The lamps used for flat fields have an uneven spectral emissivity. Combined with the uneven spectral sensitivity of the spectrograph and detector, this results in flat fields which usually have very different intensities in different spectral regions. Therefore a direct flat-fielding, i.e., division by the original dark subtracted flat-field, produces spectra which are artificially enhanced in some parts and depressed in others. In principle, this should not be a problem because the instrumental response curve should include these variations and permit them to be corrected. However, the response curve is established by integration over spectral intervals of small but finite width. At this stage, strong gradients obviously introduce severe problems. Also the wish, at a later stage to evaluate the statistical significance of features argues strongly against distorting the signal scale in a basically uncontrolled way.
Therefore, it is better first to remove the low spatial frequencies along the dispersion axis (but not perpendicular to it!) from the flat-field, using the command NORMALIZE/FLAT. This command first averages the original image along the slit (assumed to be along the Y-axis) and fits the resulting one-dimensional image by a polynomial of specified degree. Then the fitted polynomial grows to a two-dimensional image and divides the original flat-field by this image to obtain a ``normalized'' flat-field.
In some cases, e.g., EFOSC in its B300 mode which gives a very low intensity flat-field at the blue end, the polynomial fit is not satisfactory and it is advisable to do the sequence manually. Instead of fitting a polynomial one could fit a spline using the command INTERPOLATE/II or NORMALIZE/SPECTRUM. The latter belongs to the low-level context SPEC and is interactively operated on a graphical display of the spectrum.