Sky Subtraction with Fiber-Spectrographs

The sky subtraction performances of multi-fiber spectrographs are discussed, analysing in detail the case of the OPTOPUS system at the 3.6 meter ESO telescope at La Silla. A standard technique, based on flat-fields obtained with a uniformly illuminated screen on the dome, provides poor results. A new method has been developed, using the [OI] emission line at 5577 \AA\ as a calibrator of the fiber transmittance, taking into account the diffuse light and the influence of each fiber on the adjacent ones, and correcting for the effects of the image distortions on the sky sampling. In this way the accuracy of the sky subtraction improves from $2-8 \%$ to $1.3-1.6 \%$.