## 2. The basic principles of Tycho de-censoringThe mean and magnitudes
must be derived by taking the missing measurements into account.
Various techniques for solving statistical problems involving missing
data were developped during the last decades (Little & Rubin
1987). Some of them were successfully applied in astronomy (Feigelson
& Nelson 1985, Isobe et al. 1986). According to the definition
given by Kendall & Stuart (1961), the data in Tycho photometry are
affected by a "Type I censoring", since the measurements smaller
than a threshold were lost. However, their treatment is rather
complicated, because two different censorings must be considered: the
censoring coming from detection, that depends on the photon counts in
the The algorithm is based on a model describing the data acquisition. The model is used to compute the mean luminosity corresponding to each censored measurement, assuming the and magnitudes of the star are known. The mean luminosities are imputed to the censored measurements. The next step is the calculation of the mean and magnitude of the star, taking all transits into account. The values thus obtained are input for another iteration, until the calculation has converged. © European Southern Observatory (ESO) 1997 Online publication: May 5, 1998 |