If you have used DEFINE/SLIT there are no offsets copied to .tbl automatically and you have to determine the offsets yourself using the command OFFSET/MOS on a wavelength calibration frame. This command will take the calibration spectra of the different slitlets and search for arc lines with the parameters given by (200,5). Correlating these line lists with that of the first slitlet gives the offsets relative to the first slitlet. The resulting offsets may be wrong if there are not enough or too many lines to get unambiguous correlation results. The safest way is to use this command after you are satisfied with the results of SEARCH/MOS and you have stored the parameters for threshold and width to . In this case you will NOT have the offsets from the center of the CCD but relative to the position of the first slitlet! The changing image scale over the detector can make the method unappropiate. Therefore it is strongly recommended to copy the offsets from descriptor data to the .tbl table, but not from OFFSET/MOS - if possible.
NoteBefore starting the calibration, a carefully selected line catalog must be prepared. Reject any line that is below the detection limit and any line which is blended!