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IRAF’s identify error – center not found

During the last few days I have struggled a bit with the wavelength calibration of some long-slit spectra. When using the identify task of IRAF I found that I could select (“m”ark) only some of the lines, without any clear indication why the rest were not recognized (some were indeed stronger but others were equal or weaker). The actual error that kept popping out was “Center not found: check cursor position”. Although I went on to investigate all parameters (even the aidpars !) the one thing that solved the problem was finally to increase a bit the full-width of the features to be identified (fwidth), from the default value of 4.0 to 10 pixels. Apparently some of the lines seems too wide to be determined as real features by the task.