Correlation coefficients between the proper motion in Right Ascension and the parallax, averaged over HEALPix level 5 pixels. Top: for stars with first-epoch Hipparcos positions; bottom: for stars with first-epoch Tycho-2 positions. Correlations in the bottom graph are clearly systematic over the sky (linked to scan coverage) and can reach values over ±0.9. Similar correlations, but differently distributed, are observed between all the astrometric parameters for data with first epoch Tycho-2 positions.
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