Source detection and association

Moving objects are detected as part of Difference image analysis (DIA). For the DP2 Solar System products, DiaSource detections were associated with the predicted positions of previously known small bodies.

Association input

The association input was a 2026 March 13 snapshot of the MPC orbit database, limited to objects whose observational arcs exceeded two days. Small bodies discovered after that date, including those found in Rubin data, are therefore absent from the delivered SSSource table. The input orbit catalog did not contain comets, so no comets were available for association in DP2.

Ephemerides at the visit epochs were generated with Sorcha (see Merritt et al. 2025 and Holman et al. 2025). The same MPC snapshot is delivered as the auxiliary DP2 mpc_orbits table.

Matching algorithm

Associations were made independently for each detector and visit. Predicted objects were first restricted to the detector footprint. For each predicted position, the algorithm found all DiaSources within 1 arcsecond. For each visit-detector, all candidate pairs were considered in order of increasing separation and accepted only if neither the predicted object nor the detection had already been assigned. The result is therefore a one-to-one positional match.

The matching did not use positional uncertainties, brightness, color, morphology, motion rate, or a probabilistic association score. The delivered diaDistanceRank is 1 for every SSSource row and must not be interpreted as a ranking of alternative matches. Each diaSourceId occurs at most once in SSSource; unassociated DiaSource detections have no corresponding row.

Contents and interpretation

Each SSSource row contains selected measured quantities from the DiaSource and the predicted ephemeris at the observation epoch. These include measured astrometry and PSF photometry, predicted position and apparent Johnson-V magnitude, observing geometry, rates, and Cartesian position and velocity components. The ephemeris coordinates and state vectors are predictions from the input MPC orbit, not a Rubin-derived orbital solution. Users needing the complete detection record or detection-quality fields should join SSSource to DiaSource on diaSourceId.

The ephOffset column gives the total measured-minus-predicted angular separation. The coordinate, along-track, and cross-track components are available in ephOffsetRa, ephOffsetDec, ephOffsetAlongTrack, and ephOffsetCrossTrack. These residuals are useful diagnostics, but the hard 1-arcsecond selection radius truncates their distribution.

Chance associations remain possible, especially in regions with high DiaSource density, because the match does not use brightness or detection reliability. Users constructing a high-purity sample should consider detection reliability, angular and cross-track residuals, consistency with the predicted magnitude, and the uncertainty of the input orbit.

At this time, most of the associations delivered here have not yet been reported to the Minor Planet Center (MPC). Rubin will submit the astrometric measurements represented in SSSource to the MPC in stages, so that association quality can be validated before each submission and the risk of reporting misassociations can be minimized.