ИСТИНА

We present results of study of seeing and atmospheric extinction at Mount Maidanak Observatory. Mount Maidanak Observatory at Ulugh Beg Astronomical Institute of Uzbek Academy of Sciences located in the South Uzbekistan at an altitude of 2600 m. We have determined the realistic seeing of the 1.5 m AZT-22 telescope of the Maidanak Observatory using more than 20 000 CCD frames with stellar images in the UBVRI bands acquired in 1996–2005. In total, we reduced data for 440 nights between July 1996 and November 2005. The median seeing, εmed, ranges from 1.23 arcsec in U to 0.91 arcsec in I. The seeing in V varies from 0.5 to 2.4 arcsec, with ε(V)med = 1.065 arcsec. During the entire observation period, ε(V)med was better than 0.7 arcsec on only two nights (0.5%) and better than 0.8 arcsec on 31 nights (7%). Median seeing reduced to unit air mass, ε(V)med(M(z)=1) = 0.945 arcsec. We find that the seeing did not change during the studied 10 yr period, on average. The range of ε(V)med was 0.97 arcsec (2005) - 1.15 arcsec (1999). The best seeing is achieved in Autumn (with ε(V)med = 1.01 arcsec for October observations), while the worst seeing is obtained in the Winter-Spring (with ε(V)med reaching 1.44 arcsec in April). To estimate the time for relaxation to stationary atmospheric conditions under the AZT–22 dome with the ventilation on, we analyzed the variations of the CCD V and R seeing during the course of a night for eight nights with ε(V)med ≤ 0.85 arcsec. The results show that the time scale for relaxation to stationary conditions under the dome is 2-2.5 hours after the end of astronomical twilight. Simultaneous seeing estimates with the differential image motion monitor (DIMM) and at the AZT–22 were used to estimate the influence of the space under the telescope dome. The seeing in the free atmosphere differs from that under the dome by about 0.1 arcsec. The difference between the DIMM and AZT–22 estimates remains constant. Atmospheric extinction coefficients in different photometric bands were also determined. The mean V band atmospheric extinction is 0.20±0.04 mag.