The LSST is a 8m wide-field photometric telescope that will run at Cerro Pachón in Chile, starting in 2020. As part of the calibration of the 6 large photometric bands ugrizy, there is a need to monitor the atmospheric transmittance in time and across the field of view (~10 deg2). An auxiliary telescope with a spectrometer will be dedicated to that task, fitting an atmospheric model to the...