Detection of Arc lines

At first you have to find the calibration lines in the wavelength calibration frame belonging to your object. This is achieved by SEARCH/MOS which detects all lines separately for each slitlet and centers them either with center of gravity or with a gaussian fit. The search threshold and window are stored in (200,5). The positions of the lines, the CCD rows and the slitlets where the lines have been detected are stored in the table .tbl (linpos). First the median of ( (3)) rows in the middle of each slitlet is calculated to search in this average spectrum for lines. 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 order to speed up the search you may decide not to take every row, but only some ( (3)) and for the detection of weak lines you may bin several row together ( (3)). 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 $\sigma$ and the center of the line.