The next frame taken after the flat field will generally be an arc spectrum, which should be reduced using REDUCE_ARC. A raw arc spectrum looks like this (this is the HK grism with the argon lamp, most grisms will not show quite as many lines as this):
Again the slices are extracted and approximately aligned in the dispersion axis (with no resampling):
A static calibration file is now used to give a list of arc lines. The Figaro Iarc routine is used to wavelength calibrate every row of the spectrum individually, and the resulting calibrations are filed. All rows are now scrunched to a common linear wavelength scale to test the new calibration. If calibration has been successful then all spectral lines should now be straight and the spectrum should be virtually indistinguishable from a normal long slit spectrum. (Note that the dispersion axis is flipped at this point. Raw IFU spectra have increasing wavelength to the left, but the scrunching applies a wavelength scale increasing linearly to the right.)
The resulting calibration, which is filed in the index.iar file, is a text file giving the wavelength calibration to be applied to each row of the 2d spectrum. This is the output of the Figaro Iarc routine.