The RA_ERR and DEC_ERR values are approximations and become increasing inaccurate the closer to the poles, DEC=+/-90.
Users may also see incorrect values for these errors for RA values very near 0|360 such that the error bar crosses the boundary and wraps around.
The X_ERR and Y_ERR values in physical pixels is correct. Users can use these together with the known plate scale to compute their own error estimates.
A bug was introduced in CIAO 4.17 that now always creates the regfile ASCII region. The default parameter value is none, so the tool will try to create a file named none in the current directory. If the tool is unable to create the file (e.g. due to permission problems or clobbering) the tool will error out.
The values for the BKG_RATE and BKG_RATE_ERR columns are always set equal to 1.0.
The way the vtpdetect algorithm works the exposure corrected background rate is indeed constant for all sources. There is one background level above which Voronoi cells may be merged to create sources. The ESTBKG header keyword has the estimated number of background counts in the area covered by the tessellation, ie the TOT_AREA keyword.
The BKG_RATE and BKG_RATE_ERR column values are not used by vtpdetect.