THE QUALITY OF PHOTOMETRIC INFORMATION IN THE HIPPARCOS-INPUT-CATALOG

Comparison with independent photoelectric photometry is used to assess the quality of magnitudes and colors in the Hipparcos Input Catalog. These magnitudes and colors will play an important role in the luminos ity calibration of Hipparcos stars. We conclude that photoelectric pho tometry, available for about 37% of the catalog, is of uniformly good quality, and the error estimates in the Catalog are very accurate. On the other hand, colors determined from poor (one-dimensional) spectral classes suffer from very significant errors, also correctly estimated m the Catalog, which make them essentially unsuited for the purpose o f luminosity calibration. Only stars with either photoelectric colors or good two-dimensional spectral classes can be used in a high-accurac y luminosity calibration.