Type c to centroid a predefined aperture. On weakly exposed spectra, the object may not be detectable in the full slit profile. Alternatively, you can set the width of the extraction aperture using the height of the cursor as a threshold to define new upper and lower bounds and type y . Type r to redraw the graph. The cursor commands then available are those of the interactive curve fitting task icfit that were listed earlier.
Type z to delete the sample region nearest the cursor.
Type s to define one side of a new sample region, and s again to define the other side. Type f for a new fit. Type r to redraw the graph. Type q to exit from the interactive curve fitting task. When the extraction and background apertures have been defined, type q to exit the aperture editor.