DEFINE/MOS helps you to localize your objects and sky regions
and by default works automatically.
It averages 
columns around the position
(in world-coordinates!). In the averaged frame it tries to detect objects
above the threshold (
)
within the search window 
and fits a gaussian to any detected object.
The limits of the object are defined at the position where the gaussian has
reached 
of the central intensity. A safety margin of 3 pixels
is taken on both sides of each object where no sky is automatically defined
(can be overridden manually later) and the rest of the slitlets is taken
as sky region. The results are stored in 
.tbl and can be displayed
in the display and/or the graphics window. If you are not satisfied with
the results you can change them interactively. You may also choose the
SET/MOS def_mode=i
Then no automatic search is performed; instead you define the objects and sky regions for each slitlet manually with graphics cursor.
The threshold may be given in absolute (> 0.0) or relative (< 0.0) numbers. It may be advisable to do at least a rough sky subtraction ahead of this command to facilitate the detection of the objects.
One may also think about rebinning the object frame to constant wavelength steps because then the search could be done in the same wavelength region for all slitlets.
Table: Commands of the context MOS