A photometric calibration of the Guide Star Catalog

Using the Tycho Catalogue as a photometric reference catalogue, the magnitudes of the published Guide Star Catalog 1.1 have been re-calibrated. It is possible to bring the systematic errors of the GSC well below 0.1 mag, at least for stars brighter than 11.5 mag. For each plate, an individual colour term and magnitude dependant correction was applied. Only 0.6 per cent of the plates were unfit for calibration. The systematic corrections are normally below 0.4 mag for stars of magnitude 10. The one sigma equivalent of the resulting residuals is 0.18 mag for the Palomar Quick-V plates used in the northern sky and 0.14 mag for the UKST IIIa-J plates used in the south.