Sky subtraction with fibres

The precision and reliability of sky subtraction is the limiting factor in faint-object spectroscopy. It has been believed that precise sky subtraction cannot be achieved with multi-object fibre spectrograph systems, so that slit observations are most suitable for study of the faintest objects, in spite of their very large loss in multiplex advantage over multifibre systems. In general, the sky signal recorded together with that of an object of interest has both astronomical and instrumental contributions. These two backgrounds have fundamentally different properties that necessitate a clear distinction in their handling to achieve precise and reliable sky subtraction. We determine the fundamental intrinsic limit to sky subtraction in general, set by the inherent lumpiness of the astronomical sky, and present a practical algorithm which allows data from fibre-fed spectrographs to be sky-subtracted with both precision and reliability close to this limit. The general considerations presented here are of relevance to all techniques of faint-object spectroscopy, including long-slit and multislit systems.