Fiber-fed spectrographs can now have throughputs equivalent to slit spectrographs. However, the sky subtraction accuracy that can be reached on such instruments has often been pinpointed as one of their major issues, in relation to difficulties in scattered light and flat-field corrections or throughput losses associated with fibers. Using technical time observations with FLAMES-GIRAFFE, two observing techniques, namely dual staring and cross beam switching modes, were tested and the resulting sky subtraction accuracy reached in both cases was quantified. Results indicate that an accuracy of 0.6% on the sky subtraction can be reached, provided that the cross beam switching mode is used. This is very encouraging regarding the detection of very faint sources with future fiber-fed spectrographs such as VLT/MOONS or ELT/MOSAIC.

On-sky tests of cross-beam switching operations with FLAMES/GIRAFFE at the VLT. Left panel: raw frame with individual spectra showed as individual columns and wavelength along the vertical axis; the sky lines can be seen as horizontal bright regions. Right panel: results after direct subtraction of the raw and sky frames after switching to science and sky fibers: no significant residuals can been seen. The bottom-right panel shows the accuracy of the method, which can reach a 0.6% accuracy. Optimized systems with advanced algorithms are expected to reach to ~ 0.1 % accuracy level.