Order definition

Due to the use of an image slicer for the two fibers, the FEROS order definition uses a special technique for finding and centering orders. The definition of the orders is done in several steps.

• First, the position of the orders is defined near the middle of the frame. A central cut through the orders is cross-correlated with a template which should match as good as possible the cross-order profile. Centering the peaks of the correlation function gives the centers of the orders near the center of the CCD. If the template consists of the profile of a double fiber, both fibers are centered simultaneously. This is the normal way of operation.
• Second, the orders are followed along the main dispersion direction by cross-correlating cuts taken at varying y-positions with a secondary template extracted from the central cut.
• Finally, the detected x-y positions of all orders are fitted with a polynomial. This is done for each order individually. The polynomial coefficients are kept in a table for the further steps.

Order definition is done with the command DEFINE/FEROS.