PENETRATING THE FOG - CORRECTING GROUND-BASED CCD SPECTROSCOPY FOR TELLURIC ABSORPTION

A method has been developed to correct for the effects of telluric abs orption on CCD spectroscopy in the visible and near-infrared, and is a pplied specifically to near-infrared spectroscopy of extragalactic H I I regions. The technique involves calculating the detailed telluric sp ectrum in the vicinity of emission or absorption lines using the data base of molecular absorption lines HITRAN92 (1992). A comparison of ca lculated absorption bands with those measured in observations of metal -poor 'telluric reference' stars enables the quantity of water vapour to be self-consistently set. With the reasonable assumption that the s ame telluric band strengths occur in object data collected at similar times and locations in the sky, telluric effects can be determined and removed line by line. It is shown that, if a nebular emission line is narrow enough, it can be massively depleted by chance coincidence wit h a single telluric line. A popular technique for 'correction' of tell uric absorption in astronomical CCD spectroscopy is to divide the abso rption-affected object spectrum by the continuum-normalized spectrum o f a 'telluric reference' star. This is shown to be an adequate approac h only for sources with velocity dispersions greater than about 40 km s-1 for data where the telluric lines themselves are not cleanly resol ved. For many objects of interest, however, such as the great majority of extragalactic H II regions, velocity dispersions are commonly less than 25 km s-1, so a more careful treatment is necessary to effect an accurate correction. As an example, the method is applied to the comp act H II region SMC-N9.