A very small number of ref_id values for Gaia sources appear twice in the Tractor catalogs. One of the duplicates corresponds to the "correct" Gaia source and the second duplicate has all of the correct information for a Gaia source but with an incorrect ref_id. The ref_id that would be correct for the second duplicate is missing entirely from the Tractor catalogs.
An example from DR9 is ref_id 303293444934132224, which appears in Tractor catalog tractor-0238p302.fits with correct information (RA, Dec, etc.) and in Tractor catalog tractor-0238p305.fits with information that actually corresponds to the Gaia source with ref_id 303315950562852864.
The cause of this problem has yet to be fully diagnosed, although the discrepant Gaia sources tend to lie very close to the boundaries of a brick.
As Adam wrote,
Duplicate ref_id values
A very small number of ref_id values for Gaia sources appear twice in the Tractor catalogs. One of the duplicates corresponds to the "correct" Gaia source and the second duplicate has all of the correct information for a Gaia source but with an incorrect ref_id. The ref_id that would be correct for the second duplicate is missing entirely from the Tractor catalogs.
An example from DR9 is ref_id 303293444934132224, which appears in Tractor catalog tractor-0238p302.fits with correct information (RA, Dec, etc.) and in Tractor catalog tractor-0238p305.fits with information that actually corresponds to the Gaia source with ref_id 303315950562852864.
The cause of this problem has yet to be fully diagnosed, although the discrepant Gaia sources tend to lie very close to the boundaries of a brick.