The Orion field is an interesting case. It has many identified sources (though I'm not sure how many are in Tycho). From past discussions there is at least one Tycho match that was observed 22 times and is a known double. So you have two things happening:
- One oft-observed source with a systematically bad counterpart putting a lot of bad samples into the dataset.
- One observation with a lot of sources putting a lot of repeat values of offset into the dataset.
Issue 1 can presumably be addressed by making a table of bad source positions, so X-ray sources within 5 arcsec of a bad source position is not used in statistics or most plots (or gets marked as bad in any visualizations).
Issue 2 may come down to generating a per-obsid offset that represents the weighted average for that obsid. We might have talked about that before. This does open a new source of problems, say you have 4 X-ray sources in a field and 3 of them have small radial offsets < 0.5 arcsec and one is an outlier at 2.0 arcsec, but there is no obvious justification for throwing that one out.
Old Orion discussion
TA
"Yes, this works best for named sources. Orion nebula has hundreds of sources (https://www.nasa.gov/mission_pages/chandra/images/the-orion-nebula.html) with good astrometry but again it is possible that one could be dealing with a binary. A positional search is reasonably likely to give the right source most of the time in this case.
JC
In the Orion nebula observations, it looks like what we're probably matching is this double star:
http://simbad.u-strasbg.fr/simbad/sim-id?Ident=%40810775&Name=*%20tet01%20Ori%20C&submit=submit
TA
This does point out that the single star in Orion was apparently contributing 22 of the 107 data points, which clearly indicates a need to control for repeats. This is not entirely trivial because it is difficult to separate out systematic offsets that are real (due to the X-ray emission coming from a different place than the identified optical source in our catalog) vs. systematic celestial location errors (i.e. all using the same fid set) vs. statistical cel. loc. errors. I don't have any immediately great answer to that.
cc: @javierggt @jeanconn
The Orion field is an interesting case. It has many identified sources (though I'm not sure how many are in Tycho). From past discussions there is at least one Tycho match that was observed 22 times and is a known double. So you have two things happening:
Issue 1 can presumably be addressed by making a table of bad source positions, so X-ray sources within 5 arcsec of a bad source position is not used in statistics or most plots (or gets marked as bad in any visualizations).
Issue 2 may come down to generating a per-obsid offset that represents the weighted average for that obsid. We might have talked about that before. This does open a new source of problems, say you have 4 X-ray sources in a field and 3 of them have small radial offsets < 0.5 arcsec and one is an outlier at 2.0 arcsec, but there is no obvious justification for throwing that one out.
Old Orion discussion
TA
"Yes, this works best for named sources. Orion nebula has hundreds of sources (https://www.nasa.gov/mission_pages/chandra/images/the-orion-nebula.html) with good astrometry but again it is possible that one could be dealing with a binary. A positional search is reasonably likely to give the right source most of the time in this case.
JC
In the Orion nebula observations, it looks like what we're probably matching is this double star:
http://simbad.u-strasbg.fr/simbad/sim-id?Ident=%40810775&Name=*%20tet01%20Ori%20C&submit=submit
TA
This does point out that the single star in Orion was apparently contributing 22 of the 107 data points, which clearly indicates a need to control for repeats. This is not entirely trivial because it is difficult to separate out systematic offsets that are real (due to the X-ray emission coming from a different place than the identified optical source in our catalog) vs. systematic celestial location errors (i.e. all using the same fid set) vs. statistical cel. loc. errors. I don't have any immediately great answer to that.
cc: @javierggt @jeanconn