Abstract:
The atmospheric extinction coefficients in Johnson U, B, V, R and I photometric passbands are determined for the first time at Devasthal, Naini Tal during 1998 and 1999. A solid state stellar photometer mounted on a 52-cm reflector was used for the measurements. The minimum atmospheric extinction was observed on Jan 21, 1998 with the values of coefficients 0.38, 0.22, 0.12 and 0.06 mag in U, B, V and R respectively. However, mean values of the coefficients during the observations are 0.49±0.09, 0.32±0.06, 0.21±0.05, 0.13±0.04 and 0.08±0.04 mag in U, B, V, R and I respectively. A comparision of the atmospheric extinction observed at Devasthal with those at other optical astronomical sites in India as well as abroad indicates that Earth's atmosphere at Devasthala site is fairly transparent and is suitable for precise photometric observations. The observed extinction values also agree fairly well with those expected due to molecular absorption, Rayleigh and aerosol scatterings by molecules and dust particles present in the Earth's atmosphere at an altitude of ~ 2.45 km. Variation in aerosol content of the Earth's atmosphere seems to produce the observed night to night variation in extinction at Devasthal.
