In cross matches, the nearest within 1.5, what are false match rate, missed match rate?

Question

I have a general question about cross-matching. For example,  ls_dr10.x1p5__tractor__gaia_dr3__gaia_source is for matching legacy DR10 and Gaia DR3. I see that x1p5 indicates the nearest counterpart inside 1.5". Because there may be other possible counterparts nearby, I am looking for a description of false matches, confusing matches, and missed real matches if the position error is larger than 1.5. Where can I find such information? 

ls_dr10.tractor has wise_coadd_id, ref_id (mostly GAIA ID). Are these WISE and GAIA matches the same as in the corresponing x1p5 tables? In other words, are they also the nearest counterpart within 1.5"?   It is possible that the listed WISE and GAIA sources are separately more than 1.5", and they are not related. Is this something we need to worry about?

Answer 1

Hi, 

Our x1p5 crossmatch tables are simply catalog-matched convenience tables and do not cover all science cases. We use the distance and join function from q3c to determine the single nearest neighbor within 1.5" and do not retain any mismatches or information about false matches, confusing matches, or missed real matches. The Legacy Surveys may not crossmatch in this same way, and may contain more or less matches depending on how they did their crossmatching. You could run your own crossmatch query with your own constraints to figure out the selection that you're looking for. Here are some example notebooks on how to do that: https://github.com/astro-datalab/notebooks-latest/tree/master/04_HowTos/CrossmatchTables