"Corrected GSC: Reference Catalogue for CCD Observations. Now"
      A.Lopez Garcia, J.M.Martinez Gonzales, A.Ortiz Gil, L.I.Yagudin

Astrometrical properties of the Hubble Space Telescope Guide Star Catalogue
(GSC) were investigated by its comparison with the PPM catalogue. Three
kinds of systematic errors were found in the GSC: sky-based errors produced
by reference catalogues, plate-based errors produced by telescope optics
and plate-based magnitude dependent errors. Subroutine for correction of
all these errors is developed. With this subroutine the GSC can be used as
reference catalogue on about 0.4" accuracy level.