  Fibre-fed spectrographs now have throughputs equivalent to slit
spectrographs. However, the sky subtraction accuracy that can be reached has
often been pinpointed as one of the major issues associated with the use of
fibres. Using technical time observations with FLAMES-GIRAFFE, two observing
techniques, namely dual staring and cross beam-switching, were tested and the
resulting sky subtraction accuracy reached in both cases was quantified.
Results indicate that an accuracy of 0.6% on sky subtraction can be reached,
provided that the cross beam-switching mode is used. This is very encouraging
with regard to the detection of very faint sources with future fibre-fed
spectrographs, such as VLT/MOONS or E-ELT/MOSAIC.
