4. Photometric accuracy
Internal standard errors of the Tycho-2 photometry are shown in Fig. 2b. It is concluded in H2000a, from a comparison with ground-based photometry, that the ratio of the external and internal standard errors is close to 1.0 for stars brighter than magnitude 7.5 and then gradually rises to at most 1.5 at magnitude 10 and fainter. The Tycho-2 magnitudes are systematically correct down to magnitudes 13 and 12 in and , respectively. Fainter magnitudes are not trustworthy, and magnitudes in the catalogue fainter than respectively 15 and 14 should be understood to mean that the star was not detected in that colour.
© European Southern Observatory (ESO) 2000
Online publication: March 9, 2000