Atmospheric Extinction Derived from Cometary Observations

Abstract

Special filters for photoelectric photometry of comets enable the study of atmospheric extinction not only in the broad band photometric systems but also in the intermediate band IHW/IAU system covering wavelengths from 365 to 514 nm. Extinction coefficients, which are characteristics of an atmospheric state, are by-products of observations of comets. Contrary to the observation of variable stars, comets have to be very frequently observed a few degrees above the horizon and so wide intervals of air masses are used in the calculation of characteristic values. Extinction measurements obtained during the observations of bright comets 1P/Halley in 1985/86, 23P/Brorsen-Metcalf in 1989 and C/Austin in 1990 are used in this study. 78 measurments in 21 observational nights cover all the parts of year with the exception of spring with cloudy weather in our mountain site and June with the shortest observational nights. Average extinction coefficients are derived in the paper and some conclusions of common validity (in central European conditions) are formulated. The results of IHW/IAU extinction study have been compared with the results of UBV extinction coefficients gained mostly as a by-product of the photoelectric photometry of variable stars (407 nights in the period from 1962 to 1995). Seasonal variations of atmospheric extinction are discussed.