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.