The de-censoring of the faint stars in Tycho photometry

The Tycho catalogue provides astrometric and photometric data for more than 1 million stars. The derivation of the mean magnitudes of stars fainter than about 8 mag required a dedicated treatment, in order to take into account the observations that were too faint for providing a measurable signal. This treatment, called "de-censoring", is based on a statistical model permitting to substitute mean values to the missing data. Faint photometric standard stars are used to demonstrate that the de-censoring produces reliable magnitudes and uncertainties.