FBS blue stellar objects DSS1/DSS2 astrometry

To establish the accuracy of the DSS1 and DSS2, measurements have been made for 153 AGN for which absolute VLBI coordinates have been published. The rms errors are: 0.45" for DSS1, 0.33" for DSS2 red, and 0.59" for DSS2 blue in each coordinate, the corresponding total positional errors being 0.64", 0.46", and 0.83", respectively.

The highest accuracy (0.42") is obtained by weighted averaging of the DSS1 and DSS2 red positions. It is shown that by using all three DSS images accidental errors can be significantly reduced. The comparison of DSS2 and DSS1 images made it possible to reveal positional differences and proper motions for 78 objects (for 62 of these for the first time), including new high-probability candidate white dwarfs, and to find objects showing strong variability, i.e. high-probability candidate cataclysmic variables.