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You can repeat steps 1-5 from B.3 using the flat field frames; this time without smoothing as you are interested in the pixel to pixel variations. Then subtract the ``scaled'' preflash (if used) from the resulting frame. You will have obtained a flat field frame; divide it by its mean value and you have a normalized flat field that is now ready to be used in the data reduction.
For extended objects, it is important to obtain during twighlight several well exposed ( counts) sky frames, because the illumination of the slit by a lamp might not be uniform. These flat skies are reduced as flat fields.