results obtained in this work with seeing conditions at ESO Observatories of La Silla and Paranal in Chile and at Roque de los Muchachos Observatory (ORM) at La Palma.

When the high-quality atmospheric conditions of Mt. Maidanak have been established with a modern instrumentation, the next step was to measure other atmospheric parameters relevant to adaptive optics and interferometry, such as wavefront outer scale, isoplanatic angle, high altitude wind. In July 1998 during a site testing campaign, supported by INTAS, all these parameters were measured with the Generalized Seeing Monitor (GSM) developed at the University of Nice Martin et al.1994. A brief description of the method used and results of the campaign, as well as cross-calibration of DIMM and GSM, are presented in Sect. 3. In Sect. 4 general meteorological information, estimation of wind speed and direction at different levels and correlation of seeing with ground wind speed are presented. In Sect. 5 the relative contribution of the different atmospheric layers to the seeing at Mt. Maidanak is discussed. The summary and final remarks are given in Sect. 6.