  The main scientific characteristic of VIMOS (VIsible Multi Object
Spectrograph, to be placed on ESO Very Large Telescope) is its high
multiplexing capability, allowing astronomers to obtain up to 800 spectra per
exposure. To fully exploit such a potential a dedicated tool, the VIMOS Mask
Preparation Software (MPS), has been designed. The MPS provides the astronomer
with tools for the selection of the objects to be spectroscopically observed,
including interactive object selection, handling of curved slits, and an
algorithm for automatic slit positioning that derives the most effective
solution in terms of number of object selected per field. The slit positioning
algorithm must take into account both the initial list of user's preferred
objects, and the constraints imposed either by the instrument characteristics
or by the requirement of having easily reduceable data. The number of possible
slit combinations in a field is in any case very high (10^73), and the task of
slits maximization cannot be solved through a purely combinational approach. We
have introduced an innovative approach, based on the analysis of the function
(Number of slits)/(slit length) vs. (slit length). The algorithm has been fully
tested with good results, and it will be distributed to the astronomical
community for the observation preparation phase.
