Astronomical Observing Conditions at the Xinglong Station in 1995–2001

This paper summarizes the measurements of astronomical observing conditions with a fixed pointing to the North Pole over 6 years at the Xinglong Station, where the BATC Multicolor Sky Survey has been conducted since 1995. Three major effects that influence the sky quality the most have been studied: the atmospheric transparency, the sky brightness, and the seeing. No obvious annual variation of the transparency was detected, but seasonal changes on a regular basis have been observed. The transparency is better and more stable in the winter than in the summer, which points to the fact that winter is the season for precise photometric work in Xinglong. The relations of sky brightness versus lunar phase and versus lunar altitude are derived and discussed. We find, on average, that the value of sky brightness in moonlight-corrected V magnitude of the North Pole field is about 21.0 mag arcsec 2. The sky is darker in the fall and winter than in the spring and summer, and the transparency is better. It is conjectured that there are fewer particles in the air in the winter, thus both absorption and scattering are minimal. The seeing in the winter is worse than in the summer. This is probably due to the gusty wind from the northwest in the winter. The results of the North Pole monitoring presented in this paper show that the Xinglong Station is an astronomically good observing site. The North Pole observations will continue as a part of the BATC sky survey.