A method to reprocess the Hipparcos Intermediate Astrometry Data that reduces the propagation of the along-scan spacecraft attitude errors was developed and successfully used to obtain a more consistent parallax of the Pleiades (Makarov, published in 2002). The same technique is used now to correct the Hipparcos parallaxes of the Coma Berenices and NGC 6231 open clusters, which are also in error. The new mean parallax of Coma is 12.40 ± 0.17 mas (against previously 11.43 mas) and of NGC 6231 1.7 ± 0.4 mas (against previously -0.8 mas). The new data for Coma are in excellent agreement with the pre-Hipparcos main-sequence fitting estimates. All six members of NGC 6231 that have negative parallaxes in Hipparcos obtain positive parallaxes. These results suggest that the main source of astrometric error has been correctly identified, and that a more accurate Hipparcos catalog can be computed.
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