After identifying the arc lines you may now start the actual wavelength
calibration with
SET/MOS wlcmtd=IC
CALIBRATE/MOS
This program fits a dispersion relation at first to the row with the identified
lines, then to the slitlet, and afterwards to all slitlets (row by row,
separately for each slitlet).
The fitted line positions are compared to those listed in 
.tbl
( hear).
The type of polynomials (Legendre or Chebyshev) is read from
( CHEBYSHEV), the fit order from 
( 2), and
the tolerance for automatic line identification from 
( 1 pix).
can be given in pixel (> 0) or wavelength units (< 0).
In case you know the central wavelength and the mean dispersion of the
grism you used you have to correct the central wavelength for the offset
between the center of the CCD and the reference slitlet used by
OFFSET/MOS. Then you can use the following command
SET/MOS wcenter=central wavelength
SET/MOS avdisp=mean linear dispersion
SET/MOS wlcmtd=LC
CALIBRATE/MOS
which will either read the two parameters from 
and 
.
If you have FORS data use
SET/MOS wlcmtd=FC
CALIBRATE/MOS ? ? FC
This will read the grism number from the header of 
and then take
the parameters stored in the respective keyword.
The plot option 
( N)
decides whether or not you get a plot of the residuals
and the display option 
( 0) influences the amount of intermediate
results displayed on screen.
CALIBRATE/MOS stores the results in 
.tbl ( coerbr)
in the columns
and adds to 
.tbl ( linpos) the columns
Slitlets where no dispersion relation could be fitted are marked with '0'
in column :SLIT.
With
RESPLOT/MOS ? slit
you can also get a plot of the residuals for a certain slit (number =
slit) that are stored in
.tbl ( linpos)
(or in the first parameter given on the command line).