In the description of the Chandra catalogue, the column related to the positional error is defined as
Position error radius at 95% confidence level // arcsec // Position error 95% confidence level circle radius (column also called err_ellipse_r0)
While in the Chandra web page, err_ellipse_r0 is said to be something different
Three of the parameters specifying the geometry of each error ellipse are the semi-major and semi-minor radii (/err_ellipse_r0/, /err_ellipse_r1/), and the position angle θ that the major axis of the ellipse makes with respect to the tangent plane /y/ axis (/err_ellipse_ang/).
What is not actually the same. The difference can be specially relevant when cross matching several set of catalogues/source lists at once.
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