Source detection and association#
Moving objects are detected as part of difference image analysis.
For the DP2 Solar System processing and data 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 Minor Planet Center (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 DP2 Solar System processing association and the delivered DP2 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 DP2 mpc_orbits table contains a snapshot of the Minor Planet Center’s catalog of orbital elements, as used for Solar System object association.
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 DP2 delivered diaDistanceRank field is 1 for every SSSource table row and should not be interpreted as a ranking of alternative matches.
Each diaSourceId occurs at most once in the SSSource table; unassociated DiaSource detections have no corresponding row in the SSSource table.
SSSource table contents and interpretation#
A DP2 “Solar System source” is a signal-to-noise ratio > 5 moving object detection in a difference image that has been associated with a previously known small body.
Each row in the DP2 SSSource table represents a one-to-one positional association between a DiaSource and the predicted position of a known small body.
Each SSSource table contains additional useful computed quantities for previously known Solar System objects that have been linked to measured DIASource photometry and astrometry.
These include 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 the SSSource table to the DiaSource table 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, not all measurements of known objects have yet been reported to the Minor Planet Center. Rubin is submitting the astrometric measurements represented in the DP2 SSSource table to the MPC in stages, so that association quality can be validated before each submission and the risk of reporting misassociations can be minimized. The associations delivered in the DP2 data set are currently being reported to the Minor Planet Center (MPC) in this manner.