The main disadvantage of the thin CCD-chips used currently as detectors in some instruments is the generation of interference fringes, that is, intensity fluctuations in the spectra which can be as high as 30% at Å (York, 1981). These fringes arise from interferences within the silicon for long wavelengths, while for shorter wavelengths they can be due to the interfaces silicon-glass on the back side of the chip. This effect is constant for a given setting and it can be effectively corrected by dividing the object image by a flat-field exposure taken with the same instrument configuration. Before the actual division is carried out, the background levels, both in the object image and flat-field, are subtracted and the flat-field is normalised.
The flat-field correction is done with the command COMPUTE/IMAGE, this command divides the background subtracted OBJ by the normalised flat-field as computed by the command FLAT/ECHELLE.