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2 The Hipparcos processing

Before proceeding with the new solutions it is worth recalling the various categories of astrometric solutions published in the Hipparcos Catalogue. The Hipparcos Catalogue is the primary result of the observations and reductions of the satellite data acquired over 37 months between November 1989 and March 1993. The Catalogue comprises 118218 entries with median astrometric positions of the order of 1 milliarcsec and specific results (separation, magnitude difference) for double and multiple systems. For single stars the standard astrometric model yields the five astrometric parameters (position, parallax and the two components of the proper motion) together with their full covariance matrix.

Many catalogue entries were known to be, or found to be, components of a double or multiple system. The final astrometric solution for these complex entries, given either for the photocenter of the system or for the brighter component, is fully independent of the relative position of the two components. In cases of detected duplicity the observation model had to be extended to account for resolved systems or for systems exhibiting a significant motion on the sky over the mission. One major difficulty in the double star processing was the consequence of the large variety of categories of systems and the difficulty to solve simultaneously for the relative and absolute astrometry.

It was clear when the time came to publish the results that several hundred problem stars were left and their solution could not be given with the same level of reliability as for the bulk of the Catalogue. The main problems were primarily linked to the double and multiple star solutions and much more time would have been required to clear the remaining difficulties for a subset no larger than a thousand stars. The main sources of problems were:


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