The performance of the CMS ECAL Preshower detector at LHC

The Preshower detector is part of the CMS endcap electromagnetic calorimeter, located in front of the lead tungstate crystals. It is comprises two orthogonal planes of silicon strip sensors each preceded by a thin layer of lead. The purpose of the Preshower is to identify two closely spaced photons from $\pi^{0}$ decays, allowing a reduction of one of the backgrounds to the two-photon decay channel of the intermediate mass Higgs boson - $\pi^{0}$s in jets faking photons. Following a brief overview of the design of the Preshower we present, - occupancy and first in-situ absolute calibration.