2 Observational method

The observational programme ($30^{\rm o}$ or $45^{\rm o}$) was changed every 3 or 4 weeks. All the star groups must be sufficiently observed to make the internal and the external agreements and to have the significative mean residuals.

At Calern Observatory there are near 200 nights of observation per year that allow to obtain more or less 600 groups. In summer this quantity is smaller (2 or 3 groups) but the observational quality is better. The autocolimation is used to position the optical square with precision.