Table of Contents
RA_ERR and DEC_ERR values near poles
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.
The BKG_RATE and BKG_RATE_ERR columns are always equal to 1.0.
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.
vtpdetect with negative coordinates (16 May 2007)
Running vtpdetect on a level 1 event file may fail with an error that all the events are at the same location and the tool cannot tessellate. This is related to the (normal) bad coordinates at the beginning of the ObsID where there is no aspect data.
If you must work with the evt1.fits file, apply a time filter before running vtpdetect.