Order extraction

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.

• First, the pixels are re-ordered (rectified) in a new 2-D frame. It is important to note that this step involves no resampling. The fractional pixel offsets are kept in a separate file. The corresponding command is RECTIFY/FEROS.
• Second, the fluxes are summed up, producing a new 2-D frame (pixel-order space) for each fiber. The corresponding command is EXTRACT/FEROS.

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.