After background subtraction, the spectral orders can be extracted. This is done by defining offsets in x-direction (with respect to the center of both fibers) and summing up the flux within a defined slit-width around these offsets. Both fibers are treated completely separately.
For reason of efficiency and simplicity, this is done in two steps.
The extraction can be done by straight summation or by using an optimum extraction algorithm which also detects and removes cosmic ray events. Optimum extraction requires a good knowledge of the cross-order profile (COP). For FEROS, this profile is - due to the image slicer - very complicated. It is approximated by fitting the fractional flux per pixel at several distances from the center of the COP as a function of position along the order with a polynomial. Since rebinning has to be avoided, this step is complex and CPU intensive. Thus, the COP can be saved for further use. For FEROS, the COP is typically determined from a flat-field exposure and later used for extraction of object spectra. This assumes that the COP does not change its profile, which can reasonably be assumed only for fiber-linked spectrographs.