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\item Le survey : As stated in REF, ``The fundamental basis of the LSST concept is to scan the sky deep, wide, and fast with a single observing strategy, giving rise to a data set that simultaneously satisfies the majority of the science  goals. This concept, the so-called “universal cadence”, will yield the main deep-wide-fast survey  (typical single visit depth of r ∼ 24.5) and use about 90\% of the observing time.'' This main survey will consist in series of pairs of 15-second exposures with each filter, optimized to cover the southern sky (20000 deg^2) deg$^2$)  in about 3 to 4 days, with a single visit reaching magnitude ~24.5 in the r band. As aften as possible, each field will be observed twice, with visits separated by 15 to 60 minutes, so as to optimize sampling of short-period variability. Over the planned 10 years of observations, this will represent about 2.8 million visits, or 5.6 15-secon long exposures. ``The remaining 10\% of the observing time will be used to obtain improved coverage of parameter space such as  very deep (r ∼ 26) observations, observations with very short revisit times (∼ 1 minute), and  observations of “special” regions such as the ecliptic, Galactic plane, and the Large and Small