9.2 Table generation
The Gaia EDR3-Gaia DR2 neighbourhood table has been generated according to an ADQL query shown in Section 9.4.1. It applies a simple linear proper motion correction (when possible) to all Gaia EDR3 objects and looks for Gaia DR2 matches within a 2” radius. The angular distance and magnitude difference is computed for each pair. No single value rank column has been determined, as explained in Section 9.3. The query is split into several chunks using the Gaia EDR3 random index (only 100 sources are used in the query example).
A more accurate proper motion propagation was used in Gaia DR2 against Gaia DR1. However, it was much slower. We have verified the differences with respect to the approximate calculation are negligible for cross-match purposes due to the small average epoch difference between catalogues (half a year). See more details in the Archive on-line cross-match tutorial. The simpler propagation allowed us to use the same query for all stars. All objects are proper motion corrected and the same search radius is always used, as opposed to Gaia DR2. In the end, this added robustness to the procedure and prevented us to commit the same error as for Gaia DR2, where the high proper motion stars were inadvertently not added to the combined table.