  Although relative errors can readily be calculated, the absolute astrometric
accuracy of the source positions in the Chandra Source Catalog (CSC), Version
1.0, is a priori unknown. However, the cross-match with stellar objects from
the Sloan Digital Sky Survey (SDSS) offers the opportunity to compare the
apparent separations of the cross-matched pairs with the formally calculated
errors. The analysis of these data allowed us to derive a value of 0.16" for
the residual absolute astrometric error in CSC positions. This error will be
added to the published position errors in the CSC from now on, starting with
CSC, Version 1.1.
