The very first thing you should do now is to locate the limits of your slitlets
because this information is needed for all further commands. Therefore you
type
LOCATE/MOS
This should produce the table 
.tbl ( mos) with the columns
and write the total number of slitlets to 
( 0).
It may be that the threshold defined by 
( 500)
is either too low
(e.g. below bias value) or too high for your data. Then you should have a
look at your flat field and try again with
SET/MOS flatlim=new_threshold
LOCATE/MOS
If you do not have FORS data the column :xoffset will be set to zero.
To determine the offsets of your slitlets relative to the first one
(which is not necessarily identical with the center of the CCD) use
OFFSET/MOS
This command will take the calibration spectra of the different slitlets and by correlating them with the arc spectrum of the first slitlet will derive their offsets.
Normally spectroscopic flat fields show the spectral signature of the lamp
with which they were taken. You can take out this spectral intensity
distribution with
NORMALIZE/MOS
By default this command will fit a polynomial
of 
( 3
)
order to the averaged
(along the slitlet) spectral intensity of the flat field (separately
for each slitlet) and divide it
by these fits. The results are stored in the frame 
. If you have not
written anything to this keyword it will derive the name of the normalized
flat field by adding an `n' to the name of the input flat
(e.g. nmosff1)
and write the name to 
.
To perform the actual flatfield correction type
FLAT/MOS
This command will take the frame 
.bdf, divide it by 
.bdf
and write the result to 
.bdf.
If you have not
written anything to this keyword it will derive the name of the
flat field corrected object frame by adding an `f'
to the name of the input
frame (e.g. ffors0001).