Parallaxes and proper motions for 20 open clusters as based on the new
  Hipparcos catalogue
  A new reduction of the astrometric data as produced by the Hipparcos mission
has been published, claiming that the accuracies for nearly all stars brighter
than magnitude $\mathrm{Hp}=8$ are improved, by up to a factor 4, compared to
the original catalogue. As correlations between the underlying abscissa
residuals have also been reduced by more than an order of magnitude to an
insignificant level, our ability to determine reliable parallaxes and proper
motions for open clusters should be improved. The new Hipparcos astrometric
catalogue is used to derive mean parallax and proper motion estimates for 20
open clusters. The HR-diagrams of the nearest clusters are compared and
combined to provide future input to sets of observational isochrones. The
positions of the cluster HR diagrams are consistent within different groups of
clusters shown for example by the near-perfect alignment of the sequences for
the Hyades and Praesepe, for Coma Ber and UMa, and for the Pleiades, NGC 2516,
and Blanco 1. The groups are mutually consistent when systematic differences in
$\Delta c_0$ are taken into account, where the effect of these differences on
the absolute magnitudes has been calibrated using field-star observations.
