How in situ atmospheric transmission can affect cosmological constraints
  from type Ia supernovae ?
  The measurement of type Ia supernova colours in photometric surveys is the
key to access to cosmological distances. But for future large surveys like the
Large Survey of Space and Time undertaken by the Vera Rubin Observatory in
Chile, the large statistical power of the promised catalogues will make the
photometric calibration uncertainties dominant in the error budget and will
limit our ability to use it for precision cosmology. The knowledge of the
on-site atmospheric transmission on average for the full survey, or for season
or each exposure can help reaching the sub-percent precision for magnitudes. We
will show that measuring the local atmospheric transmission allows to correct
the raw magnitudes to reduce the photometric systematic uncertainties. Then we
will present how this strategy is implemented at the Rubin Observatory via the
Auxiliary Telescope and its slitless spectrograph.
