At first you have to find the calibration lines in the wavelength calibration frame belonging to your object. Then in all rows the detected lines are centered using the line list obtained by the search algorithm. In this way we want to avoid unstable dispersion solutions which might arise from different line lists for different rows of the slitlet. In the case of the Gaussian centering method, blends could prevent the proper convergence of the non-linear fitting algorithm. The accuracy of the centering is significantly improved, if the peak flux of the Gaussian is hold fixed to be the meassured maximum value in the central pixel.
Free parameters are and the center of the line.