Calibration of the CMS Electromagnetic Calorimeter at LHC startup

The first 7 TeV LHC collisions recorded with the CMS detector have been used to perform a channel-by-channel calibration of the electromagnetic calorimeter (ECAL). Decays of $\pi^0$ and $\eta$ into two photons as well as the azimuthal symmetry of the average energy deposition at a given pseudorapidity are utilized to equalize the response of the individual channels. The ECAL comprises a central {\it barrel} section and two {\it endcaps}. Based on an integrated luminosity of $\sim$ 100~nb$^{-1}$, a channel-by-channel {\it in-situ} calibration precision of 1.15\% has been achieved in the barrel ECAL in the pseudorapidity region abs($\eta$) $< 0.8$. The energy scale of the ECAL has been investigated and found to agree with the simulation to within 1\% in the barrel and 3\% in the endcaps.